Mercs and megaphones--thinking the unthinkable?

[Dixon263]

There is something that I've never been able to figure out and never heard a good explanation either. Here's the deal, I own a Stuska water brake dynamometer (130 hp max, 14,000 rpm) and have dyno'ed many four cylinder 44 cu in Mercs in quiet form. They dyno real close to 40-41 hp in stock, quiet form with a great horsepower curve. I chose to dyno directly off the driveshaft to reduce unneeded wear on the "D" Quicksilver or 45SS unit. A friend with a 12' 6" hydro saw 79 mph many times but never broke 80. We put his motor on the dyno and pulled 40.6 hp in the 6500 rpm range consistently. Then we installed a set of Parker megaphones and saw 57 hp with a wide power curve that was right in the range the motor would be run at for top end. A trip to the water saw some odd results, 79 mph and not a tiny bit faster. The pipes still made good hp to well over 7000 so we were definitely in the higher hp range. With megaphones, at top speed, the motor was developing close to 50% more hp, but was not able to go ANY faster.

Before I present a possible theory, here is another example. I modified a '44 with single ring Turner pistons, mild porting used the larger KC carb instead of the KA's. The dyno showed 46 hp, in quiet form again. A trip to the water showed a max top speed of 85 mph, quite a difference from my usual 78-80 bests. So what is going on here? These tests are not flukes but consistently this is the way it will go every time. So here is a possible theory. After decades of racers using megaphones on Mercs, is it actually possible that somehow the high speed air that flows across the pipes is cancelling the 50% gain shown on the dyno? I realize that this a big pill to swallow, but something is really wrong with this picture. 50% more power being applied to the prop at top end and not any more speed at all? I have a friend that manufactures the 'Hurricane' 27 hp industrial leaf blower that will move a large volume on air at 160 mph, I guess it would be interesting to set it up in front of the dyno with a megaphone Merc running at 57 hp and turn the air on. Any other ideas on this one?

[Bill Van Steenwyk]

Normally that type of HP gain SHOULD result in a speed increase, BUT, did you take advantage of the HP increase you showed by doing anything else to take advantage of it?

In most cases a prop with more pitch would help, or even be required, to increase speed so as to take advantage of the HP increase. If you did nothing to take care of the increase and translate it to increased thrust in the water, it would not be surprising that little or no gain would be registered in speed. I have absolutely no experience with open megaphone testing versus a closed exhaust but that increase is considerable, approaching 50%. Are you positive no mistake was made. I am sure there was some increase, but that is really considerable. Not doubting your word, just seems a lot to gain with just that one change.

You do not mention any changes made other than just the pipes. HP at the prop and the props ability to put the increase into increased "thrust" into the water is one of the keys to increased speed, with any improvement in power. During the time I raced any increase in power from the engine ALWAYS required more pitch or some changes to the prop to take full advantage of the HP increase. I seem to remember from other discussions about HP that there is some loss between the driveshaft and the propshaft, so possibly you are not seeing the entire increase at the prop, although if the increase you show is actual, that should not be the entire cause of the discrepancy. I seem to remember the major outboard mfgrs. changed the way they rate HP a few years ago to "propshaft HP" from powerhead HP to more closely indicate what was actually getting to the water.

[motormike]

Hi Bill

Maybe I can help you understand a little but I have to know more detail questions.
Lets start at the beginning. you state 40.6 hp and you saw 79 mph.

What rpm was the 40.6 rpm?

What rpm was the boat at 79mph?

Note from your the peak of 40.6hp, what is power 500 rpms higher and 500 rpms lower?

Need to understand the power band


With your mod engine, did it make more torque at higher end of the rpm range? This guides you on props needed.

It takes torgue to get the boat up and going, once the boat is off the water, it only takes rpm to go faster. most race boat speeds are best way past the peak power. If the torque drops to quick after peak power, the engine won't turn more rpms to make you speed.

Some pipes make more power, but can limit the rpm. at that point you must harness the more power by going up in pitch.
If you have 50 hp with 12 pitch you will so fast. With 75 hp and the same prop at the same rpm you will be same speed. to go faster you must increase the pitch to carry the load. More rpms alway make more speed, but with high rpm setup motors, they are soft on torque and thats why little props are needed.

The true measurement of power is torque. Horse power is just a math equation. Where the torque is and how fast it fars off is the key.

I have been doing dyno development since 1980 on stock, mod and pro engines. I have 3 active computerized dynos.
The first thing you learn when you start doing dyno work is: Everything you thought you knew, is wrong. You must go back to the basics, and understand each step as you take it.

Lets discuss.

Mike Wienandt
wptracing.com

[Dixon263]

Bill and Mike, you are both using sensible logic that I have also found to be true in pretty much all other motor performance situations. But this case is different, somehow everyone seems to be missing some phenomena with the hp made with megaphones. Let's take for instance the 4 cyl Merc 65, on the dyno it makes around 62 hp with great bottom and mid range power, but falls very rapidly at a certain rpm. When applied to the water on a hydro, everything makes perfect sense there also. It pulls hard up to the mid to high seventies then almost like a governor it falls on its face as expected. The 44 in quiet form has a very good power curve. With megaphones the curve is not quite the same, but it is safe to say that at high speed say 80 mph, the motor will be around 6500-7000 in my case with a 13 pitch prop. On the dyno it will be putting out in the mid to high fifty hp range at that rpm. This is pushing 50% more hp, but will not push the hydro ANY faster.

I have run a turbocharger on a 44 and saw 90+ hp at a boost level that was safe on 100 av gas, if I were to set the boost at a point where I made the same 57 hp, I would have positively went much faster, as expected with that set up. I'm not trying to be stubborn or argumentative, but something is being missed here with pipes. If I was still obsessed with my 44, I would devise a way to check hp output through the driveshaft while on the water. Possibly a spring loaded coupler that would flex a certain amount and a way for it to 'remember' that point after a run. I feel it is safe to say that at 7000 on the dyno, the hp would not match the hp at speed on the water at the same rpm.

[Bill Van Steenwyk]

Dixon263:

THEORY is wonderful and can provide many hours of thoughtful questions/answers, BUT nothing beats testing, especially with boats, motors, and most importantly, propellers, when questions like you have are posed.

One of the most thoughtful, talented motor builders I have ever known who was responsible for many engines that set records and won championships, including several built for me, used to have a saying. It went this way. "If everything was known about the two "P's", everyone would be a World Champion". The "two P's" are pipes and props!! Theory tells little about these "P's". Testing tells a lot more than just formulas and numbers, but not everything, and even then there are still mysteries to be unraveled. If you were to run a poll amongst boat racers, or even average boaters, they would probably without fail tell you what has them scratching their head most is props. That is why you have to either spend the money with a good prop guy, beat on them to learn yourself, or be forever lacking in probably THE most single important thing regarding knowledge about boats and making them go fast. That is of course if you first have the HP to achieve what you are striving for. If you tell us anywhere in your original question you have gone thru the process I am describing, I missed it. Also I did not see where you mentioned the type boat involved. That as I am sure you realize is also very important in your problem.

I am sure Mike Wiendandt, who has built a bunch of those championship motors would agree. Unless you have actually TESTED, with larger props, or different configurations of same, you are spinning your wheels. (no PUN intended) If you like to do that, fine. If not, it would be helpful to try to answer Mike's questions. Even the family Dr. needs a little help to determine whether you just have an ingrown toenail, or the problem is terminal. Few on this forum are as knowledgeable about MOD engines as he, both as a builder and as a competitor himself.

[Dixon263]

I've spent several years drag racing snowmobiles where a dyno was almost a must for finding the very small range that the horsepower peaked. This range on our very narrow power band of a hundred or so rpm's was so critical that we took into consideration the pipes heating over a 5-6 second run and used a varying helix clutch/converter that would follow the horsepower peak throughout the run, only changing slightly from starting line to finish line. As the pipe heated over that 5-6 seconds the peak hp would rise around 2-3 hundred rpm's. I am very aware of propping to achieve this effect and I have a wide selection from 11 to 15 pitch in several styles and diameters and it is pretty easy to match dyno peak rpm on the water. I believe the answer to my question can only be answered by thinking out of the box on this one, as I have exhausted the simple basics covered here that seem to work on other applications.

[champ20B]

I've spent several years drag racing snowmobiles where a dyno was almost a must for finding the very small range that the horsepower peaked. This range on our very narrow power band of a hundred or so rpm's was so critical that we took into consideration the pipes heating over a 5-6 second run and used a varying helix clutch/converter that would follow the horsepower peak throughout the run, only changing slightly from starting line to finish line. As the pipe heated over that 5-6 seconds the peak hp would rise around 2-3 hundred rpm's. I am very aware of propping to achieve this effect and I have a wide selection from 11 to 15 pitch in several styles and diameters and it is pretty easy to match dyno peak rpm on the water. I believe the answer to my question can only be answered by thinking out of the box on this one, as I have exhausted the simple basics covered here that seem to work on other applications.

Have you tried wrapping the pipes with insulation? Lower temps when running in the wind can cool the pipes and slow down the sound wave effect. I'd try keeping those pipes hot. Lucky for me, my Yamato megaphone is encased in a mid tower where it doesn't get too cooled. But for any outside pipes on any other kind of engine, I would wrap them up.

Also, a slight less prop pitch will free up the power curve with megaphones. When the Mark20H popper conversion was offered, 16:21 gears was included to replace the 1:1 so it would help the tuned exhaust do its job and survive the competition of hotrods. Anyway, Lesser pitch props somewhat accomplish the same effect. Maybe these ideas might work.

[Dixon263]

Champ, I've never tried insulating pipes on an outboard only on the drag snowmobile. The pipe temp would be a variable that would be different from dyno to real world though. First off there is no arguing that pipes help acceleration in the mid range. There are only a few things that I can come up with that could somehow cancel the large gains you see with pipes on the dyno at top end speeds. The pipe temp will be cooler, high speed air flowing across the pipe that could change pressure waves and the slight amount of weight that the pipes change the motors center of gravity rearward.

[Fastjeff57]

Love to know what RPM each setup was made at the boat's top speed. If each set up used the same prop, then the top speed should be the same, right?

Jeff

[GHMiller]

This thread is pretty interesting. I would like to ask a question about megaphones. What would be the change in performance, if any, if you had a narrow cone vs. a wider cone? For example, an 8 degree cone vs. a 12 degree cone? Or a longer cone vs. a shorter cone? Again for example, an 8" cone vs. a 14" cone? And then any combination of said examples what would be the performance advantage? Thanks for any input, just trying to learn.

[Fastjeff57]

In expansion chambers, a narrow cone produces a wider power band than a a narrow one. Not sure if this is also true with megaphones, but...

Jeff

[Dixon263]

This thread is pretty interesting. I would like to ask a question about megaphones. What would be the change in performance, if any, if you had a narrow cone vs. a wider cone? For example, an 8 degree cone vs. a 12 degree cone? Or a longer cone vs. a shorter cone? Again for example, an 8" cone vs. a 14" cone? And then any combination of said examples what would be the performance advantage? Thanks for any input, just trying to learn.

I'm sure there are others here that would be better qualified to answer that one on megaphones. From dyno experience I have found that with expansion chambers , the convergent cone, at the far end from the engine, controls the power band width. A very sharp angled cone sends the return pulse back to the exhaust port for a very short duration for very peaky power band. The highest hp numbers should be seen this way but are just about useless except maybe on a snowmobile where the clutch can be set to keep the engine within a couple of hundred rpm's. A very slow taper gives a much broader peak, but sends a weaker pulse back, so less actual hp, but still better in most situations.

[GHMiller]

Thanks for the reply's. But....I'm looking for a bit more on the technical side. Like the how's and why's of the megaphone. What I mean is, I've always just run whatever length and degree of cone that came from whoever made them. And accepted it as the best possible combination. Is there a better combination? I don't know and thus my questions. I've never tried to tune them and want to know how and why they were made or came to the final dimensions of the finished product. And how those final dimensions affect the performance of the motor. Please keep this discussion going and I think it will tie into the original questions that started this thread. Mr. Dixon263, I hope you don't mind, if you do I will move this to a thread of it's own.
Thanks,

[Lil' Blue Rude]

GHmiller, if your interested in a good read that might help explain the effect of different tuner lengths you should look up Gordon Jennings two stroke tuner handbook. Should be able to find it through google search in PDF form. It has a chapter about the expansion chambers and how they work and what changes affect what and why. Definitely worth a look though.

[GHMiller]

GHmiller, if your interested in a good read that might help explain the effect of different tuner lengths you should look up Gordon Jennings two stroke tuner handbook. Should be able to find it through google search in PDF form. It has a chapter about the expansion chambers and how they work and what changes affect what and why. Definitely worth a look though.

Thanks for the heads up. I've read it and understand expansion chambers but it doesn't go into megaphones and how to tune them. Does the same theory apply? It's probably close but not the same as there are I'm sure some different variables to address. This is what I'm trying to find out.

[Seagull 170]

Jennings is a very good first book to read on this subject, but there are several more that offer a slightly different viewpoint & are more up to date.

Try, Two-stroke Performance Tuning by A Graham Bell, next

Then, Two-stroke High Performance Engine Design & tuning by Cesare Bossaglia

Then, The High-Performance Two-Stroke Engine by Dr John C Dixon

Then, Design and Simulation of Two-stroke Engines by Gordon P. Blair if you can do the maths.

The Two Stroke Engine it's Design & Tuning by K.G. Draper, Two Stroke Power Units by P.E. Irvin & Motorcycle Tuning two stroke by John Robinson, are also worth a read.

The only one of the above that deals well with megaphones & mini pipes is John Dixon, if I remember right.

I'd be interested to know how much the powerband moves if you spray water on the pipes, during the dyno run.

[zul8tr]

Jennings is a very good first book to read on this subject, but there are several more that offer a slightly different viewpoint & are more up to date.

Try, Two-stroke Performance Tuning by A Graham Bell, next

Then, Two-stroke High Performance Engine Design & tuning by Cesare Bossaglia

Then, The High-Performance Two-Stroke Engine by Dr John C Dixon

Then, Design and Simulation of Two-stroke Engines by Gordon P. Blair if you can do the maths.

The Two Stroke Engine it's Design & Tuning by K.G. Draper, Two Stroke Power Units by P.E. Irvin & Motorcycle Tuning two stroke by John Robinson, are also worth a read.

The only one of the above that deals well with megaphones & mini pipes is John Dixon, if I remember right.

I'd be interested to know how much the powerband moves if you spray water on the pipes, during the dyno run.

Here is the Graham Bell book in pdf. Chapter 4 on exhaust but nothing on magaphones:

http://iheartstella.com/resources/manuals/tuning/Graham-Bell-Two-Stroke-Performance-Tuning.pdf

Here is something more specific on megaphone design for Victory cycles (not 2 strokes but informative):

http://victorylibrary.com/brit/mega-c.htm

A bit more on megaphones:

http://www.motorcycle.com/how-to/how-twostroke-expansion-chambers-work-and-why-you-should-care-3423.html

Expansion chamber exhaust has more design flexibility than megaphones.

[GHMiller]

Mr. Zul8tr,
Thanks for the links. I realize that expansion chambers are better in most cases, however, we typically use megaphones with the Mercury 44 in the modified classes. I would like to experiment a little, to see what might improve or get worse, but I need to know where my starting point is and why. I'm trying to educate myself a little and I realize that using a dyno would be ideal but I don't have access to one. I will have to do this with trial and error and seat of your pants feel.

[GHMiller]

The article at the Victory library is the most useful one yet that I've read. Thanks again for the link Zul8tr.

[rumleyfips]

The article at the Victory library is the most useful one yet that I've read. Thanks again for the link Zul8tr.

Harry Brinkman's manual ( google Boatsport.org) has an explanation and 2 diagrammes ; page s 56 - 62.

Frank Volker has some thoughts on another BRF thread.

[Art Kampen]

Having built plenty of successful 4 cly. Mercs over the years. I have found out that expansion chambers are not my cup of tea on deflector Merc`s. Megaphones are still somewhat of mystery, just a ton of variables that can effect performance,porting and other internal mods, how you cool the engine and pipes. I have found that the hotter you can run the engine and pipes the better the performance. I have actually melted an aluminum megaphone off with no damage to the engine. Pipe length and the angle the exhaust exits also has a lot to do with performance. Witness the McAfee sidewinder pipies on their E. Experimenting with cooling can gain you a lot, but can be expensive. The ignition that we run now has an effect.

[GHMiller]

Harry Brinkman's manual ( google Boatsport.org) has an explanation and 2 diagrammes ; page s 56 - 62.

Frank Volker has some thoughts on another BRF thread.

Thank you, I found the thread, interesting read.

[Fastjeff57]

Cooling the exhaust causes it to contract, reducing the pressure. Hummm...

Slow Jeff

[champ20B]

Thanks for the reply's. But....I'm looking for a bit more on the technical side. Like the how's and why's of the megaphone. What I mean is, I've always just run whatever length and degree of cone that came from whoever made them. And accepted it as the best possible combination. Is there a better combination? I don't know and thus my questions. I've never tried to tune them and want to know how and why they were made or came to the final dimensions of the finished product. And how those final dimensions affect the performance of the motor. Please keep this discussion going and I think it will tie into the original questions that started this thread. Mr. Dixon263, I hope you don't mind, if you do I will move this to a thread of it's own.
Thanks,

One thing I used to think about what really happens inside a megaphone is......

As the sound waves move outward as the cone diameter increases, the waves divide thinner and thinner, more and more. It is like if you drop a pebble in water, you will notice an initial wave made that spreads out around and divides into multiple ripples as it goes outward from the disturbance. This effect basically creates a barrier of waves that act like an expansion chamber's belly, deflector and stinger pipe. Now, revisiting the "pebble dropped in water theory", the next point is that as the pebble enters the water, air follows in with it naturally. But as the pebble is encased by water, the water then collapses around the trail of submerged air and resonates it back into the atmosphere. Basically, imagining all of this, the wave ripple theory explains how the sound waves can create a situation of a density like the water is to air in the fore-mentioned figurative scenario. The pebble represents the sudden shot of powerful exhaust gasses piercing this accumulation of waves and hot gas. The excess fuel that is wasted out behind the exhaust is like the air above the dropped pebble that enters behind it and the air being pushed back is basically like the collapse of the waves /gases resonating the fuel back into the cylinder. This can probably take place as the initial exhaust gases are going to be strong exiting whereas the fuel/air following will be weaker and easy to drive back. Maybe this could be a bit off the wall though.

[champ20B]

Thanks for the heads up. I've read it and understand expansion chambers but it doesn't go into megaphones and how to tune them. Does the same theory apply? It's probably close but not the same as there are I'm sure some different variables to address. This is what I'm trying to find out.

Somewhat relative to my first theory, I have a second possible idea of the true nature of these tuned pipes....

Now, in this possible scenario, as the sound waves multiply into higher-frequency/thinner waves they move slower exiting, thus the exhaust barrier is created. As the shot of exhaust collides into these waves from behind, a springing action occurs. Basically, the inner waves are pushed against waves toward the open end of the funnel therefor being compressed together. As this happens, heat between the waves causes a return resistance/expansion that drives the excess air/fuel that was scavenged behind the exhaust to be resonated back into the cylinder. Think of it like a spring....

[champ20B]

Thanks for the heads up. I've read it and understand expansion chambers but it doesn't go into megaphones and how to tune them. Does the same theory apply? It's probably close but not the same as there are I'm sure some different variables to address. This is what I'm trying to find out.

I have been thinking of a clear way to finally address the reason of the cone's general design, as to how it's various characteristics affect the physics involved.....so here it goes.

generally, the whole idea of a "cone" design is what I wrote of earlier. The cone affects the division of the wave into more waves. For example, If you have a straight pipe the exhaust will blast through in a wave like a shunt. However, if the pipe increases in diameter from beginning to end, the wave will break and divide (one into two ect) as it tracks along the wall of the pipe naturally. That is the purpose of the angle of the cone. As you increase the relative angle, the wave multiplies more actively in shorter distance. The only drawback is that the angle can become less affective in excess. Basically, if the angle increases too much the sound waves will not track along the inner wall and will affectively break loose. At this point, it will be no better than a vintage open exhaust. The diameter ( at the start) of the pipe is relative to the area of the exhaust port opening. This also affects length particularly taking "duration of relative time open", into account. The cone angle ultimately affects your power band. If you create a barrier of mega-waves closer to the port, your resonation of scavenged fuel will take place quickly, good for high RPM / lower port duration engines like the modified class. This can be accomplished with a higher angle cone. The needed length of the cone is due to duration of the port to ensure that the full desired function takes place before the waves exit or desipate in the pipe thus loosing the barrier, particularly due to the limit of RPMs the engine can handle or what the carburator can efficiently supply due to CFM for effective power affecting the exhaust strength.

[Seagull 170]

Cooling the Megaphone & the exhaust gas, increases the gas density & changes the speed of sound causing the exhaust to appear longer than it is, lowering the powerband revs, just the same as an expansion pipe.

There are 3 sound "reflection" points, a minor one at the start of the taper & the major one at the end of the pipe & on the return wave at the exhaust port.

[Master Oil Racing Team]

Here's a couple of experiences not directly related to the subject matter, but maybe in a way that can scramble your brains enough you can "see" a little bit differently.

A lot of people know this, but probably most don't. You can calibrate a sound making device (don't remember the name) to a certain ID diameter pipe and weight per foot, and when it is open at boat ends, when you shoot the gun, it can tell you very accurately how long that piece of pipe is. Oilfield pipe dealers use such devices to get shipments out to pipe yards quickly when hand measuring takes up so much time. This machine works on linear pipe only, but when the sound spreads out at the opposite in the machine knows how far the sound had to travel to get there and records it at, I guess you might say "the speed of sound".

Another phenomena that I have witnessed is boiling water at a little over 100 degrees Fahrenheit in a table top distiller. You can see the water boiling at say 103 degrees in 25 inches of vacuum, but you cannot see some of what is going on before boiling begins. This kind of goes back to the old saying "a watched pot never boils". This is what I learned from the chemist that set this glass still up. As the water heats up, it thins some and the hotter water starts rising. As the temperature climbs so does this hot water climb and the water inside starts moving around because of the water being displaced from bottom and pushing up on the upper layers as it rises, then the layers on top start cooling and have to start back down as they are being displaced. So they water is moving around, but the heat is continuing to rise, and still no boiling. Everyone has probably seen how when you try to bring on a full boil under the hottest setting very quickly, once boiling starts you have to turn it down, sometimes four or five points to the desired slow, full boil roll. This happens because as the hot water starts rising, there becomes a tough "skin" on the surface that prevents the water on the bottom where the heat is from evaporating. The weight of the water and the skin on top is not allowing the water to evaporate into steam and air, thus preventing the bubbles from forming. The hotter the water gets, the tougher the skin is until at some point the skin is penetrated by a bubble. At that point lots of bubbles can now form and rise to the top, then not only is their less pressure from the skin, but a whole bunch of smaller bubbles have lightened the water, then the pot erupts into a full boil.

Got it so far? The vacuum allows boiling at a lower temperature due to atmospheric pressure has been reduced so much, but the skin still forms. Watching this experiment one day, we were bringing up the heat and as the skin was tightening (you can see little surface ripples and a fog forming in the glass bowl) one of the guys accidentally dropped a half inch combination wrench on the concrete floor and the water immediately started boiling. We all looked at each other and were amazed. It had to be the "ding" of the wrench that broke the skin. We cooled the still down a little. You can cool it very quickly by reducing heat and leaving the vacuum on. Then we brought the temp back up to where the skin would be formed and we figured it was getting close to boiling and dropped the wrench from about the same height and once again at the "ding" the water started boiling instantly. So the experiment was repeatable. We never got our project finished because the 1993 economy collapsed and killed our project, but we were going to experiment with harmonics when we got everything working reasonably commercial.

While these two examples don't directly play into the discussion they do include sound waves, an open pipe, heating and cooling and harmonics. Thought it might help stimulate some thoughts and theories. I just drove boats and took pictures and never studied any of this, but was always curious. Great thread.

[Art Kampen]

All of these theory`s make for good reading and I find interesting. But finding a way to put them to practical use when building a 4 cyl. Merc is the trick. Their are so many variables. Even duplicating a really good engine, which you would think would be easy to do is very difficult. So i don`t think there is a hard & fast formula that you can apply to megaphone exhaust. Sometimes in the real world of building engines things just work out. And then you woinder how come that engine is so good. Is it the exhaust? Is the block and internals straight & true? Induction, carbs & reeds better? Did you do some little mod that that gave it that little edge.

[GHMiller]

Art, I agree there seems to be a lot of variables when building a 44. I think this is what might be playing into what Dixon was getting at in the beginning of this thread. I believe that what works really great with one engine might be "off" a little with another even with duplicated porting. Why is this? I think it's anyone's guess really. But, that is why I asked my questions. I really think that you have to approach each engine as having its own personality and then tune it to what it likes. Not only with a dyno (if you have one) but it really needs to be tuned on the boat in real world conditions with lots of testing. Sure it is nice to say that your motor was tuned on the dyno but I believe the real proof is on the back of the boat. Some of our best snowmobile engines were never run on the dyno and were real hard to beat.

[Fastjeff57]

Power BAND, not power alone, is what makes a racer fast.


Jeff

[GHMiller]

Power BAND, not power alone, is what makes a racer fast.


Jeff
Jeff, your are correct. That is what I'm trying to figure out. I think that is where the "black magic" comes into tuning with megaphones. Once you hit on the "right" porting, flow, carburetion of your set up, now how does your megaphones affect all of this? By changing length vs. cone degree you need to get the power band tuned into where you desire the rpm range to be. With our snowmobiles this was easier because all we had to do was change the clutch settings to obtain the desired power band to rpm ratio. But with outboards you are somewhat limited to what changes you can do. I believe that is the obstacle with tuning an outboard whether it is an expansion chamber or megaphones.

So if I understand this right, by having a short wider cone you raise or have a narrower power band with higher rpm's? Thus increasing horse power but not torque? By having a narrow longer cone then you would have a lower or flatter power band with lower rpm's? This would increase your torque but not horse power? Where would your compromise be? Maybe a longer but wider cone?

[Fastjeff57]

Back (way back!) in my motocross days, I built hundreds of expansion chambers to pay for my racing expenses. One thing I learned is that narrow angles equals more torque and a wider power band, where sharper angles does the opposite. Not sure how this relates to megaphones, but my best guess is that wider angles MIGHT make more peak horse power though with a narrower power band--great for top speed, but lousy out of the corners. The length of the megaphones, generally speaking, should effect the rpm where the horse power peaks: longer equals lower.

Hope that helps.

Jeff

PS With just a pipe (and bolt on flywheel) I turned by ferocious TM 400 motorcrosser into a mild mannered endure bike. It's amazing what a chamber can do.

[champ20B]

simple solution to megaphones:

The fact is, there is only so much one can do in modified class as far as porting ect is allowed. Everybody doing this that has done it, even for a little while, are doing these things. All the Mercs out there are the same engines that have been ran since way way back. Everything that could be done has gone all the way for these older motors. Unless it is a looper, its no good for anything like pro and even loopers are more-over outdated. A good Rossi or Konny would destroy them in any race. Mercs are still pretty good for mod classes though. I believe driving technique and instinct are more attributed to champion drivers than "magic motors". I don't believe in Magic motors, just good set up and proper specs, and good driving. The cones that have won on these engines can simply be copied and they will do the job the same. Nothing new or any improvements can be made beyond what already exist.

[Art Kampen]

Wrong,Wrong, Wrong

[Seagull 170]

The problem with copying the megaphone lengh from another similar motor is that the powerheads are similar, not identical.

The length of the pipe is dependant on the exhaust port duration, & the exhaust gas temperature which is affected by the mixture, compression ratio, ignition timing, megaphone taper angle & scavenging efficiency.

The answer to all these variables is testing, testing & more testing!

[champ20B]

I have been thinking of a clear way to finally address the reason of the cone's general design, as to how it's various characteristics affect the physics involved.....so here it goes.

generally, the whole idea of a "cone" design is what I wrote of earlier. The cone affects the division of the wave into more waves. For example, If you have a straight pipe the exhaust will blast through in a wave like a shunt. However, if the pipe increases in diameter from beginning to end, the wave will break and divide (one into two ect) as it tracks along the wall of the pipe naturally. That is the purpose of the angle of the cone. As you increase the relative angle, the wave multiplies more actively in shorter distance. The only drawback is that the angle can become less affective in excess. Basically, if the angle increases too much the sound waves will not track along the inner wall and will affectively break loose. At this point, it will be no better than a vintage open exhaust. The diameter ( at the start) of the pipe is relative to the area of the exhaust port opening. This also affects length particularly taking "duration of relative time open", into account. The cone angle ultimately affects your power band. If you create a barrier of mega-waves closer to the port, your resonation of scavenged fuel will take place quickly, good for high RPM / lower port duration engines like the modified class. This can be accomplished with a higher angle cone. The needed length of the cone is due to duration of the port to ensure that the full desired function takes place before the waves exit or desipate in the pipe thus loosing the barrier, particularly due to the limit of RPMs the engine can handle or what the carburator can efficiently supply due to CFM for effective power affecting the exhaust strength.

Just as I already explained as quoted above, with feasible theories to support before...... I know about the areas of tuning in design.

The only thing left to explore though, is to take advantage of the loopholes in the tech rules to build faster hulls and (gearcase customizing, if possible to allow). That is something it seems a lot of racers aren't really focusing on enough. Those things seem to stay the same as "tried and true". All there is, is engines and trying to magically squeeze more power from them all the time. I say, try to make more speed of the horsepower you've got, and win the race. I just don't see how (with the limitations on technical rules), much more could be done to engines that are the same to start with as what has been on the field for 30-50 years and has been raced and built, improved, and again raced and built by the best of the best time and time again. Advancements can only go so far until you have to come up with a whole new kind of engine altogether. I just don't see how one can keep reinventing what has already been invented for too long, until everything has been thought of that would make any possible positive difference.......... This is more interesting to me than thermodynamics associated with soundwave technology!

[GHMiller]

champ20B,
I'm not trying to "reinvent anything" as you say, I'm only trying to learn how to make what I've got more efficient. As far as hulls go I've got a pretty good one that has won championships. Now my goal is to put together a complete package. Thank you for your input as it does add to the information that I've compiled.

[champ20B]

champ20B,
I'm not trying to "reinvent anything" as you say, I'm only trying to learn how to make what I've got more efficient. As far as hulls go I've got a pretty good one that has won championships. Now my goal is to put together a complete package. Thank you for your input as it does add to the information that I've compiled.

I have an idea for a quick adjustable angle cone that might or might not work just for experimentation.....

One way you could test is to take thin sheet steel and roll it like a paper snow-cone cup in a sort of way. Cut some rings of plywood (one for small and one for large end of cone) to hold it in a rolled position. Next, spot weld some little tabs around the big end, the center and then the small end. With these tabs, you can put adjustable hose clamps around these three points and they wont ride or slip back. To seal the seam, just use metal tape. With this make-shift set up, you can adjust the angle of your cone in a minute with the turn of a socket driver and find the perfect angle for tuning.........I hope this would help if it hasn't already been tried.

[GHMiller]

I have an idea for a quick adjustable angle cone that might or might not work just for experimentation.....

One way you could test is to take thin sheet steel and roll it like a paper snow-cone cup in a sort of way. Cut some rings of plywood (one for small and one for large end of cone) to hold it in a rolled position. Next, spot weld some little tabs around the big end, the center and then the small end. With these tabs, you can put adjustable hose clamps around these three points and they wont ride or slip back. To seal the seam, just use metal tape. With this make-shift set up, you can adjust the angle of your cone in a minute with the turn of a socket driver and find the perfect angle for tuning.........I hope this would help if it hasn't already been tried.

Neat idea....

[Fastjeff57]

Brilliant! The length can also be adjusted with sliding sections, as kart racers do.

On the subject of "it's already been done" and "can't make any more improvements", this is historically untrue. Some smart person out there is sure to come up with a trick no one thought about before (or didn't act on/ do it right) and another small increase is achieved.

Look at emissions in cars as an example. When this all began--in the mid-70s--car engines were God awful! Not only did they barely run, they were powerless slugs. Bleeck! (You older guys know what I'm talking about.) But over the years the horsepower has come back--big time!--and new cars are so clean they are air cleaners in some smoggy areas: the air coming out the back is cleaner than the air coming in! Amazing. But all this improvement took time--one little thing at a time.

Jeff

[champ20B]

Brilliant! The length can also be adjusted with sliding sections, as kart racers do.

On the subject of "it's already been done" and "can't make any more improvements", this is historically untrue. Some smart person out there is sure to come up with a trick no one thought about before (or didn't act on/ do it right) and another small increase is achieved.

Look at emissions in cars as an example. When this all began--in the mid-70s--car engines were God awful! Not only did they barely run, they were powerless slugs. Bleeck! (You older guys know what I'm talking about.) But over the years the horsepower has come back--big time!--and new cars are so clean they are air cleaners in some smoggy areas: the air coming out the back is cleaner than the air coming in! Amazing. But all this improvement took time--one little thing at a time.

Jeff

Look out, Charlie Strange and Mr. Christner!!! (Champ20B) just come up with a way to help make mod motors their fastest!!;)

[OUTBOARDER]

There is something that I've never been able to figure out and never heard a good explanation either. Here's the deal, I own a Stuska water brake dynamometer (130 hp max, 14,000 rpm) and have dyno'ed many four cylinder 44 cu in Mercs in quiet form. They dyno real close to 40-41 hp in stock, quiet form with a great horsepower curve. I chose to dyno directly off the driveshaft to reduce unneeded wear on the "D" Quicksilver or 45SS unit. A friend with a 12' 6" hydro saw 79 mph many times but never broke 80. We put his motor on the dyno and pulled 40.6 hp in the 6500 rpm range consistently. Then we installed a set of Parker megaphones and saw 57 hp with a wide power curve that was right in the range the motor would be run at for top end. A trip to the water saw some odd results, 79 mph and not a tiny bit faster. The pipes still made good hp to well over 7000 so we were definitely in the higher hp range. With megaphones, at top speed, the motor was developing close to 50% more hp, but was not able to go ANY faster.

Before I present a possible theory, here is another example. I modified a '44 with single ring Turner pistons, mild porting used the larger KC carb instead of the KA's. The dyno showed 46 hp, in quiet form again. A trip to the water showed a max top speed of 85 mph, quite a difference from my usual 78-80 bests. So what is going on here? These tests are not flukes but consistently this is the way it will go every time. So here is a possible theory. After decades of racers using megaphones on Mercs, is it actually possible that somehow the high speed air that flows across the pipes is cancelling the 50% gain shown on the dyno? I realize that this a big pill to swallow, but something is really wrong with this picture. 50% more power being applied to the prop at top end and not any more speed at all? I have a friend that manufactures the 'Hurricane' 27 hp industrial leaf blower that will move a large volume on air at 160 mph, I guess it would be interesting to set it up in front of the dyno with a megaphone Merc running at 57 hp and turn the air on. Any other ideas on this one?

as speed increases Resisteance "Drag" from air increases exponentially so does resistance to lower unit.
Speed as a function of Horse Power thru air alone can be expressed by

V(hp)air = K * (square root of HP)

example 14' Gw invader goes 72 mph with 90 hp aired out.
so K = V/sqrt hp = 7.8
with 100 hp goes as follows
V(hp) = 7.8 * sqrt 100 = 78mph very close to reality
this is very simplified and will still be very close over a small range.
(if you look at any function/formula at a small enough interval a straight line will approximate)

with all this said the prop may not be taking advantage of hp increase at certain speeds/conditions etc............test test tes

[zul8tr]

as speed increases Resisteance "Drag" from air increases exponentially so does resistance to lower unit.
Speed as a function of Horse Power thru air alone can be expressed by

V(hp)air = K * (square root of HP)

example 14' Gw invader goes 72 mph with 90 hp aired out.
so K = V/sqrt hp = 7.8
with 100 hp goes as follows
V(hp) = 7.8 * sqrt 100 = 78mph very close to reality
this is very simplified and will still be very close over a small range.
(if you look at any function/formula at a small enough interval a straight line will approximate)

with all this said the prop may not be taking advantage of hp increase at certain speeds/conditions etc............test test tes

Thru air or water per basic physics, Drag (a force) varies as the Velocity squared, Power varies as force x velocity, so Power (Hp) varies as Velocity cubed ======>> or Hp varies as V^^3

or in your form, V = K *(cube root Hp)

Running the numbers with a new K value your 10 hp increase (90 to 100) will not yield a 6 mph increase (all else remaining constant) but about 2.6 mph increase or 74.6 mph.

As always testing is the last word

[champ20B]

To simplify this........72mph divided by the square root of 90hp equals 7.6

now, we take 7.6 and multiply it times the square root of 100hp (since we are souping up the boat) and we then get 76mph......all answers rounded off.


72/sqrt of 90=7.589 7.589 X [sqrt(100)]=75.89mph (75.89)-72=an increase of 3.89mph going from 90 to 100 hp

[zul8tr]

To simplify this........72mph divided by the square root of 90hp equals 7.6

now, we take 7.6 and multiply it times the square root of 100hp (since we are souping up the boat) and we then get 76mph......all answers rounded off.


72/sqrt of 90=7.589 7.589 X [sqrt(100)]=75.89mph (75.89)-72=an increase of 3.89mph going from 90 to 100 hp

Not quite you are not simplifying anything with the wrong relation, as I note above the relation from basic physics is a cube root function:

V = K *(cube root Hp)

K = 72/ (cube root 90) = 72/4.48 = 16.07

V = 16.07 x (cube root 100) = 16.07 x 4.64 =74.6 mph a 2.6 mph increase all else being equal which generally never happens. As always testing is the final answer.

[champ20B]

Not quite you are not simplifying anything with the wrong relation, as I note above the relation from basic physics is a cube root function:

V = K *(cube root Hp)

K = 72/ (cube root 90) = 72/4.48 = 16.07

V = 16.07 x (cube root 100) = 16.07 x 4.64 =74.6 mph a 2.6 mph increase all else being equal which generally never happens. As always testing is the final answer.

I know.......but I was referring to the first formula mentioned.

I agree, testing is the final truth and/or proof of it all as well. But I did think that resistance is simply the measure of force alone as to determine/estimate "energy(horsepower)". Without resistance, force couldn't be recognized in the coarse of movement.

The function is: W=f(D)

this is ........work= force X distance

If you calculate work in unit weight with time it takes , then you find horsepower. There is only one force factor and if the horsepower is known as well as speed, then you get an efficiency factor as to use to estimate possible results of greater hp.

Another way to describe horsepower is rotating force in number of rotations in given time (minutes) when referring to motors. That would be: f(d)/t..... I hope I got it right.

[OUTBOARDER]

14' gw Invader 90 hp = 72 100 = 78. The boat is dragging less in water.
So V(hp) = k*(hp)^.5 works on planing hulls (racing) over small intervals.
air resistance increases proportionally to Velocity Squared. recall your basic diferential equation course word problem.

water resitance increases proportinally to velocity cubed. used for discplacemant hulls like the USS United States.

any way my 14' john did 25mph with 15hp and did 36mph with a 30hp. it works on planing hulls that are not draggin in the water.

yes very simplistic indeed but useful

[OUTBOARDER]

Thru air or water per basic physics, Drag (a force) varies as the Velocity squared, Power varies as force x velocity, so Power (Hp) varies as Velocity cubed ======>> or Hp varies as V^^3

or in your form, V = K *(cube root Hp)

Running the numbers with a new K value your 10 hp increase (90 to 100) will not yield a 6 mph increase (all else remaining constant) but about 2.6 mph increase or 74.6 mph.

As always testing is the last word

sqrt did not mention cubed

[zul8tr]

14' gw Invader 90 hp = 72 100 = 78. The boat is dragging less in water.
So V(hp) = k*(hp)^.5 works on planing hulls (racing) over small intervals.
air resistance increases proportionally to Velocity Squared. recall your basic diferential equation course word problem.

water resitance increases proportinally to velocity cubed. used for discplacemant hulls like the USS United States.

any way my 14' john did 25mph with 15hp and did 36mph with a 30hp. it works on planing hulls that are not draggin in the water.

yes very simplistic indeed but useful

The basic physics relation (Drag proportional Velocity squared and Power proportional to Velocity cubed) I stated still applies, it's the K that is variable with different conditions of boat operation. You stated that "The boat is dragging less in water" which is contradictory to what I stated in my posts above and implied when using the formula that "all else remaining constant". "K" is NOT constant. The calculated value based on a single velocity and Hp was used for the increased Hp to estimate velocity to keep consistent with your approach. There are many variable conditions going on to apply a single K value to all operation conditions.

Relations noted here (K usually known as Cd - coefficient of drag):

http://en.wikipedia.org/wiki/Drag_(physics)

Examples for drag and power (air drag used here but same applies to water drag and Cd still not constant but used that way for easy relative to compare calculations)

http://phors.locost7.info/phors06.htm

Let's get this post back on track regarding megaphones!

[OUTBOARDER]

The basic physics relation (Drag proportional Velocity squared and Power proportional to Velocity cubed) I stated still applies, it's the K that is variable with different conditions of boat operation. You stated that "The boat is dragging less in water" which is contradictory to what I stated in my posts above and implied when using the formula that "all else remaining constant". "K" is NOT constant. The calculated value based on a single velocity and Hp was used for the increased Hp to estimate velocity to keep consistent with your approach. There are many variable conditions going on to apply a single K value to all operation conditions.

Relations noted here (K usually known as Cd - coefficient of drag):

http://en.wikipedia.org/wiki/Drag_(, physics)


Examples for drag and power (air drag used here but same applies to water drag and Cd still not constant but used that way for easy relative to compare calculations)

http://phors.locost7.info/phors06.htm

Let's get this post back on track regarding megaphones!


once again for planing hulls that run high speed = k * sqrt of hp
more than one boat builder has used it to predict speed of a boat for a change in hp

let me go one step further with my real life john boat experince
15 hp = 25 mph
25 hp = 36 mph
50hp = 44mph over powered Carolina skiff J16 with 1998 Johnson 50 hp went 44 mph

as in real life John Boat example
k can be treated as constant over a small interval

the cubed HP is for Discplacement hulls like Ships.

[rumleyfips]

Jeff, your are correct. That is what I'm trying to figure out. I think that is where the "black magic" comes into tuning with megaphones. Once you hit on the "right" porting, flow, carburetion of your set up, now how does your megaphones affect all of this? By changing length vs. cone degree you need to get the power band tuned into where you desire the rpm range to be. With our snowmobiles this was easier because all we had to do was change the clutch settings to obtain the desired power band to rpm ratio. But with outboards you are somewhat limited to what changes you can do. I believe that is the obstacle with tuning an outboard whether it is an expansion chamber or megaphones.

So if I understand this right, by having a short wider cone you raise or have a narrower power band with higher rpm's? Thus increasing horse power but not torque? By having a narrow longer cone then you would have a lower or flatter power band with lower rpm's? This would increase your torque but not horse power? Where would your compromise be? Maybe a longer but wider cone?

Frank Volker has a new thread about a flathead on the Quincy dyno. He says the pipe temperature fluctate between high on the first pulse to low on the suction wave. He thinks temperature readings are to problematic to be much use.

Yesterday Mercedec Formula 1 tried a megaphone on the turbo outlet ( looks short with wide divergence ) to increase the noise. It didn't do much.

It does however beg the question, Ars all racers just 4 year old boys, sitting in a cardboard box and going vroom, vroom,vroom.

John

[OUTBOARDER]

There is something that I've never been able to figure out and never heard a good explanation either. Here's the deal, I own a Stuska water brake dynamometer (130 hp max, 14,000 rpm) and have dyno'ed many four cylinder 44 cu in Mercs in quiet form. They dyno real close to 40-41 hp in stock, quiet form with a great horsepower curve. I chose to dyno directly off the driveshaft to reduce unneeded wear on the "D" Quicksilver or 45SS unit. A friend with a 12' 6" hydro saw 79 mph many times but never broke 80. We put his motor on the dyno and pulled 40.6 hp in the 6500 rpm range consistently. Then we installed a set of Parker megaphones and saw 57 hp with a wide power curve that was right in the range the motor would be run at for top end. A trip to the water saw some odd results, 79 mph and not a tiny bit faster. The pipes still made good hp to well over 7000 so we were definitely in the higher hp range. With megaphones, at top speed, the motor was developing close to 50% more hp, but was not able to go ANY faster.

Before I present a possible theory, here is another example. I modified a '44 with single ring Turner pistons, mild porting used the larger KC carb instead of the KA's. The dyno showed 46 hp, in quiet form again. A trip to the water showed a max top speed of 85 mph, quite a difference from my usual 78-80 bests. So what is going on here? These tests are not flukes but consistently this is the way it will go every time. So here is a possible theory. After decades of racers using megaphones on Mercs, is it actually possible that somehow the high speed air that flows across the pipes is cancelling the 50% gain shown on the dyno? I realize that this a big pill to swallow, but something is really wrong with this picture. 50% more power being applied to the prop at top end and not any more speed at all? I have a friend that manufactures the 'Hurricane' 27 hp industrial leaf blower that will move a large volume on air at 160 mph, I guess it would be interesting to set it up in front of the dyno with a megaphone Merc running at 57 hp and turn the air on. Any other ideas on this one?

spray some water vapor also???
Pipes are Colder on water [(apart from water injection) watercraft tune jacketed exhaust by manipulating water temp surrounding pipe]
one way to correlate engine performance under both conditions aside from conjecture would be a egt gauge.

[Mark75H]

There is something that I've never been able to figure out and never heard a good explanation either. Here's the deal, I own a Stuska water brake dynamometer (130 hp max, 14,000 rpm) and have dyno'ed many four cylinder 44 cu in Mercs in quiet form. They dyno real close to 40-41 hp in stock, quiet form with a great horsepower curve. I chose to dyno directly off the driveshaft to reduce unneeded wear on the "D" Quicksilver or 45SS unit. A friend with a 12' 6" hydro saw 79 mph many times but never broke 80. We put his motor on the dyno and pulled 40.6 hp in the 6500 rpm range consistently. Then we installed a set of Parker megaphones and saw 57 hp with a wide power curve that was right in the range the motor would be run at for top end. A trip to the water saw some odd results, 79 mph and not a tiny bit faster. The pipes still made good hp to well over 7000 so we were definitely in the higher hp range. With megaphones, at top speed, the motor was developing close to 50% more hp, but was not able to go ANY faster.

Before I present a possible theory, here is another example. I modified a '44 with single ring Turner pistons, mild porting used the larger KC carb instead of the KA's. The dyno showed 46 hp, in quiet form again. A trip to the water showed a max top speed of 85 mph, quite a difference from my usual 78-80 bests. So what is going on here? These tests are not flukes but consistently this is the way it will go every time. So here is a possible theory. After decades of racers using megaphones on Mercs, is it actually possible that somehow the high speed air that flows across the pipes is cancelling the 50% gain shown on the dyno? I realize that this a big pill to swallow, but something is really wrong with this picture. 50% more power being applied to the prop at top end and not any more speed at all? I have a friend that manufactures the 'Hurricane' 27 hp industrial leaf blower that will move a large volume on air at 160 mph, I guess it would be interesting to set it up in front of the dyno with a megaphone Merc running at 57 hp and turn the air on. Any other ideas on this one?


I agree, the most likely culprit is the prop ... or possibly something else in the set up. It doesn't have to be a speed induced drag problem; there are lots of things that can cause a boat to "hit a wall" speed-wise.

Set up for circle competition most 44's are propped for 8400 rpm ... might be running above the power peak to gain some acceleration.

If you propped for exactly the power peak, you are probably propped too slow. If you are not running a spray shield/baffle in front of the carbs, 80 mph intake might be affecting the carbs.

[Mark75H]

One thing I used to think about what really happens inside a megaphone is......

I think you should read the Brinkman explanation until you understand it.

[champ20B]

I think you should read the Brinkman explanation until you understand it.

Key words: (I used to think), as to what happened. I have other ideas too. But no one really knows the exact answer.

Afterall, it is still as good of a theory as any. They are all really no more than theories when one gets down to it, particularly when it comes to megaphones. As similarly stated on another reply, "if the two (Ps) [props and megaphones] were understood, one could win every world championship".

I did, however, come up with a nifty way to find the right exhaust tune potentially.