above water exhaust outlet mod

[microbream]

Hi guys,
just ran the 10ci Suzuki on my microbass boat and my god did she fly!
Topped 30 :)easy @ 7000RPM:(. Yeh while i get my prop repitched i am thinking of doing some exhaust mods. ATM the exhaust exits below the cav plate, plus their are three small exhaust holes further up which exit to the atmosphere. The main outlet kinda exits directly behind the prop from a tube that protudes 2 inches below the cav plate. I was thinking of moving the outlet above the cav plate or even above the water pump. Now i now noise may be an issue, but surely this will improve mid range response and result in less drag once i have removed the protruding exhaust tube.
should i aim for same outlet area around 30mm or smaller considering it is above water
cheers
Microbream

[Mark75H]

It will not make any performance increase

[Ron Hill]

I used to drill two 3/8 holes, just so the motor made enough noise that I would know where the boat (s) were....But speed advantage??.....NO....

[JohnsonM50]

It could be that the below water outlet improves power a bit. I agree that no performance gain can be expected & think it might even slow ya. Your turning alot of R's :eek:.. whats it rated for?

[microbream]

It could be that the below water outlet improves power a bit. I agree that no performance gain can be expected & think it might even slow ya. Your turning alot of R's :eek:.. whats it rated for?

i realy can not find any info on this engine?
its an 81 dt8 and yes 7000 does seem a bit high, but the way it pulled from 5-7k was awesome! I will look to go to a 2" higher pitch to bring it down to 6 ish.
Interestingly the mid section is almost a perfect diffuser. Then it hits the water pump and runs out thu a straight 25 mm tube at 30 deg exiting below the prop.

[JohnsonM50]

i realy can not find any info on this engine?
its an 81 dt8 and yes 7000 does seem a bit high, but the way it pulled from 5-7k was awesome! I will look to go to a 2" higher pitch to bring it down to 6 ish.
Interestingly the mid section is almost a perfect diffuser. Then it hits the water pump and runs out thu a straight 25 mm tube at 30 deg exiting below the prop. How much pitch change has to do with gearing so don't know about 2p more or less but targeting 6 sounds good, better to over rev a little than to lug it under stress.

[microbream]

It could be that the below water outlet improves power a bit. I agree that no performance gain can be expected & think it might even slow ya. Your turning alot of R's :eek:.. whats it rated for?

i am curious about say a merc 25xs v standard 25. The xs has the racing lower unit and above cav exhaust outlet. where as the stock 25 has thu prop exhaust. Would seem like the different exhaust configs would have an effect on running good or bad, any ideas.

[JohnsonM50]

Its possible that it does, [perform best above water] but not likely because of it in itself. Im not that familiar with the 25XS but would guess whatever it uses for a tuner has more to do with it than the position of the outlet. The stock motor has noise to contend with & ais probably the best it can be as is. Improving on the tuner would be more likely a place to succeed but even there its probably optimal. If you go ahead & do it, let us know if it proves to work.:cool:

[microbream]

i guess when you look at it above water exhaust has constant backpressure regards of boat speed. where as thru prop exhaust backpressure changes with boat speed. As no or low speed backpressure = water pressue. But as speed increases the moving water will actually reduce this back pressue. I beleive this is best illustrated by gunning the engine from a standing start, the motor output definitly improves as speed increases.
interesting may put on a pressure gauge and do some testing

[JohnsonM50]

i guess when you look at it above water exhaust has constant backpressure regards of boat speed. where as thru prop exhaust backpressure changes with boat speed. As no or low speed backpressure = water pressue. But as speed increases the moving water will actually reduce this back pressue. I beleive this is best illustrated by gunning the engine from a standing start, the motor output definitly improves as speed increases.
interesting may put on a pressure gauge and do some testing.
A small race motor is a different animal in several aspects, Exhaust being one of them. They still suffer the same semi submerged start out above or below tho. Race motors generally use locked time so don't have the full range time calibration as a stock motor. Then the race props are of smaller dia. than their stock counterpart. There's a host of other possible differences too like different ports, carb changes, padding, reeds & last but not least exhaust tuning. 1 change to be more like a racer is for those possible reasons alone unlikely to make a difference. Not to say don't try anything, just not to expect a dramatic boost of power, its likely not there. [Radical change is best enjoyed with spare parts-n-donor motors ;)] Good Luck

[Mark75H]

You can not choke a 2 stroke on the exhaust side without burning pistons.

There is no production outboard being made that will go faster by relieving the exhaust and doing no other mods. They just are not made that way because of burning pistons ...

[JohnsonM50]

"You can not choke a 2 stroke on the exhaust side without burning pistons" Agreed, what I meant by that is if you cut open an above water passage it will likely relieve too much pressure so closing the opening a little to get back some of what may be released is all, not to close it in till it burns.:eek:

[Mark75H]

Also agreed.

The above water exhaust on the newer race motors is just a styling throwback from when it was assumed no back pressure is better than some.

[Powerabout]

what you can do is burn a piston due to the lean out usually accompanied by less back pressure

[fs5]

i dunno guys,i reckon if your turning big rpms on a stock motor it gives you a gain.i've got a stock merc seapro 55 ,i modded the gearbox to let most of the exhaust exit above the prop.i made a little pate up to bolt on when i was fishing to shut it up.
i found running big pitch props where the motor only revs around 5500/5800 it made no noticable difference,but when i bolted on a smaller prop and the motor reved over 6100 it made a definate difference to the topend revs ,i reckon it picked up an easy 100+ rpms.
just my 2cents worth.

[JohnsonM50]

i dunno guys,i reckon if your turning big rpms on a stock motor it gives you a gain.i've got a stock merc seapro 55 ,i modded the gearbox to let most of the exhaust exit above the prop.i made a little pate up to bolt on when i was fishing to shut it up.
i found running big pitch props where the motor only revs around 5500/5800 it made no noticable difference,but when i bolted on a smaller prop and the motor reved over 6100 it made a definate difference to the topend revs ,i reckon it picked up an easy 100+ rpms.
just my 2cents worth.
What might work to some degree on 1 motor might not on another. 100R's might seem alot on the tach & if it helps ya pass your pal/rival even better but realistically 100R's is about 1.6 - 2% of 6000 or 5500 equaling about 1mph. [unless the prop is faster anyway]. If you net the same percent on a smaller motor it becomes less yet. So for the effort & risk you get little if any.

[Mark75H]

If you did not go back and block the hole and retest you don't know if the rpm came from that or something else.

Both results have to be repeated or its not scientific

[hydroplay]

As a kid I used to play with Mark 20 Mercury motors before I knew much about the race motors. I cut the exhaust snout below the cavitation plate off and then lopped off the end of the main tower housing to make my version of a quicksilver tower. Nope, it wasn't much faster if at all but people heard me. Next was a project to bore large holes in the tower housing with a hole saw. Lighter is faster and noisier, right? Neighbors hated me because the best water is always early morning. All were happy when a friend of my dad's gave me an old D marathon boat with a KG9-H. Now I was faster and quieter. No substitute for a bigger motor.

[sean181]

try finding some bath tub racers from canada on the forum they still race the dt 8. I used to race against the dt 8 with merc 8 and the suzukis where allways the quicker of the 2

[microbream]

thanks for the replies guys,
i ended up leaving the exhaust tube outlet below the cav plate. I just opened up the three holes on the midsection slightly from 4-6mm. Should make it slightly louder if nothing else, may help holeshot, we will see.

Further on that engine , i have just resprayed it and was very impressed by the exhaust profile, even has a diffuser built into the mid section.
Although the port matching between the cylinder sleeve and the case needs some work.

[JohnsonM50]

thanks for the replies guys,
i ended up leaving the exhaust tube outlet below the cav plate. I just opened up the three holes on the midsection slightly from 4-6mm. Should make it slightly louder if nothing else, may help holeshot, we will see.

Further on that engine , i have just resprayed it and was very impressed by the exhaust profile, even has a diffuser built into the mid section.
Although the port matching between the cylinder sleeve and the case needs some work.

Would be interesting to add an accelerometer to the standard speedo & tach test data normally gathered :cool:

[Mark75H]

There's an ap for that ... an iPhone can do it

[JohnsonM50]

[QUOTE=Mark75H;109992]There's an ap for that ... an iPhone can do it[
/QUOTE] I had no idea, no I phone either.:cool: