Compression variables

[smiley]

What is a good compression for the 15 Mercury A and the 15 OMC A motors? I have ben told that many guys do not consider compression very important. I disagree and think that compression does have an affect on acceleration not necesarily top end. Anyway I am trying to establish a base line to follow when considering a re-ring or bore job on a block. Any advice would be helpful. Also does anyone have any idea what the rpm range should be on the straights and corners to achieve optimum performance?

What prompts this post is I took a compression on an OMC and had 80psi. A leak down test showed a 40psi drop above the ports and a quick seal before the ports opened. Mesuring the no 2 cylinder showed .003 out of round, .003 taper and .009 clearance. WOW. No wonder I was 1.st, 2nd or 3rd across the line then still ended up last in every race.


There is now a new block and pistons on top of the lower unit.

Bob Smiley

[zul8tr]

Obtain the cubic centimeters (cc) volume of the Combustion Chamber with the piston at TDC and convert to cubic inches by dividing by 16.39. This can be measured or maybe from manufacture specs. Obtain the cubic inches volume of 1 cylindere from manufacture specs or calculate as cylindere bore Area x piston stroke. Calc an approx cold compression gage pressure in psi from:

Compression ratio =
(Vol of 1 cylindere + Combustion chamber vol) / Combustion Chamber vol

Compression pressure = (Compression ratio - 1) x 14.7

This will give a generally lower value for a hand crank measuring with no account of the heated compressed air.

Note all compression gages are different so use the same gage therefore relative readings can be monitored over time.

Cylinderes should not differ by more than 10%

Note the readings at each pull and how many pulls to reach a max pressure.

Redo measurements by adding some oil to the cylinders to give an idea of ring leakage.

Best to measure the engine stroke and bore diameter and combustion chamber volume if you want engine specific results

Example: 1973 Merc 25ss per 1978 APBA specs has a minimum allowed combustion chamber vol of 22 cc and a cylindere volume of 11 cubic inches for a compression ratio of 9.21 and a calculated hand crank pressure of 121 psi.





























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[Mark75H]

As stated, gages vary too much for a specific number to be quoted.


Compression will not work out as described because 2 stroke exhaust ports are open for much of the first half of the compression stroke.

[smiley]

Thank you for the replies.

Sam regards to two stroke compression. If I calculated cylinder volume above the ports that should correct the problem you stated?

Team member, does the K factor 14.7 correct for the concern that Sam stated?

After a bore job, new pistons, 40 minutes in a barrell, 6 test runs and two race heats the OMC compression is 115 and 110. Much better than 80 psi. The leakdown tests are fine, no leaks throughout piston travel. RPMS and speed is up. Adding airtraps added another 200 rpm and increased the speed some more. Going in the right direction.

Bob

[Mark75H]

14.7 corrects atmospheric zero to absolute pressure

[zul8tr]

As stated, gages vary too much for a specific number to be quoted.


Compression will not work out as described because 2 stroke exhaust ports are open for much of the first half of the compression stroke.

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Sam as you correctly state :), and I am aware of, the exhaust timing will effect the compression psi and the formula was stated for the full cylindere volume. As you know the compression ratio will be lower if based on cylindere volume from TDC to the top of the exhaust port thus a lower compression psi will be the result. The formula derives from the general gas law and is used in many forms by others.

No formulas that I am aware of are completly accurate because they rely on some degree of emperical measurement. The intent of the formula was to state a means to calculate a relative expected measure of compression psi to be compared to a gage. Certainly different compression ratios can be used in the formula.

Based on my experience with stock 2 cycle outboards the formula yields fairly close results after several hand pulls compared to compression gage readings. Also I have noted that the first pull compression gage reading comes fairly close to the calculated compression pressure based on the exhaust port compression ratio. Which is why I always note the gage readings at each pull.

The 14.7 is local atmospheric pressure and the minus one ( -1) in the formula adjusts the calculated pressure from absolute pressure to gage pressure to match what a pressure gage is scaled to read.

[smiley]

Based on my experience with stock 2 cycle outboards the formula yields fairly close results after several hand pulls compared to compression gage readings. Also I have noted that the first pull compression gage reading comes fairly close to the calculated compression pressure based on the exhaust port compression ratio. Which is why I always note the gage readings at each pull..

FROM THE ABOVE PARAGRAPH, THERE ARE TWO PRESSURES TO OBSERVE. WHAT USEFUL ANALYSIS OR CONCLUSION CAN YOU GLEEN FROM THE TWO DIFFERENT RESULTS?

Bob Smiley

[zul8tr]

Bob

When engines are new :) and when broke in :eek: I take the readings noted to use as a reference to compare to compression testing at a later date. The compression readings at each pull and number of pulls provide an indication of how compression builds in the combustion chamber. At future compression testing a lower first reading compared to the initial reading would indicate more ring leakage and reduced ability to build compression more quickly.

In race engines it is important that each cylindere have fairly close compression readings at each pull and at max readings with the same number of pulls. I find 4 pulls to be adaquate to reach a max value if engine is in good condition. I do cold engine compression testing because that is easier to do and it keeps the variable of ambient temperature equal to engine temperature an easily measured value.

Your latest compression readings are more inline with good values but without specs on your engine hard to determine expected values.:confused:

Note that carbon buildup can greatly effect results:eek: so get the carbon out:). Also note that carbon will sometimes build on the exhaust ports at the exit side of the ports and effect port timing and performance so that needs to be cleaned as well.


































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As stated compression gages are not absolutly accurate but relative devices so keep the same gage and protect it.

[Mark75H]

I agree on all points. Very good advice. :)

[Powerabout]

You can always cc the combustion chamber and do the piston crown as well then the stroke above the exhaust ports and use the ratio as a working guide.
This eliminates all other errors
( as explained to me by an OMC race engineer)

[smiley]

Here are the OMC specs

Minimum combustion chamber at T.D.C to top of spark plug hole 13.5 cm cubed. I do not know the volume of the spark plug hole.

cylinder bore .030 over at 2.218 in.

Crank stroke is 1.760 inch +/- .008
effective cylinder travel above exhaust port to top of stroke 1.203 in.

I have documented cold and after run compression but have found cold tests to be easier and more frequent to do and record.

How can one lap rings to pistons where there are pins in the ring lands, do you just clean out the ring lands with a broken/filed ring or do you just let it be?

Also there is a discussion about minimum oil vs more oil in the gas. Minimum gets you more fuel for combustion; more oil gives a better seal? Is this logic correct? What have your tests shown? I have heard of fuel rations ranging from 20:1 to 50:1.

Also, what is the ideal mix ratio of say 110 racing fuel to standard marine pump gas? Does higher octane provide a longer power stroke duration? Is this beneficial? What is the overiding factors to shoot for? I know that OMCs are very low on torque so energy management in the turns is critical.

Bob Smiley

[zul8tr]

Here are the OMC specs

Minimum combustion chamber at T.D.C to top of spark plug hole 13.5 cm cubed. I do not know the volume of the spark plug hole.

cylinder bore .030 over at 2.218 in.

Crank stroke is 1.760 inch +/- .008
effective cylinder travel above exhaust port to top of stroke 1.203 in.

I have documented cold and after run compression but have found cold tests to be easier and more frequent to do and record.

How can one lap rings to pistons where there are pins in the ring lands, do you just clean out the ring lands with a broken/filed ring or do you just let it be?

Also there is a discussion about minimum oil vs more oil in the gas. Minimum gets you more fuel for combustion; more oil gives a better seal? Is this logic correct? What have your tests shown? I have heard of fuel rations ranging from 20:1 to 50:1.

Also, what is the ideal mix ratio of say 110 racing fuel to standard marine pump gas? Does higher octane provide a longer power stroke duration? Is this beneficial? What is the overiding factors to shoot for? I know that OMCs are very low on torque so energy management in the turns is critical.

Bob Smiley

Based on your numbers for the full stroke cylindere volume:

pi = 3 142 ....

Cyl vol = 1.76 x [pi (2.218)^2} / 4 = 6.8 cubic inches ,this is a small engine at 13.6 c.i.

Combustion chamber vol = 13.5 / 16.387 = 0.824 c.i.


Compression ratio = (6.8 + 0.824) / 0.824 = 9.25 : 1

Cold crank expected max pressure = 14.7 (9.25 - 1) = 121 psi

--------------------------------------------------
Exhaust port cyl volume = 6.8 x 1.203/1.76 = 4.65 c.i.

Exhaust port compression ratio = (4.65 + 0.824) / 0.824 = 6.64 : 1 This is 72% of full stroke ratio. Typ stock engines have a exhaust port compression ratio about of 65 to 75%of full stroke value

Cold crank pressure at 1st pull = 14.7(6.64 - 1) = 83 psi

Check my math,;) All numbers are approx but you should read similiar values from your compression gage. If you don't have a good gage get one and protect it.

When you do a cold crank compression test squirt a gas oil mix the engine will run on into cylinderes and hand pull 6 or so times to get cylindere walls coated then do compression test. Keep sparkies out of the plug wires and pull the plug wires away from the open plug holes or :eek::eek:.
I always use more oil to get the seal as less oil lhan recommended ratio is bad for the rings, cyl walls and bearings. I never noticed a performance drop with more oil.

I mix at 20:1 for revs at 7000+ for my 25ss and Yamato Y80. What is recommended for your OMC as a fishing engine ? What ever it is just double the oil should be ok.

These are low compression engines and IMO in stock trim they really don't benifit from 110 av gas. just use regular non ethanol and you will be fine. That's what I use. Of course do your own tests with a tach and gps and see results.

Pay attention to the mag timing for each cylindere and spark plugs that's where good performance can be gained. And each cylindere needs to fire at exactly 180 degrees apart at the assigned pistion position BTDC. I do not know what BTDC inches works for your engine. You need to ask other racers. But in the final analysis Test, Test ....... and learn how to read spark plugs. Always start with the colder plug and move toward hotter plug if needed. Do you have a dial indicator and a buzz box?

Just carefully clean the ring grooves of all carbon with a chemical decarbonizer no scraping, and lap both sides of each ring on a glass surface with 600 grit wet or dry paper with light machine oil or marvel mystery oil. I use a circular motion. Hold each ring with a flat shape (like a wood block that will not mar the surface ) that covers the ring to keep it flat to the glass. Don't overdo the lap you just want to get a finish that is clean.

Check the reed settings and adjust as needed.

Carburation and jetting also needs attention.

[Mark75H]

I agree on all points, good advice :)

[smiley]

I thought hi octane fuel would not benefit these motors much. No I do not have a buzz box nor a dial indicator to check timing. The flywheel is marked so that is good.I need to get a depth guage, I agree.

Since these are electronic ignition can you set the timing without running the engine. There are no points to open. I haven't tried it. The manual says you need a timing light and run it. Need to look at that closer. I do have a test barrell but the test wheel in the barrell gives erattic tach readings.

50:1 oil is recommended for fishing motors. 25:1 sounds about right. Some guys do 32:1. I like the idea of a better seal. I will check the plugs after a heat and see if they are clean at that ratio.

There is more to do. I will find out more about timing from other racers as you suggested.

Bob Smiley

[zul8tr]

I thought hi octane fuel would not benefit these motors much. No I do not have a buzz box nor a dial indicator to check timing. The flywheel is marked so that is good.I need to get a depth guage, I agree.

Since these are electronic ignition can you set the timing without running the engine. There are no points to open. I haven't tried it. The manual says you need a timing light and run it. Need to look at that closer. I do have a test barrell but the test wheel in the barrell gives erattic tach readings.

50:1 oil is recommended for fishing motors. 25:1 sounds about right. Some guys do 32:1. I like the idea of a better seal. I will check the plugs after a heat and see if they are clean at that ratio.

There is more to do. I will find out more about timing from other racers as you suggested.

Bob Smiley

On CDI systems (no points) a timing light is needed with engine running. If you lock the mag at full advance timing can be set at idle rpm.Get an inductive type light that clamps around the spark plug wire for the signal and is powered by 12v battery. This way it can be used for other things like autos, cycles, etc.

The dial indicator can be helpful to check the factory marks on the flywheel for TDC and max timing by piston position. I did this on my 25ss which is CDI system and made a series of timing marks on the flywheel rim and a stationary index pointer to set timing.

Don't understand erratic tach readings:confused:, if tach properly hooked up barrel running should not make a didderence. What type of tach?

[zul8tr]

I agree on all points, good advice :)

2nds are appreciated. Thank you :)

[Powerabout]

Yes you will need a timing light and hence set up a timing mark with a piston stop.
I would also degree your flywheel and check the timing of both cylinders.
Cross flows can have a wide range of advance from runs fine but is gutless to instant detonate.
110 or avgas just makes life safe.
Remeber timing at idle will be differetn at wot

[smiley]

I made a dial indicator for my antique KG4 and KG7s and a battery with a light bulb for breaking points. I will obtrain a timing light and get a depth guage. my problem is the spark plugs are angled into the holes. A depth guage may not work. UGH So it is important to use the degree plate and mark the flywheel.

Regards to erratic tach readings. The tach is fine. I have two. The Tiny Tach and the Aim Data acquisition system. What I meant is that at full throttle the water in the barrel sloshes around and presents an uneven work load on the motor. If I understand you correctly; obtain a tach reading at a certain timing setting at Idle. That will work in the barrell. Go out and test the boat and performance at that setting and use the idle tach reading as your base. Is that what you mean?

Can you define cross flows for me? I have not heard that term before.

I did goof when I assembled the engine. I put in new postions, rings, gaskets, seals and one bearing but forgot to lap the rings. Oh well. next time.

This is a very informative discussion for me. Thanks for you input.

Bob Smiley

ps 6 days to Lake Lawrence.