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How To Find Ideal Engine Compression

Boats & Engines > How To Find Ideal Engine Compression


Ever wish you could sit around in your shop or garage with a bunch of boat lovers and pick each other’s brains about perplexing engine, maintenance or boat-use questions? The iboats Boating Forums let you do just that. This important part of the iboats community brings experts, novices and users at all levels together to jaw about almost everything related to boating and life (an inseparable connection for many of us.)

In the October 2006 Transom™ we spotlighted some general categories or topics you can explore in the Forums. Winter is a good time to curl up with a good computer and surf through a few of them. Heck, you might even find yourself contributing to the Forums pool of knowledge . . . and thereby helping others!

This month we highlight a thread or series of messages that have been posted as replies to a classic question on engine compression. Ten different users contributed to the discussion within a period of 38 hours, several within an hour and a half. Names have been left out and most spelling errors corrected to protect the innocent.

In a shameless plug for iboats.com we remind you that a plethora of manuals, engines, power heads and engine parts can be purchased from us online. Not surprisingly, we also feature propellers, boat covers and all sorts of boating accessories . . all of which helps keep our Forums free to ALL users.

Ideal Engine Compression

User 1 – Question
I’ve read many threads with questions of correct compression. Standard answer always is 100+ psi and within 10% on all cylinders. My Clymer manual says same thing. Is there no place to look up exact new compression and factory allowed tolerances, as in many car manuals.

I tested compression on 8-10 motors now, never met any with 100, but 150, 125 and 135 PSI. What would factory compression be, for example, on my 1991 Evinrude 115 HP (now 3x125 and 1x135 ) and my 1988 40 HP Evinrude ( now 148/150 )? Isn’t wear from 150 PSI down to 100 much to accept?

User 2 – Response
K, I'm an idiot with respect to your first question. Don't even know where to find "Factory Spec" on compression. And, yup, 125+ is common to see on a good motor.

All of that said, I've got a couple of OLD Rudes that seem to read 70psi cold compression and guess what? They run GREAT!

I know that this doesn't help answer your question. On the contrary, it seems to add confusion. In the other direction. Had a 25 Merc that was reading 125/125. One bad-day-on-the-lake later it read 125/90. After the OOPS, it ran like crap.

So I'm pretty convinced on the 10% thing.

User 3 – Response
I bet it would be interesting to see a monthly graph of compression on an engine starting from new and not broken in - to old age. My guess is they would all have a similar shape but the numbers would be different and occupy a certain range. They would increase as the engine broke in and slowly decrease with age unless some event like a stuck ring occurred. Probably would take a dip after winter storage and pop up a little after some use and a decarb.

User 4 – Response
While even compression on cylinders is most important, enough compression must be next in line. Most people, including myself, believe that any cylinder needs at least 100psi to operate satisfactorily. Automobile engines are a little different, but I race street stock dirt cars and I wouldn’t race a motor with anything less than 125psi.

User 1 – 2nd Input and Question
Thanks for your replies - fine thoughts - but - the standard compression figures must be available some place, so if you inspect a motor, its possible to see where on the lifetime it is.

I remember hauling in a diesel truck for service. I was told compression was within 10% from the lower margin of acceptable wear - good information and it must be possible on a two stroke too. Any dealers out there to answer my second question?

User 6 – Response
I hope that you are a patient man because I have not seen any factory compression "specifications" in 30 years. These ain't car motors.

User 6 – Response
I happen to have the OMC manual for your motor. Sorry, no good news. All OMC says under "Compression Check" is that variation between cylinders should not exceed 15psi. No specifications on compression anywhere.

User 7 – Response
I don’t think OMC every printed compression specs, older engines had higher numbers. The Mercury manuals I own have the compression numbers. One thing on the OMC engines is the numbers is not always the same on the same year HP engine, probably due to quality control, and may be reason they did not print numbers.

User 1 – 3rd Input
Thanks for all your replies. If compression varies much on new motors, output HP will vary too - kind of disappoints me - not the picture I had of Evinrude (my engine)!! Thought they were the best till Yamaha and the 4stroke Suzukis showed up.

User 8 – Response
The reason there aren't standard compression ratings listed in any manuals is that there isn't any standard compression gauge. Plus there are lots of factors that can alter the readings - temperature of the air, temp of motor, throttle open/closed, etc. It's likely that no two gauges will read the same motor at the same PSI. Allow me to illustrate...

Boat speedometers run on the same pressure principle. Ever have your 40 mph boat zip by someone else's 45 mph boat, even though the other guy swears his boat does 45 mph?? Marine speedometers are just as notoriously inaccurate for the same reason...

A compression gauge is a diagnostic tool, used to determine the health of your motor. Cylinders rarely wear out equally, as one usually lets go when that time comes. Most gauges will read within 20% of actual and as long as they are consistently accurate to themselves, they can tell you a lot about your motor...

User 9 – Response
Keep in mind that compression numbers at cranking speed have a lot to do with port timing and configuration. Its not unusual for a 40 HP E/J to have 140 lbs and a V6 Looper to have 100. A V4 Crossflow will have around 125 and a V4 Looper may be around 100 but the Looper makes much more torque.

Its also not unusual to modify a motor (porting) and the numbers stay the same or may even come down some. Especially if the exhaust port timing is raised but it makes more power. You can also change the timing edge on the piston. The easiest way to raise the numbers is with the heads. Isn’t this fun?

Good luck!

User 1 – 4th Input
Thanks for replies.

Not to start arguing, I just think it is interesting and would like to know stock compression figures, as it gives a hint of operation hours on an old motor.

I just gave it a couple of more thoughts. If lack of accuracy in gauges was the reason, it would be as hard to determine if compression was 100 or 80 as 120 or 140. I did try to compare my gauge to other and only got < 5 psi difference. Can it be for troubleshooting purposes only the "100+" comes in? If a misbehaving motor shows up with compression beyond 100+ on all, better look for reason somewhere else than compression, as all motors will be able to run with 100. Still, a compression at 100 must be worse on a motor originally designed to run with static compression at 150, than one designed with 125.

I know there are better ways of boosting power than increasing compression, especially if you ask bearings and rods!!

User 3 – 2nd Response
Just an aside to your last comment the compression readings we're talking about here are non-firing or static compression. Force on bearings and other internal components have little to do with static cylinder pressure and the difference between 100psi and 150psi is probably negligible. Any improvement in an engine that results in a power improvement whether it is simply a better mixture, better ignition, breathing (porting), or compression ratio will increase the dynamic (firing) pressure when that cylinder lights off. Dynamic cylinder pressure during firing is going to be orders of magnitude higher than the static pressure during a compression test.

User 1 – 5th Input
Thanks for all your replies.

I like to learn more about outboards and 2strokes, and I really learned a lot from the experienced people on this board. It is the most interesting board I ever joined, and lot of helpful people. Though I messed with motors since I got my first motorbike as 13 years old, 35 years ago, I learned a lot here.

User 10 – Response
I’ve got a 1970 Mercury Thunderbolt 65 HP 650cc and apart from the oil leak I think I can do about 40-45 mph at the motor (speedometer is reading 50). Cold compression is within 8 and around 95.

Runs like a beast, apart from the lower water intake gasket thing leaking water into my case bowl :/

[Transom Editor’s note: Please come and join the fun of reading and adding your comments to our Forums. We have specific Forums for different engines, fishing, fixing boats, different boat types, and MUCH more. Come and see - for FREE – at iboats Boating Forums.]

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