Piston Compression...

[ngentile]

Mariner 25hp 1987 Compression

What I'm looking for is what is the ideal compression for my engin model. I just want to know how far off I am from the norm.

For example if I brought my engine to a shop and the mechanic with 40 years experience checks the compression, he has a number in his head of what a good PSI for my year/mk/model is. What is that magic number? I know all engines are different.

The engine runs perfect and powerful.. just would like to know the PSI range for the 25hp 1987 Mariner. Thanks

 

[achris]

Re: Piston Compression...

If the engine is running well, check your compressions... That's your magic number... I like to see more than 120psi... and both cylinders close to each other.

 

[emckelvy]

Re: Piston Compression...

Every compression gage is different and results taken on a different day with different weather conditions can even differ.

That being said, if both cylinders show even compression and are between 100-150 psi they're in a 'normal' range of compression.

I'd be much more concerned if one cylinder was, say, 140 psi and the other one was 115-120, than I would be if both cylinders measured 125 psi, for example.

It's not common to find a motor with extremely low compression in all cylinders. One cyl or the other will typically go bad first. Some exceptions to that would be in a multi-cylinder motor with a head gasket blown between cylinders, or one with severe overheating problems. And your motor doesn't have a head gasket anyway.

If the variance between cylinders is less than 5 psi and the motor runs very well, I woudn't worry. If you have a mechanic with a leak-down tester, better still. Leak-down is a much more accurate test. A leak-down tester is pretty good at catching a marginal cylinder that might still have a decent compression reading.

HTH.........ed

 

[ngentile]

Re: Piston Compression...

Ok.. well my compression is 95/95. The motor runs exceptionally well and gives me more power than I could ever dream of. The compression seems lower than the norm but it runs so well never any problems with it. This is why I'm wondering if the level compression is normal for this certain motor.

What do you think?

 

[achris]

Re: Piston Compression...

The low reading could be a characteristic of your gauge...

If the engine starts easily, runs well and smooth and give good power... All is well... Record the compressions for future reference and check them again in 6 months... If they are the same... All good.

Chris......

 

[frwhlr38]

Re: Piston Compression...

Like Others have said weather, barometric pressure can play havic on ones mind also no 2 gauges read exactly the same even the high dollar ones like snapon macor matco i have several high end ones and take a couple reading then average them out anything above 100 is good and maybe 95 is good for your engine as like i say many verables in it

 

[emckelvy]

Re: Piston Compression...

Some motors, due to their design and the way they're ported, have lower compression readings at cranking speeds. This doesn't mean there's something wrong with them.

If it runs great and has lots of power, not an issue.

And the compression values you're getting are even, more's the better!