Another Compression question

[klyross]

I have read a number of threads re: interpreting compression readings and I get that
normally > 100 psi and within 10% on all cyls is the rule of thumb.

My question is, (all things being equal), if we are talking about the same two stroke outboard motor (make and hp), compression tester, etc.,
could/should one expect the engine with readings of 125 on all cyls to have better performance
than one with, say, 110 across all cyls? (i.e. does higher compression readings matter, or is it truly just
being above 100 and even across all cyls).

Thanks ...

 

[Frank Acampora]

Re: Another Compression question

To answer the question simply: Yes, higher compression ratio equates to a higher horsepower output. That is why hot-rodders always "Milled" the heads of their engines and "decked" the block.

Now, the simplified explanation: ALL internal combustion engines operate by the pressure of expanding gases heated by burning fuel (in our case, gasoline) pushing down the piston. This pressure can be measured and averaged. It is called the BMEP for Base Mean Effective Pressure. It is an average of the pressure pushing on the piston. It can be drawn as a curve, just like torque and horsepower curves.

When you increase compression ratio, the whole BMEP curve rises. Even though it takes more horsepower to compress to a higher ratio, since the whole curve has been raised and more torque is made, more horsepower is generated.

However, at above approximately 9-1 compression ratio, gasoline tends to self ignite from heat of compression (pre-ignition, pinging, or knock) so we must use premium fuel. This is why many larger outboards designed to use regular gasoline are limited to about 145 PSI compression.

Yes, a lot of people on the forum use 100 PSI as a base. This is because many compression testers vary and will give different readings from each other. SO: generally speaking for larger engines, if compression is over 100 PSI, the engine is probably healthy. However, a little knowledge is a dangerous thing: Some of the big engines are low compression--that is, about 90 PSI and most small engines are 90 or lower. SO: You need to know factory new specs before you make a decision as to the health of the engine.

 

[klyross]

Re: Another Compression question

Thanks !!!!

I asked a specific question for Yamaha compression values in the Yamah Suzuki forum ...

 

[JB]

Re: Another Compression question

You did, and it was answered there. This thread violates rule #11. Please respect our rules.

 

[klyross]

Re: Another Compression question

Sorry JB - that was not my intent.

My initial question was a general question about compression - i.e. is higher and even better than lower and even.
A response there indicated knowing factory specs before making a final decision. So,
I took that recommendation and asked the question in the Yamaha/Suzuki forum as the
motor I was referring to was a Yamaha.

Anyway, that was my reasoning - I did not mean to cross-post.

Thanks ...