Compression Test

[SpeedJunkee's Carbs]

Anyone know where I can find a calculator to determine compression ratio on a motor that is already put together. Or the formula to determine it?

The motor has 170lbs of compression when tested

Thanks in advance

[TC]

Re: Compression Test

Anyone know where I can find a calculator to determine compression ratio on a motor that is already put together. Or the formula to determine it?

The motor has 170lbs of compression when tested

Thanks in advance

Cranking compression has no bearing on static compression, you can have an 8:1 motor with 170lbs of compression just like you can have a 12:1 motor with 170lbs of compression. A lot of it has to do with the cam specs and valve timing, along with the LSA of the cam.

[BlackoutSteve]

Re: Compression Test

There are programs that can predict cranking compression with the cam and static comp details.
There accuracy is something I wouldn't have a great deal of faith in though..

I have a well known and popular HP program that suggests my cranking psi should 200. In reality it's 220-225, which I think is a lot to be wrong by.

[DanStokes]

Re: Compression Test

I have to agree with the other guys - really no relationship.

Go to your local NASCAR track and see if you can borrow their whistler. They don't know it but it can be used to determine displacement as well as CR. Do a cylinder at BDC and one at TDC then do the math. You'll get values in displacement (I think, CC's if I recall correctly) - subtract the combustion chamber volume from the total volume. So you get displacement but not bore and stroke, which is something you would like to know on many combos. Just a thought.

I haven't used one of these in YEARS but I know we did it at EPA.

Dan