Compression numbers - need to estimate my CR

[notch johnson]

I bought a used 03 Predator 500. Apparently the engine was modified by Top Gun..... I though they only sold exhaust kits. ???

Anyway can I approximate the compression ratio by performing a compression check?

What would stock be?

What about 12.5:1?

This is assuming all is well piston ring/ valve wise.....

Thanks!

[Bradracer18]

its going to be tough to estimate that, because it could vary a lot between engines, especially with not a perfect ring seat or valve seat.


it could be roughly figued....by the volume change between BDC and TDC.....

[notch johnson]

Yeah - in theory if all was equal I assume this number would be high than the 90 psi my manual states I should have cranking with the starter at 400 rpm.

If its higher than 90 great I suppose, could be a 12.5 or up piston. Either that or my valve timing/ adjustment could be off LOL

Just trying to see what I have without tearing the engine down.

If anyone has compression test numbers from a 12.5 on up CR would love to hear them

Thanks Notch

[Fred55]

You cannot determine CR by using a compression tester, a compression tester tests the actual pressure in the cylinder in PSI and a CR is the ratio of the cylinder volume at BDC over the cylinder volume at TDC.

[notch johnson]

Originally posted by Fred55
You cannot determine CR by using a compression tester, a compression tester tests the actual pressure in the cylinder in PSI and a CR is the ratio of the cylinder volume at BDC over the cylinder volume at TDC.

Techinically speaking yes - but a higher cylinder volume will equate to higher cylinder pressure (ie quench) if all else is equivalent besides the CR or at least one would think applying basic physics.

Eitherway - does all "race" gas contain a dye to distinguish it from pump gas? Prob easier to determine this route - lol

The gas has a odd smell and the exhaust smells a little sweet. From reading around on here since my intial post it seems even if this was 12.5 (which some claim is really 11.8) that 93 pump gas works fine. Prob nothing to worry about ;)

[Fred55]

The last race fuel I had used did have a "sweet" smell to it, it didnt seem to have a die in it rather it just looked a lot cleaner than pump.

[Bradracer18]

My race gas is a purple color....and does smell much better(like a race car)! You could always dip in and take a look at the piston.