Is there any way to determine compression ratio with an engine in the car? My buddy's engine was supposed to be built to 9.5:1 compression, and it ran great out in Colorado with 10psi of intercooled boost on pump gas. Now, down here in Mississippi, we can't get the detonation to stop even with the 6BTM dialed all the way up. I think the shop that built his engine put the wrong pistons in and messed up his compression ratio, but without taking the engine back out we'll never know. Is there any way we can find out if it's a compression ratio problem?

 

How much boost are you seeing now?

 

Check the compression using a regular compression tester. This won't tell you compression ratio, but you can compare to other cars.

For instance:

200 psi - Optimum performance naturally aspirated, marginal for use on pump gas.

180 psi - Typical performance natural aspiration, pump gas.

150 psi - Medium performance for natural aspiration. Marginal for supercharging.

120 psi - Good for supercharging. Mine tested at this and I'm about 9.0 / 1 compression.

This amount of compression in psi depends on compression ratio AND cam timing, so you can't equate it to a ratio.

Let us know what you find.

Mike

 

Don't forget that you went from one end of the spectrum to the other in terms of elevation. Chances are, you're pretty close to sea level in Mississippi, whereas you were several thousand feet up in Colorado. Not sure if you've compensated for that or not.

 

I'm with mongse_1 ... the altitude drop from Colorado down to Mississippi should have the car running pretty darn lean. If it was tuned correctly for Colorado then the fuel pulse widths should be pretty lean to match the thin air. Bring those same fuel tables (WOT) down to some thicker air (Miss) and the fuel pulse widths at WOT stay the same .... the A/F ratio in Miss should be showing very lean with rattles and etonation a major concern.

Later ... Larry S.

 

Well, we finally ran the compression test...his was around 185 on every cylinder. Then we did mine for comparison. The only internal mod I have is a hotcam and mine was 200ish. So, does this mean that the compression is way too high?

 

In my opinion, 185 psi is too high for alot of boost. He could pulley down to 6 psi and probably be OK.

That doesn't necessarily mean it isn't 9.5/1. A low compression motor will have alot of cranking compression with a small cam.

Mike

 

Yeah but the cam is way too big for his application anyway. 230/244 114lsa. That alone should bleed down alot of cylinder presure.