Compression testing a High Comp engine

[Wobba]

HI! As some of you know, I have hit a weird issue. I am getting quite good with these kind of things, but this is the first time with this engine. It is a forged engine with high compression parts.

When doing a compression test I suspected the compression figures would be lower from cold than a standard engine due to the clearances between the piston and bores with these engine types when cold. I had results from 85 to 100 PSI on all 4...very low in normal circumstances.

When I did a wet test on each one it was back up to 180-225 with variability for how much oil i dropped into the bores etc...its not an exact science.

Is this normal? Please only reply if you have a clue about forged engines and high comp stuff. My guess is they are crap with holding compression at low temps, so low compression results are not that unusual? Would make sense.

[chip]

What were your piston to bore clearances?

On N/A with forged pistons I would typically be looking at building to around the 2-3 thou ballpark, so not greatly dissimilar to standard pistons.

There is no way that you should be seeing results as low as you are, for example my nova was running a relatively huge piston to bore clearance of 4.5 thou (to be safe at over 2 bar of boost and 600bhp ish), and only 8.5:1 compression and was still showing compression tests results of 10bar+

I would say that you have a problem personally. And the difference between cylinders is worrying too.

Do a leak down test, you can do this on a warm engine to rule out cold pistons causing any issues too.

[Wobba]

Hmm...I actually suspect the compression tester may be at fault. I had strange readings on my last engine too using the same tester, but when the engine was checked over and taken apart, tolerances checked etc it was mechanically fine.

I don't think valves are damaged, but they could be. Perhaps cambelt has slipped?

[LEIGH-ANNE]

Do a test on one of mine to make sure?

[Wobba]

Ah yea could try that. I will check timing on my car this weekend then perhaps test yours sometime with Robi's comp. tester :)

[LEIGH-ANNE]

So you have it then? Werent i supposed to post that? Yes thats fine, which one do you want to do it on?

[sideways danny]

not forgetting to open the throttle again are you? ;)

[chip]

If the cam timing was far enough out to lose your half your compression you would fecking know it when you drive it.

Throttle not open is a good suggestion, you havent made a schoolboy error have you?