Compression test results - question?

[cstreit]

Long story short...

New race motor. (3.0L case, 100mm pistons) 10.5:1 compression. About 1.5 hours on the engine.. I selected the wrong gear and hit 8500RPM. Valve springs are good to 8500.

Two thoughts:

1. ARP rod bolts SHOULD be good to 8500. Anyone ever experienced otherwise?

2. EBS Springs are good for 8500 also. COmpression test came up as follows...

1 - 183
2 - 185
3 - 187
4 - 185
5 - 185
6 - 165

So six is a bit low... Hmmm, leakdown test in order I guess.... But any thoughts?

[JeremyD]

Ouch - I'd drive 20 miles and do another leakdown. If they are consistent with the first one - maybe kissed enough to lightly effect the valve seal...??

[cstreit]

Well I have a leak-down tester coming tomorrow, but here's the thing...

1. I ran 25 laps full bore after the "zing" with no loss of power or issue.

2. I just hooked up 100psi of shop air to the cylinder, there was no air going by those valves...

I'm calling it good.

[304065]

Chris, try the leakdown test with 20 pounds, the % of 20 that a leak represents (airflow through an orifice is proportional to the diameter of the orifice and the pressure of the air) will be more revealing than 100 psi air.

As a final test put a synchrometer on that barrel without adjusting the carb.

[2.7RACER]

Chris,
I agree with you. 100 psi into the combustion chamber with no leak through the valves, means the valves aren't leaking.
Maybe another comp. check when you have a few more miles/laps on the car just for reference.

[cstreit]

Thanks guys

John I will do that tomorrow relative to the other cylinders.

I suspect that 6 is just going to be lower than the others... Not that uncommon really, but if I can't hear 100psi of air escaping, I can't imagine I have an issue.


Doug, Absolutely. I've got 4 sessions and two dyno runs on the motor. It's possible that six is lagging in breakin too.


Anyone have feelings about ARP rod bolts at 8500RPM? I have balanced rods (withing 1/2 gram) and lightweight (465 gram) JE pistons so I I'm in better shape with accell/deccel stress than a stock setup for sure...

[Mr Beau]

Any compression test numbers from before the incident? Seems strange that 6 is so far from the others.

Are you sure you didn't hit 8510 RPM on the 8500 RPM springs? The RPM of the spring really depends on the mass of the valve and cam profile. But with that said, it doesn't sound like you have a bent valve.

[cstreit]

No numbers. There was only 10 laps and 3 dyno runs on the engine before this.

The springs are "conservatively good" to 8500 so a slight peak to the 8533 (actual logged) is still okay.