1% Leakdown realistic on 80k mile car?

[flightlead404]

In preparation for selling my '86 930 I did a compression and a leakdown test on the engine today.

This is on a cold engine with 80k miles that has not been run in a few weeks.

My compressions were performed simply by connecting the pressure tester with all plugs out, and cranking the engine a few seconds with WOT. All were 105-110psi.

I measured the leakdown with my ATS differential pressure tester that I use on my airplanes. Rotated the engine to TDC compression stroke cylinder by cylinder, and used 100psi as the control pressure. Every cylinder was 98 or 99 psi. That is 1-2% leakdown. I did this twice around to make sure I did it correctly.

These numbers aren't what I was expecting. I was expecting the compression to be a little higher, and the leakdown to be a lot lower.

Is it really reasonable to expect 1-2% leakdown or did I do something wrong?

To validate the tester was working correctly I watched the gauge as I (carefully) went past TDC and it did suddenly drop as the exhaust valve opened.

I'm just surprised by the numbers I suppose.

[patina]

What’s the problem? 1-2% leakdown is perfect.

[slow&rusty]

Originally Posted by patina

What’s the problem? 1-2% leakdown is perfect.

Agreed.

[flightlead404]

Originally Posted by patina

What’s the problem? 1-2% leakdown is perfect.

I guess I was just feeling it was a little too perfect

I was expecting in the mid-90% range.

Its pretty hard to mess that test up though. At least, its hard to mess it up to make the pressure read higher (leakdown be lower).

I just never see that kind of result on airplane engines, especially not mid-time engines. Never done it on a car before.

[dannobee]

Leakdown is fine. Compression gauges vary. Specifically, the amount of tension on the spring inside the shrader valve in the end of the plug adapter. Try a different gauge.

[MikeD930]

It's possible that since your car sat for several weeks some engine oil seeped into the cylinders. This may contribute to 'false' comp readings. I think it's best to do a compression test shortly after warming up the engine.

[flightlead404]

My compression might be lower because I have a higher lift, longer duration cam in there.

Oil in the cylinder would see out, not in, but would generally give both a higher compression and better leakdown. The compression and leakdown numbers being consistent across the board is good.

IDK why I'm complaining about such good numbers lol

[scottrx7tt]

On most other cars even 10% leak down is great