engine and drivetrain Weird compression test

[ShaneUtah02]

From reading C.D.'s compression test write up I found the numbers on my engine to be quite a bit lower than the 185 listed in his write up.

I used an extension to make the connection a little easier (it came with the guage) and got a low of 110 high of 117-120 and one in between. C.D.'s write up gave 185 as the target mark.

Car doesn't run like it is low comp, but the numbers don't match 185. Will 5000 ft elevation, and an extension do that? I have heard that you basically want the numbers close to each other.

What do you guys think?

Thanks,

Shane

[Guest Anonymous]

The air's thinner the higher you go and so compression numbers will go down given the same # of engine cranks. I used to get similar numbers to what you've posted when I lived in SLC, UT. You're right when you say it's the variance between cylinders that matters. You'll typically want all cylinders within a 10psi variance.

[c.d.iesel]

WHAT MATERIAL IS THE GAUGE extension?

flexing rubber WILL result in lower readings

your altitude will also result in slightly lower readings

and 185 was not the spec for a 2002 motor - just an example

for the explaination

if you have 155 to 175 - yer excellent to go

if below 130 in any cylinders - start taking action

in a number of different ways - depending on 99

conditions.

[Napes]

given the same guage, same car, and same operator, does anyone know what the compression reading drop would be per 1000 ft of altitude gain.

(I live @4000' and know my car is certainly stronger when I go to San Diego).

Bob Napier

[ShaneUtah02]

Okay, the guage had a rubber extension about 10 inches plus a screw in 1 1/2 inch aluminum extension to better reach the plug holes.

Sounds like the motor should be fine compression wise given the extensions and altitude.

Thank you for your quick responses. Now what to do with my good engine since I have the m20 ready and waiting, I really don't need another 02, or do I? Maybe a cabrio with a turboed m10. Hmmm.

Shane

[zane]

Using this calculator http://www.not2fast.com/turbo/compression/cranking_pressure.shtml they listed a gauge pressure of 163psi at 4000'(1220m) and 70 Fahrenheit. I used a compression ratio of 9:1, rod length of 135mm, stroke of 80mm, and 264 duration (I think that's right for a stock 02). This obviously doesn't include the variable from the length of hose from the combustion chamber to the gauge, but I couldn't imagine that it would cause a 40-45psi drop.

[ray_]

Using this calculator http://www.not2fast.com/turbo/compression/cranking_pressure.shtml they listed a gauge pressure of 163psi at 4000'(1220m) and 70 Fahrenheit. I used a compression ratio of 9:1, rod length of 135mm, stroke of 80mm, and 264 duration (I think that's right for a stock 02). This obviously doesn't include the variable from the length of hose from the combustion chamber to the gauge, but I couldn't imagine that it would cause a 40-45psi drop.

Also on that page is:

Altitude

The altitude at which you are running compression tests has an effect on the resulting gauge numbers. This is easy to see by imagining that you are in the dead vacuum of space and running a compression test; the result will obviously be zero.

This has a relatively small effect on the results, so you can safely leave it at zero (i.e., sea level) unless you live in a high mountainous area. At my locale, Ann Arbor, MI, the elevation is about 250 meters ASL, which reduces calculated pressures on the order of 5 psi (about 2%). In Boulder, CO, with an elevation of 1650 meters you will see calculated pressures about 40 psi lower than at sea level. (Maybe I should make this local barometric pressure, as that's what is actually going on here...)

Cheers,

Ray

[Guest Anonymous]

I think you should adjust valves and retest. Test with warm engine and throttle wide open. If numbers are still abnormal put a little oil in the cylinders and retest.

Jeff Ireland