Could My Heads Be Milled Or Something???
 

I did a compression test just a couple months ago, and my compression was at a 148psi low, and 158 high.

I have been told by lots of people a stock 5.0 is around 140 stock.

Oh, BTW I have 110000 miles on the clock, so its well worn.

That's, what, a low of 10:1 and a high of 11:1? Hope you're running 93 octane. ;)

?????????????????

Anyway, what was the difference between wet and dry? There is no set answer for a stock 5.0. I've seen all kinds of readings. That's why they stopped listing a recommended number, and instead gave a range that was acceptable. Several things could affect your readings, too. On an engine with high miles, carbon build up often occurs, and that can both raise compression and encourage pinging.

You should run a test again, both wet and dry, and with all of the plugs removed during the test. Post the results.

Take care,
-Chris

Doesn't quite work like that Jeb. Static compression tests are a measure of the pumping ability of the cylinder. Many things affect it such as ring seal, ring gap, piston clearances, engine temperature, throttle (open/close), exhaust restriction, etc. You just can't take the static compression test pressure and divide by 14.7 to get the compression ratio. Remember that you've also got the volume of the test guage to consider in the process. That's why you have to crank it 5 to 10 times before the pressure reading peaks.

For example, my motor is above 12:1 so by your calculations I should see about 176psi on a static tests. My normal readings are more on the neighborhood of 250 to 260 psi per cylinder. That would say that I've got a compression ratio of 17:1! God, I wish!!!! The extra pressure comes from many factors, including the fact that there's no exhaust scavenging present to help empty the cylinders, among others. Bow Tie is right in the normal range for a stock motor. There's only 7% difference between what he saw and what he was expecting. Hot/cold, adiabatic heating, inaccuracy in the gauge, and many more things could stack up to that 7% real quick. Static compression test should be used as more of a qualitative test rather than a quantitative test. As long as all cylinders were within a reasonable range of each other, he's OK.

Jeff, I thought the term "adiabatic" meant there was no heat tranfser in the system? So what exactly is adiabatic heating? Just wantin to learn. Maybe my Thermo Proffessor doesnt know what he's talking about:D

Oh yah, how'd those pedal's work out for ya?

Yes, adiabatic heating would be an oxymoron.

My compression ratio is 9.5 to 1 and I read about 181 psi in all my cyls.

Maybe what I was reaching for was adiabatic compression. What I was thinking was that there's work put into the system in compressing the volume which would increase the temperature of the volume to some extent. Would it mean much to the system? Probably not, since there's enough thermal mass in the cylinder wall, heads and pistons to probably wick that heat away pretty quickly. But in the theoretical world, it'd be easiest to consider the compression cycle as adiabatic and ignore the heat lost to the wall. Conversely, if done when the motor is warm, the heat input would probably be something to consider. Check with your professor and see whether it'd be an adiabatic, isobaric, isometric, isothermal, isentropic or polytropic process. Gawd how I hated thermo! I'd much rather deal with 'lefty loosey, righty tighty'.

Point is though that'd just be one of the many tiny little variables that all stack up in the variability.

I was told that compression tests held way too many variables to be truly acurate and that the best way to check the cylinders was by doing a leak down test. Supposed to be more accurate anyway.

Chris-
Compression tests are very usefull. They aren't as good a diagnostic tool as a vacuum gauge, but it's still an important test. It tells you where to look for the problem, but won't tell you what the problem is.

Take care,
-Chris