compression results

drmango · 02-15-2009, 06:46 AM

I have a '74 911 2.7.....

I recently had a compression test done. The results were between 135 and 150. Is this good? Should I be concerned? What is too low and what is ideal?

Thanks,
Dave

drmango · 02-15-2009, 07:01 AM

Also...

This may explain the white smoke out of the exhaust at start up. Typically after it sits for a few weeks.

Should I have a concern with the above compression test results?

drmango · 02-15-2009, 07:24 AM

Another thought...

If these numbers are bad, is the only solution to rebuild the engine??

ossiblue · 02-15-2009, 07:28 AM

As a general rule, the difference between cylinders on a compression test should not exceed 10 percent and the results you post are at that "limit". A more informative measure would be a leak down test which would give you an idea of the top end condition. Your compression #'s aren't alarming but since it's on a 74 2.7, you do have potential issues with loose head studs and valve guides unless those have been addressed.
As to the smoke, that's normal for any car that has been sitting for a few weeks and does not necessarily imply you have a problem.
A little history on the car could help determine if you should be concerned but if the engine runs well and feels strong, it all seems to be good.

drmango · 02-15-2009, 07:36 AM

Thanks L.J....

The PO already upraded to the Carrera chain tensioners and also replaced the head studs with the Devlar (not sure on spelling) studs.
I will see about getting a leak down test. Otherwise, I guess i should just keep an eye on the oil consumption.

drmango · 02-15-2009, 07:40 AM

How complicated is replacing the valve guides???

Ruben911 · Ruben911's Garage

Dave, see the following link on compression numbers versus engine compression ratios. If you engine is a 2.7L then you are probably running 8.5:1 ratio which indicates your numbers are normal.

Compression Test Pressures

With regards to smoke on startup. I have a 2.7 L that still runs strong at 175K and i do get some smoke on startup (have been getting it since i bought the car back in 1999) evn after sitting overnight. My engine consumes the normal amount of oil.

BTW, where are you in Florida?

vreference · 02-15-2009, 07:47 AM

Seen a few white smoke threads; is there a consensus? To my mind, on an air cooled engine about the only source of white "smoke" is water from the combustion process (normal) and Brake fluid via a brake vacuum booster (which I imagine your car does not have.) It's certainly the right time of year for you to see some water vapor for the first few minutes of operation depending on your climate and humidity.

drmango · 02-15-2009, 08:01 AM

I am pretty sure the smoke is from oil leaking past the valves. I am a novice at this and am assuming based on reading all the very informative threads on this site. I think it has unfortunately increased my paranoya.

Ruben, I am located in Brooksville (Hernando County)

thanks to all for the informative feedback