Confusing compression results
 

Here’s my issue:

Just checked the compression on my newly rebuilt 79 3.0SC, I have about 500 miles on the engine since the rebuild. The results were:
#1 100 psi
#2 98 psi
#3 102 psi
#4 100 pi
#5 98 psi
#6 103 psi
The engine was warm (tested about 1 after running). The rebuild included 9.3:1 pistons and 964 cams.

First, I’m not sure exactly what pressure it should be, but 100 psi seems a bit low to me, what do you guys think?

Second, the engine bucks under very light throttle; I thought it was due to a lean condition. I removed the CIS and re-sealed the airbox and checked all the flex hoses, could find a leak anywhere. Checked the warm fuel pressure – 52 psi (where is should be), I measured the vacuum at idle – 15mm hg (The Bentley manual say 15-22 is acceptable), do you think this is a little low since I know all the seals are good?

Other thoughts:
Is the 964 cam causing the low compression and/or the bucking? Do I have the cam timing off?

Any thoughts or ideas would be most welcome….this is driving me nuts

Cheers

Stu

If you didn't remember to have the throttle butterfly all the way open your #'s would be low i.e. wrong. One of the very few mistakes I haven't made personally so I don't know how much lower.
-Chris

Chris, I pulled the fuel pomp relay and held the throttle wide open.

Any other thoughts...I'm at a loss here.

open throttle is generally for individual stacks like MFI or carbs. the CIS has a common plenum.
maybe the starter is cranking slowly. 964 cams are far from lumpy, so that wouldn't be it. cam timing off is easy enough to check. i would think about 160# with that compression. maybe the gauge is off. i like to pump each cylinder about 15 times. maybe you didn't crank long enough.

Thanks John, I'm going to check the valve clearance tomorrow along with the cam timing, then re-check the compression.

Never thought about the gauge being out of spec!

Not sure if this would make any difference, the cylinders are Alusil which were re-ringed (Goetze rings)

Thanks, Stu

that shouldn't make any difference. run it longer next time too, more than a minute.

Its possible the thing isn't broken in yet. Have you done several full throttle accelerations on the engine to get the break in started? If not do so. Note full throttle is not full rpm, just full throttle say from 2ooo to 5000 rpm in 3rd gea, 2nd gear if there is no place to run it up that fast,r and let it back off in gear. This helps seat the rings. Sometimes rings can take up to 10,000 miles if things were not right to begin with. otherwise look for a problem.

Not to doubt your statement, it does look like the throttle plate was closed during the test. Thats exactly the kind of results you might get if it was.

Jack, you could be right, I've only completed about 500 miles since the rebuild. I'll do a few full throttle pulls.

Thanks for the help, Stu

Stu,
Just a guess here, but is it possible the gauge is broken? The numbers look decent, just low. Maybe check it on another car or against another gauge.