Cylinder Pressure test low?

[TopBanana72]

I just rebuilt my 1972 318 and it runs good and I have a couple thousand miles on it now. During some tuning, I decided to do a compression test on all cylinders. Although all cylinders were even pressure, the numbers were only 127ish pounds. I would have thought a fresh engine would be about 150. Especially since I added higher compression pistons and 80's 302 heads with the smaller closed chambers. The engine is a .030 overbore and I used the correct 3.94 ring packs. The original pistons were about .080 in the hole. The new ones were pretty close to .010 in the hole.

Did I do anything wrong?

 

[slimt]

I just rebuilt my 1972 318 and it runs good and I have a couple thousand miles on it now. During some tuning, I decided to do a compression test on all cylinders. Although all cylinders were even pressure, the numbers were only 127ish pounds. I would have thought a fresh engine would be about 150. Especially since I added higher compression pistons and 80's 302 heads with the smaller closed chambers. The engine is a .030 overbore and I used the correct 3.94 ring packs. The original pistons were about .080 in the hole. The new ones were pretty close to .010 in the hole.

Did I do anything wrong?

Fresh heads? New guides?

 

[65-440]

Did you know the factory service manual has compression spec of 100psi +/- 15psi ! ..
At least your numbers are even!

 

[TopBanana72]

Fresh heads? New guides?

Fresh reman heads from Jegs

 

[TopBanana72]

Did you know the factory service manual has compression spec of 100psi +/- 15psi !

My manual states that's the minimum pressure. Before the rebuild, the 125,000 mile engine tested in the 130's

 

[TopBanana72]

Did you know the factory service manual has compression spec of 100psi +/- 15psi ! ..
At least your numbers are even!

Cam is a mild summit, just a little higher lift than stock - about 420

 

[MoparMur]

Lots of things come into play here -- cranking speed, engine temp, cam profile, even engine oil viscosity. I apprenticed on carbureted cars in the late '70's/early '80's, always doing compressions tests on tune-ups (a lost art now, it seems). In reality most American V-8s never got above 135 psi or so after 3 or 4 engine revolutions. What I watched for was how high the compression gauge jumped after the 2nd or 3rd engine revolution. If the engine was in good shape on the 2nd revolution it would be just about at its max pressure then. If it was taking 4 or 5 or 6 revolutions to make its best reading then I knew something wasn't right.

 

[MoparLeo]

Also most people do not know how to do a "proper" wet compression test to begin with so their numbers tend to be low. 2-4 revolutions max, throttle plates locked wide open. We would use a long, standard blade screw driver inserted down the throat of the carb to hold the plate wide open. Gas pedal seldom holds throttle plates wide open so not as much air being drawn into combustion chamber. engine warmed up, disconnect and ground the coil wire (no accidents!) Just a few of the highlights.

 

[PRHeads]

While the small summit cam is indeed a small cam...... it’s still quite a bit more duration than a stock 318 cam.

Longer duration = later intake valve closing point, which(all else being equal) = lower cranking compression.

You added some compression, and cam duration, and the result ended up as a wash for cranking compression.

 

[MoparMur]

Good point Leo, keeping that throttle wide open is important.

 

[TopBanana72]

Also most people do not know how to do a "proper" wet compression test to begin with so their numbers tend to be low. 2-4 revolutions max, throttle plates locked wide open. We would use a long, standard blade screw driver inserted down the throat of the carb to hold the plate wide open. Gas pedal seldom holds throttle plates wide open so not as much air being drawn into combustion chamber. engine warmed up, disconnect and ground the coil wire (no accidents!) Just a few of the highlights.

 

[TopBanana72]

Thanks.. I'll run the test again at some point.

 

[TopBanana72]

This is all good information. Also, I never degreed the cam, nor installed it advanced. I installed it dot to dot and timed it by ear. It likes about 15 degrees initial. I'm going to do some more tuning and not worry too much about the compression numbers. I am confident the engine went together well. I was expecting more performance from the higher compression (I figured it would be about 9.5 to 1), but.... after all, it is a 318 in a B body.

 

[MoparLeo]

4 important tuning tools.
#1- A good vacuum gauge
#2- A dial back timing light.
#3- Tach/dwell meter.
#4- Service manual.

 

[65-440]

Good point Leo, keeping that throttle wide open is important.

You sure about that? I have yet to see a difference in compression readings whether the throttle is open or closed.....as a matter of fact just did a compression test on a FBBO members vehicle last week and got the same numbers both ways....I never open the blade