compression check ?

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70408dart

70408Dart
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My motor is a 73 360 thats been checked for cracks, bored 30 over, decked and line honed. I'm using the KB107 pistons and have the edlebrock aluminum 60779 heads. I don't remember the thickness of the head gaskets. When doing a compression check on my engine what should the numbers be? Reason I ask is because I did do a compression check and I got 145psi on cylinders 2,4,6 and 8. Cylinder #1 140 psi, #3 165, #5 150 and #7 150
A buddy said with those numbers its more like a 8.5:1 motor instead of 10.5:1 Any comment on this would be greatly appreciated as I don't understand this stuff.
 
10:1 would be your static compression, those numbers represent dynamic. Thats all i know about it lol but dynamic is supposed to be lower
 
cam size lobe lift 1.5 ratio 524 intake 540 exhaust
duration @ .050 228 intake 232 exhaust

Rings seated motors got about 3 to 400 hundred miles on it and one 1/4 mile pass.
 
My old 360 had KB107's and the compression ratio figured out to be 9.85 to 1 and with a cam that had 232 @ .050" and .513 lift had 148-155 psi cranking compression. Without knowing the exact compression ratio and cam lobe seperation I can only guess and say it should be approx. 150-160. The big issue I see with yours is the varience. 20 psi from the lowest to the highest isn't good. Did you test it with the engine warmed up, all the spark plugs removed and the carb blocked open? If not that could drastically affect the readings.
 
Depending on the quality of the gauge, the numbers are really only representing relative pressures, which shouldn't vary more than 10% on an engine with miles..

In my shop, we have 3 compression guages, a Mac, a Snap-on, and a $39 budget guage,, they are all different, on the same engine/cyl, - by up to 18 lbs. but they all will show a cylinder imbalance..

but try to figure compression ratio,,.. anyone's guess is good.. my .02
 
It's impossible to figure compression ratio from the info given and it's impossible to equate a given cylinder pressure witha compression number. That being said, for the numbers you have supplied, it seems like it's low and I'd expect something closer to 160psi and like Fishy said - a spread that far on a fresh engine would raise my eyebrow. Lot's can affect a pressure test. For starters tell use exactly how it was done.
 
Motor was cold when I did the compression check. Plugs were all out of it and the carb was wide open. Dont no why such a variance in pressures. Will be looking into that as the motor is now pulled back out of the car for disassembly. We shall see. Just wanted to thank everyone for there input.
 
70408dart said:
Motor was cold when I did the compression check. Plugs were all out of it and the carb was wide open. Dont no why such a variance in pressures. Will be looking into that as the motor is now pulled back out of the car for disassembly. We shall see. Just wanted to thank everyone for there input.
Click to expand...

I wouldn't have pulled it a part just yet. but sense you have. look vary carefully at every part, you take out. including gasket such a head and intake.

I would have done a leak down check on the low cyl. then i would know if it was a burn/ leaking intake or exh valve, bad rings exc. It could have got the focus a lot closer to the problem

When you get it apart, post up picture, of areas that look odd or different from cyl to cyl.
 
I second what everyone has said, the compression test will come up with different results depending not only on your compression ratio, but also your cam duration, cam advance/retard, lobe separation, and tons of other stuff. Consistency is what you're looking for in a compression test, not the actual numbers. if one or a couple of cylinders are way down from the rest, you've got a problem in those cylinders.

The odd thing seems to be that most of your cylinders are low compared to a couple high cylinders. Before you take the pistons out, check the piston-to-bore clearance of all the cylinders with a feeler gauge, and once the pistons are out, take the rings off, and check the ring gaps in their respective bores. Again, you're looking for consistency.

if some pistons are tighter than others, try switching the tight pistons into loose cylinders and vice versa. If the clearances are all the same, check your ring gaps. If there're one or two that are wider or tighter than the rest, run a glaze breaker down all the cylinders, get a new set of rings, and break it back in again.

If the piston to bore clearances are all the same, and the ring gaps are all the same, I'd chock it up to engines being the fickle animals they are. and put it back together. :dontknow: (unless of course there's a reason not to put it back together.)
 
Re-do the test with it operating temp. Then post the results. Youcannot rely on a cold test and I think once it's warmed up you'll find it's ok.
 
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