I tested the compression on the 327 this morning and found i have 100-105 psi on all cylinders. What would cause this condition? I have seen engines with one or two low but never the entire engine at once.
if it was a sudden loss of power, I would guess at a cam chain being loose and jumping a couple teeth. valves still open a bit at the start of the compression stroke, if it has been something happening over a span of time, rings worn out just my opinion
what caused you to check compression in the first place? is it a high mileage engine? put a little oil in the cylinders and recheck. does the compression increase? verify timing.
There are two schools of thought on this, and I am sure someone will disagree with me, but I think compression tests need to be done with the throttle blocked wide open. This insures a good charge of air on the intake stroke. Also, the engine needs to be warmed up. EDIT: Basically, I am saying that you need to be sure of your testing technique before you start pulling timing covers and squirting oil in your cylinders. These are good things to check, but are not the way to go until you are satisfied that you performed the test correctly.
Throttle plate blocked wide open and all of the plugs out of the engine. The accepted norm is to crank it so that it hits on compression on the gauge five times per cylinder. If they are all evenly low squirt a bit of oil in each one, spin the engine over and run the test again to see if the compression comes up appreciably on all cylinders. That would tell you that the rings are pretty well worn.
An SBC won't run a couple of teeth off. But a loose chain will cause enough difference in cam timming to cause problems. ypk53, Do you have a base line on compression? IE do you know what the compression was before? Maybe it is just low. Dribble some oil in the cylinders and run your test again. Do you notice a jump in the compression? If it is a big jump than I would suggest that your rings are pretty worn.
This is exactly correct information. Close your mouth and pinch your nose. Then, take a deep breath. Can't do it? Neither can your engine.
This is a post from yesterday from the original poster ... THIS is probably why he did a comp test in the first place. "I keep a log of everything that I do to the car, and I have noticed that the intown gas mileage has dropped from a constant 14 mph to 8 mph for the last two tanks. I have also noticed that the exhaust seems to smell rich after i turn it off. I did a quick check under the hood tonight a see nothing wrong. PVC is connected , choke appears to be functioning. This is a 300/327 with a year old edelbrock 600 cfm carb. The power level seems fine. No hard start or run on issues. No back fires. Nothing has been modified to the car other than a oil change in 6 months. What could case this condition. Where should I start checking?" <!-- / message --> I'm thinking the compression has nothing to do with his issues ... a dime and a donut says the parking brake is sticking ....
Man...I don't know about your 327, but I have had plenty of sbc's over the years that ran great and only had about 100 psi per hole. What are the cranking compression specs for a brand new motor? I've had more than a couple vintage motors that spec'd out at not much better than 120 according to their tune up charts.
ypk53 I just read your sudden rich condition thread. Some info that may have helped this thread. If it is running fat you may have washed down the cylinder walls, that would also cause low compression numbers that would be fixed with a little oil in the cylinders.
1 or 2 being low is bad, all is not as bad. What was yours originaly? why did you do the check in the first place? I have seen engines run fine at 90 psi. A low reading could be as simply as a slow cranking engine from a weak battery. If there is no power loss, run it.
Time to worry is when a cylinder has more than a 10 percent differential in compression. Your cylinders are all within 5 percent of each other. I see no problem.
I'm not Phsycic,So Ill assume this just became an issue? Has it ran well in the past? I immediately think you may have a situation a lot of guys end up with when doing a Chevy engine. IF= your engine does not have 327 heads and in fact has 350 heads,the increased combustion chamber size in those heads will net you the compression you mention.... I say that 'cause you state the compression is consistent in all cylinders.... check the casting numbers on those heads and see... . the compression =RATIO= is the problem here .....it will not efficiently burn fuel at the ratio [netted from 350 heads] on a 327... It will be very sluugish and black smoke rolls out when pedal is floored-sound familiar?
I've seen a lot of high mileage engines around or even under 100psi cranking. If it's a stock 327 (not rebuilt with lower compression), then you are definitely seeing some wear. Agree that you're better off with all of them low than 1 or 2.
I also always thought SBC would not run two teeth or more out on timing,but then a guy sells me a 350 from Tenn. that he said is rebuilt and never started. I started it ,run poor low power and smoked some,but run. Well it was in a pile of parts I got from him at a low $ so all good,but funny; Well maybe in Tenn that was rebuilt?,but still had used rod and mains plus old pistons and rings but new gaskets all ,new cam and chain and a so so valve job,point is the new chain was one with the three marks= 0,triangle and a box marked on the keyway slot of crank gear and then on the chain tooth you use too line up with cam gear. There is about three teeth on crank gear between "O"keyway slot and "O"marked tooth for lining up with cam gear "0",he had over looked tooth mark and lined up on keyway only and some how it still ran,yet those are crank gear teeth wrong,WOW ,if I had not seen it run,no one could of made me beleve it And it had very low cly PSI too!
Well here's the back story. I had a small vacuum leak which caused me to run rich. After fixing the leak I thought I should check to make sure the plugs were not fouled. Then I thought as ling as I have them out I should run a compression test. I have never tested the compression on this car so I have nobase line to work from. It has the original double hump heads. I have good power. I don't burn oil. I have good oil pressure and steady high vacuum. When I did the test the car had only ran for 5 or so mins and both the choke an throttle were closed. I will most Likely leave every thing alone since they are constant across the board.
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