Compression test results

[beanhead]

Well....wanted to see what you all think. Engine has a little over 500 miles on it, a rebuild that sat for near 14 years..440, edelbrock RPM heads on plain ol' fel-pro 8519PT gaskets, cam is the compXE 21-224-4, 230/236 @ .050", 110 lobe separation, .488"/.491" lift, non-adjustable shaft-mounted rockers, 850 speed demon carb on an RPM intake. Valvetrain is somewhat clackity, when it gets warmed up. With all the flattened-lobe hysteria and such I've been wanting to get a test done, I figured it had enough miles on it (AND the tti headers did not have to be unbolted=bonus)....First firing and cam break-in was done back in February, it runs good but I'm positive it could be better if tuned better. This was done an hour or so after driving it...
#1.....160 #2.....152
#3.....155 #4.....158
#5.....152 #6.....152
#7.....150 #8.....160
What do these numbers tell me about valves operating properly, compression ratio, rings being okay etc?

 

[oldbee]

It's been yrs since I worried, but looks pretty good. Why 14yrs? I should talk, mine sat for 20 after the last young'n left the house.

 

[beanhead]

..Why 14yrs?..

Why indeed...! Got the car, started taking it apart to swap engine/trans and fix many other things...got married, kids, wham-o 14 years! (I'm in the electrical trade and it was a very good recycling year, or it would probably STILL be sitting lol!) My car's nickname is "the white rabbit"..(rabbit's the secret code name for scrap copper..shhh don't tell anyone..!)

 

[69L48Z27]

Your good. All your looking for is consistency between at the number. 8 PSI is the difference from high to low. Usually 20% is the upper limit between highest and lowest cylinder.

 

[lewtot184]

you should be fine with pump gas. static compression may be a little low but I wouldn't worry about it. that cam closes the intake valve fairly early and may be the ticket for what you have. better safe than sorry.

 

[Kern Dog]

Your good. All your looking for is consistency between at the number. 8 PSI is the difference from high to low. Usually 20% is the upper limit between highest and lowest cylinder.

I've heard that as well as 10% in other places.
In 2011, the compression numbers in my 440/493 were all within 8% but the engine smoked under acceleration. I pulled the engine and tore it down. All the ring end gaps were in the .050 and larger range, the cross hatch was gone from the cylinders and there was speckles of cam and lifter material embedded in the pistons. This was from a cam failure a few years back. I just found it odd that even with the numbers so close, the rings were worn out! I had it honed, the pistons glass beaded and installed file to fit rings like before. Back together, the compression numbers closed to within 6% and have stayed there since then.

 

[beanhead]

Thanks all. So..just for my own learning purposes, something like a sticking/hanging valve or pushrod that's a little bent would give a lower reading?

 

[Kern Dog]

Yeah, a camshaft lobe that is worn will as well. You will hear a bent pushrod though, at least the loose rocker arm clattering from the extra clearance. Unless the valve springs are soft, high mileage 318 ones, I don't recall it being very likely that an engine would have "sticky" valves. Sometimes there is carbon buildup around the valve head or seat making for a poor seal when the valve is closed. In some odd cases, a valve stem is bent and the valve head sits cocked in the seat.

 

[beanhead]

Good stuff thanks!