Big block compression numbers

[Fixable]

I did a compression check the other day on my 1967 Coronet and the numbers were not the best. I think its had a rebuild not too long ago but I do not know the specs, it has no blowby and runs great. 383, 906 heads, stock valvetrain, great sounding cam. Adding oil into the cylinders did not seem to change the numbers at all. Any thoughts?
psi and cylinder
145-8 7-110
140-6 5-140
148-4 3-150
135-2 1-120

 

[Daves69]

I think I'd retest 1 & 7 after a good highway run.
Found this. Maybe you can make out the numbers. 2bbl and 4bbl spec in there.
http://farm9.staticflickr.com/8046/8143666019_b3e203bc6b_z.jpg

 

[khryslerkid]

Carb plates blocked open when doing the test?

 

[wsutard]

What was the symptom that caused you to want to compression check your engine? I cant imagine you were just doing it for funsies.

 

[lewtot184]

do a leak down test on the low cylinders to see if it's a ring or valve issue.

 

[Fixable]

Edelbrock RPM air gap intake manifold and a Holley 750 on top.
Throttle blades we open for the test.
No symptoms, sadly enough I just wanted to check it.
I did as leakdown as well and took the rocker assy off but the tester was shot.

 

[moper]

The numbers are low, but more importantly they are spread all over. If adding oil made no change at all it's more than ring seal that's the problem. What cam is it, or do you know? a performance 383 is harder to get "right" than some others. Typically they are soft as your numbers would indicate.

 

[Fixable]

Exactly, the fact that the numbers are all over the place is what has me concerned. I do not know any internal specs but has a nice lope to it, still pulls 13 inches of vacuum at idle in neutral.

 

[not so famous bob]

Exactly, the fact that the numbers are all over the place is what has me concerned. I do not know any internal specs but has a nice lope to it, still pulls 13 inches of vacuum at idle in neutral.

I remember doing a test on my first hemi when I was going to switch to superstock. It had 205 to 210 all the way thru, after switching to cut down tent roof pistons, and going to a 785 roller, it had 160 ish all the way thru. The cam / the cam / the cam !

 

[lewtot184]

I wouldn't call those numbers "all over the place". you just need to look at the 2 that are down. is the vacuum gauge steady at 13" or is there some fluctuation?

 

[Fixable]

Thanks. The gauge dances around quite a bit. Ive had a few vehicles in the past that the vac stayed dead steady, this is not one of them lol.

 

[lewtot184]

Thanks. The gauge dances around quite a bit. Ive had a few vehicles in the past that the vac stayed dead steady, this is not one of them lol.

i'd look at valve issues.

 

[moper]

Many things will cause the "lope". ON a good running engine you can take off a plug wire or two...lol. Yours is not running well. Hence the "lope". You need ot perform a leak down. Bad rings or poorly sealing valves will show that way. I'd also suggest you pull a head and do some investigation to find what pistons it has, where they are in the bore in relation to the deck, and measure the cam to get an idea of what it is. IMO you have a sick engine.

 

[lewtot184]

383's with stock flat top pistons can get into piston to valve issues quick. higher lift than stock cams, increased duration, high pre-load on the tappet, weak springs, valve float all contribute to piston the valve contact. doesn't take much to mess up an intake valve.

 

[69L48Z27]

Search for vacuum readouts. Here's one that I found. Give symptoms of engine based on what vacuum does.

 

[Fixable]

Thanks for the input all. I was already thinking about resealing the engine over the winter, guess this gives me more reasons to pull it.

 

[Garys1969RR]

Did you ever get your engine rebuilt? Or get the compression numbers up, and more even with each other ? I once had a high-compression 451 that would produce 220 PSI on a compression test. Lowest it was 205