Any guesses?

[1969GTS]

I’m not sure if there is a formula for psi to compression ratio but was wondering if these numbers are close for a compression test on a 340 with X heads. I did not build this engine and don’t think either of the two previous owners ever got it running. It seems to run very strong and obviously has a very lumpy cam. If any of you can remember what your compression tests yielded would you say this is a stock 10.5:1 compression ratio?

#1) 140 psi
#2) 150 psi
#3) 140 psi
#4) 140 psi
#5) 145 psi
#6) 140 psi
#7) 140 psi
#8) 145 psi

Thanks for you opinions

 

[ramcharger]

The bigger the cam, the less cranking pressure there is, so it's a total crapshoot without knowing what duration cam you're running.

Generally, you can get away with 180-185 psi on premium pump gas. Keep in mind that the measured (real) compression ratio is around a full point less than the advertised or book ratio due to manufacturing tolerances.

I'd say it's most likely about a 9.5 comp ratio with a cam in the high 220's @ .050 lift.

 

[moper]

What he said...

 

[dusterdon]

A lot depends on the camshaft. I had a 440 that was an actual 10:1 with a comp cams 292-.501 hyd cam that was 165-170psi. This was with the engine warm, which will be higher than if the engine was cold.

 

[Longgone]

My stock 68 340 with A/T and stock cam measures around 175 psi per cylinder.

 

[MOPARJ]

For example:

My 318 with stock heads (3 angle valve job), Speed Pro 8:5:1 replacment pistons and Comp XE268 cam (.477/.480 lift) averages between 129-139 on all of the cylinders.

It really depends on the actual compression produced by the piston choice and the cam's profile. I get away just perfectly runnin 87-89 fuel with my 318 and these cranking numbers and it is extremely peppy for a 318. I run a lot of intial timing (15 degrees) and 36 total and it runs moderately efficient.

I am probably going to have to say that it looks like stock compression pistons, but it depends on how "lumpy" the cam is and its specs.

 

[1969GTS]

Thanks All, I'm going to do a wet compression test to make sure the rings are not on their last legs as some of your numbers are a fair bit higher than mine. The engine seems to run well though.

 

[fishy68]

Those numbers aren't much lower than my 10 to 1 360 with a pretty lumpy cam. Mine runs 152-158. When I had a cam that was 4 degrees smaller at .050" the compression was 8-10 lbs more. Just goes to show how a cam can affect things.

 

[1969GTS]

Those numbers aren't much lower than my 10 to 1 360 with a pretty lumpy cam. Mine runs 152-158. When I had a cam that was 4 degrees smaller at .050" the compression was 8-10 lbs more. Just goes to show how a cam can affect things.

I just did a wet test and the numbers averaged 10 psi more. I would say thats nothing shocking with a shot of straight 30 weight in each cylinder. I'm thinking they are all close with a wet and dry compression test so I'm not too concerned. Especially as I cant get it to hook up with 10 1/2 inch slicks. I'm guessing the engine is fairly solid. Now I just have to down size my jets on the 750 demon it came with as its RICH as hell.

Thanks