Thread: CR question on 383

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I got my 383 built again and it is running great and strong. This engine is bored .040, 5.7 rods with sealed power 0.125 domes, 76 cc heads with a comp 292 cam. It was supposed to be 10.5:1 with these pistons but a machinest told me I was running around 12:1 and would need to run 110 octane fuel. All the math I have done, I still come up with 10.5:1 with a .035 head gasket but I ran a compression test on one of the holes and it was 205 psi. I'm not sure but 205 psi seems kinda steep for a 10.5:1. What are your thoughts??

Kodiak

Originally posted by Kodiak
I got my 383 built again and it is running great and strong. This engine is bored .040, 5.7 rods with sealed power 0.125 domes, 76 cc heads with a comp 292 cam. It was supposed to be 10.5:1 with these pistons but a machinest told me I was running around 12:1 and would need to run 110 octane fuel. All the math I have done, I still come up with 10.5:1 with a .035 head gasket but I ran a compression test on one of the holes and it was 205 psi. I'm not sure but 205 psi seems kinda steep for a 10.5:1. What are your thoughts??

Kodiak

What is your piston part number?I don't see one listed for .125 dome for a 383.For a.100 dome I am showing around 11.5 with a 64 cc head.With a 76 cc head it would be more than 1 point less with a 76 cc head..

Juggling some figures around, I come up with 10.67 and yeah, 205 sounds reeeaaal healthy for that c.r. with that cam. Is the cam advanced quite a bit?? You can lower the pressure by retarding the cam a few degrees. Try retarding it 4 degrees and take another reading. What this will do is to extend the intake closing point and let some of the compression bleed off before the valve shuts. What squish figure are you running?

Sorry guys, I told you wrong, they are Speed Pro pistons. I found them in summit STL-H618CP40. The cam is advanced 4 degrees. As far as squish figuring, I didn't figure nothing, just put it together. I should have known somthing was up when I had to rebuild the starter 5 times before I ever got it started the first time............good thing I primed it good before hand. I'm just wondering if I'm trying to run too much on pump gas?

Thanks, Kodiak

If you are running a .035 gasket on an undecked block, you have approximately a .060 squish which could cause you some detonation problems. If that is the case, you may need to replace your head gasket with one that has a .015-.020 thickness.