This is the info ive gathered on my 383 short block. I need to know the compression ratio so that I can get it tuned right. It says 10.8:1 with 64cc heads but I have 55cc heads with Fel Pro 1074 head gaskets. I tried using the calulator but I come up with 12.8:1 That doesn't make sense.

JE/SRP Forged Pistons Flat Top +.030 1.125 C.H. 10.8:1 with 64cc head.
EAGLE SIR 5140 FORGED STEEL RODS6.0 Full Floating Pin
Eagle CAST STEEL/ NODULAR IRON 3.75" CRANK 1 PC REAR MAIN SEAL
INTERNAL BALANCE
Scat or Eagle CAST Crank 3.75" stroke.
2.1" journals 1rear main internal Balance
SET OF (8) 5140 lightweight I beam rods 6.0" length, 2.1 ROD JOURNAL .927 PIN floating pin design
FROM EAGLE SIR6000BBLW
SRP Forged 4.030 or 4.040 bore flat top pistons

 

Well, that CR could easily be right, depending on the parts. Are you considering deck height, piston valve relief volume, etc? If you are using zero for both in the calculator, but are actually at say .025 inch piston-to-deck, and 5cc valve reliefs, that will drop the CR. Please start by giving us ALL the values asked for by the calculator, and then we can start to figure out what is going on. How thick is that gasket?

As a comparison, my 383 at 0.038 piston-to-head (which is deck height + gasket thickness) with flattops & 5cc valve reliefs will be close to 12.5:1. With zero-decked block that combo would be about 12.9:1, so your 12.8 might not be far off.

 

The gasket is 0.039 in. thick. The reason I don't have any info on the pistons and deck height is because I can no longer reach the engine builder. The pistons are likely to have 5cc releifs in them but as far as the deck is concerned I have no idea.

 

Try putting the piston volume in as a negative and a postitive value. That can throw everything off fast.

You also have to take into account how far the piston is in the hole and how much gasket you have, that can easily sway the compression ratio a point.

Bret

 

Thanks Bret. I took 5cc's off the piston and came up with 12.0:1 with 0.010 as deck height and the .039 gasket. I have a piston at TDC right now and will attempt to measure deck height.

 

You gotta fool around a lot to find TDC, so be carefull that you actually have it.

Bret

 

I got it at TDC for sure and the piston is so even with the deck that I can barely feel a depression between the piston and deck. Maybe its zero deck and if not its probably really close.

 

It appears that 12.0:1 is what im looking at. Should be fine on 93 octane

 

depending on your cam, you might have been just groovy with the higher SCR. Did you calculate DCR?