Hi guys, I have been a little confused about what my compression ratio is sitting at. I just built a 383 using a scat Stroker crank, truck rods, and .030 over flat top pistons. I'm using edelbrock perf rpm heads (70 cc combustion Chambers) . The online calculator I tried using said my cr was like 12 to 1, and that seemed high to me. Can one of you sort me out please? Thanks very much.

 

Sounds about right. If the online calculator you were using asked for head gasket thickness, and deck height in addition to all the normal bore/stroke stuff, I'd say it probably figured it about right.

If not, that would lead to a higher than expected number

Edit: Here's the one I used: http://www.rbracing-rsr.com/compstaticcalc.html

 

"Flat top" pistons covers ALOT of ground.

The single most critical numner is left out of your description. That number is the "deck clearance", which is, how far "down in the hole" the pistons are, at TDC. Stock, that number is .025", -.000" +.015" or so, with potentially over .010" variation from least to most within the engine due to variations in stroke, block machining, and rod length. Virtually ALL cast pistons, MOST hypereutectics, and MANY forged pistons (including SpeedPro/TRW) ADD .020" to that; meaning your pistons are probably around .050" "DITH" at TDC.

Typical "flat tops" w 4 valve reliefs have about 6cc of "dish" volume.

To give you some idea of how much difference the deck clearance makes, a 383 w 6cc valve reliefs, .039" gaskets (FelPro 1010 for example), 64cc heads, and .000" deck clearance, works out to right at 11:1. With the stock .025" of deck clearance it's 10.4:1, and with .050" it's 9.8:1. With 70cc heads, that last one goes to 9.3:1. I'd guess that AT THE MOST your CR is in the 9½:1 neighborhood; perfectly fine for aluminum heads and pump gas. Maybe even a little low.

Note that I only quoted the CR out to 1 decimal place... there are SO MANY uncertainties in "calculating" it, that it's IMPOSSIBLE to be any more precise than that, without MEASURING EVERYTHING. Particularly, the head volume can easily vary ± 2cc from "nominal". Any time you see anybody quoting their CR out to 2 decimal places and telling you all about "I used so-and-so 'calculator' so it's got to be right", you can be QUITE CERTAIN that they don't have a clue what they're talking about; so if anyone does that, thank them for their concern but politely blow them off.

 

haha, thank you so much for that, you're always full of great information. I didn't know about the DITH measurement, or how to measure the valve reliefs. thanks for clarifying, and I think you're right in that my motor is about at 9.5, or 9.75 to 1. Thanks again, now on to carb and power adder questions. thanks again.

 

Measure whatever you can; that way you'll know what to expect. Deck clearance most especially, since all that takes, is a straightedge and a feeler gauge.

Personally I'd shoot for low 10s if I was building a typical street motor w aluminum heads, or mid 9s w iron. But, a tenth or 2 of a point here or there isn't such a big deal as far as a successful build, and the "success" curve looks like a gentle rise as the CR increases, to a levelling-off at "optimum" which is usually a fairly wide range anyway, to FALLING OFF A CLIFF as it's exceeded. If I had to choose between accidentally giving up a coupla HP as compared to having to put up with a pinging, detonating, premium-gas-sucking, have-to-run-retarded-timing (and not far enough BTDC besides) pig, I know which way I'd go.