Does anyone know a formula to figure out compression? I have a 327 bored out .040 over. I have 64cc heads and the normal everyday blue fel pro head gasket which is .039 I think. It has flat top aluminum pistons. I know the 327 has a 4.00 bore and since its .040 over which makes it a 333 and 4.040 bore. Also when I have the compression guage one it, it reads 150. Does anyone know a formula to figure out exact compression using this information? Do I need to get any more info? The piston makers said it should be around 10:1 with my heads but I would like to find out for sure. Thanks

 

Oh yeah the stroke on a 327 is 3.25

 

There's no way to tell the compression ratio using a compression gauge, unfortunately.

To calculate the CR, you'll also need to know how far down the bore the piston is at TDC, and either the gasket volume or the gasket bore. I don't have the formula handy here. I'd figure it out but my brain isn't up to it right now.

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1984 Camaro Z28 Astral Silver w/ 2 1/2" cowl induction hood
Engine: Decked 385 cid 4 bolt w/ oil cooler, 9.6:1 compression, ARP fasteners, 600 cfm Performer carburetor, Torker II intake, Performer RPM Heads, XE268H cam, Magnum roller tip rockers, MSD6AL ignition, Blaster2 coil, recurved HEI distributor
Exhaust: Terminator headers, Custom 3" TIG welded stainless exhaust, Twister muffler
Transmission: 4L60 w/ Corvette servo, .500 boost valve, TransGo 7-CS clutch springs, 700 PKH pump rings, 700-2 reprogramming kit, Borg-Warner High-Energy frictions & bands, 2400 RPM lockup torque converter
Drivetrain: 2.73:1 limited-slip Dana 44 w/ disc brakes
Tires: Front P225/50VR15 Rear P265/50VR15 Eagles

Because you can always sleep in your car, but you can't drive your house.

 

That combo of parts should give about 9¾:1. That was a very common stock setup; 327s with double-hump heads were exactly what you describe. As long as nothing has been decked or milled very much, you'll be about the same as a stock one was. If you've taken much metal out the CR will be higher, 10:1 isn't unrealistic in that case.

There's no way to figure the static CR from a gauge reading, too many variables affect it, from the quality of the rings & valves to the cam profile.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Thanks, 9.75 doesn't sound too bad. With the heads I don't have the double humps any more. (they were in bad shape when I was rebuilding the motor and had the smaller 1.94, 1.5 valves so I sold them) I now have the aluminum 64cc trick flow 23 degree heads with the 2.02 1.6 valves.I haven't milled the heads yet. I can't remember if the machine shop decked the block or not. I think they did a little to make a smooth sealing surface. The rods are the stock 5.7 height. But as long as the CR ratio is between 9.75:1 and 10:1 I will be happy!

 

Oh yeah If it matters the block is from a 65 vette and has the small journal crank.

 

Hey, I've got a 350 .030", stock replacement pistons, and 882 heads with '015" decked. What do you think my comp. ratio would be?

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'82 Firebird, dead stock, 9 bolt disc rear, over 200,000 miles and still going strong, more to come...
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