Compression Ratio Help
Compression Ratio Help Trying to help my son build his first stroker .
I talk with Texas Speed and they said to run a -3cc piston with a 67cc head and .051 gasket for a 11.45 compression ratio.
So I ordered up the rotating assembly from them and a set of Trick flow heads from Total Engine Airflow @ 67cc .
I have been trying to use a online calculator to see what differences a .040 head gasket would make to tighten up the quench . The C/R numbers keep coming up over 12/1 ratio .
.040 = 12.75/1
.051 = 12.38/1
What am I doing wrong ? TIA !!
I talk with Texas Speed and they said to run a -3cc piston with a 67cc head and .051 gasket for a 11.45 compression ratio.
So I ordered up the rotating assembly from them and a set of Trick flow heads from Total Engine Airflow @ 67cc .
I have been trying to use a online calculator to see what differences a .040 head gasket would make to tighten up the quench . The C/R numbers keep coming up over 12/1 ratio .
.040 = 12.75/1
.051 = 12.38/1
What am I doing wrong ? TIA !!
TECH Senior Member
Joined: Jun 2016
Posts: 22,306
Likes: 3,621
From: Central Cal.
You might be figuring the piston reliefs the wrong way, calculating them as "domes" rather than recesses. A -3cc piston has reliefs (as per TSP's spec), and a 3cc piston has a "dome", raising your compression beyond what TSP spec'd to you.
Bore: (diameter) 4.070
Stroke: 4
Cylinder Head Volume: 67
Effective Dome Volume:
Use (-) for Dome and (+) for Dish. -3
Deck Clearance: 0
Compressed Gasket Thickness: .051
Number of Cylinders: 8
CALCULATE
Compression Ratio : 12.38 : 1
Total Displacement (in.3) : 416.32
Stroke: 4
Cylinder Head Volume: 67
Effective Dome Volume:
Use (-) for Dome and (+) for Dish. -3
Deck Clearance: 0
Compressed Gasket Thickness: .051
Number of Cylinders: 8
CALCULATE
Compression Ratio : 12.38 : 1
Total Displacement (in.3) : 416.32
Last edited by 1973 STEP A SIDE; Apr 1, 2017 at 04:57 PM.
I found this
Wiseco Chevy LS Series -3cc Dome 4.070 inch Bore Piston Shelf Stock Kit - K464X7Comp Ratio @ 65CC: 11.8:1Comp Ratio @ 70CC: 11.2:1Comp Ratio @ 68CC: 11.4:1Comp Ratio @ 67CC: 11.6:1
Rod: 6.098 / 6.125 ??? I have 6.125 rods .
Wiseco Chevy LS Series -3cc Dome 4.070 inch Bore Piston Shelf Stock Kit - K464X7Comp Ratio @ 65CC: 11.8:1Comp Ratio @ 70CC: 11.2:1Comp Ratio @ 68CC: 11.4:1Comp Ratio @ 67CC: 11.6:1
Rod: 6.098 / 6.125 ??? I have 6.125 rods .
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Staging Lane
Joined: Feb 2017
Posts: 63
Likes: 14
From: Leola PA
Like G said, use a positive 3 for your valve reliefs and you should get something around the 11.4 spec. Don't worry about the .040 gasket raising the compression, the tighter quench will make the 11.7ish compression less detonation prone than the 11.4 with a bigger quench. You'll be fine on 93, mine is with 11.8, no timing retard. 91 might have the PCM pulling some timing at times. You do have knock sensors don't you? I used Felpro .041 MLS gaskets on my build, .036 quench cold with aluminum block. Iron block, you'll want more like .040.
Like G said, use a positive 3 for your valve reliefs and you should get something around the 11.4 spec. Don't worry about the .040 gasket raising the compression, the tighter quench will make the 11.7ish compression less detonation prone than the 11.4 with a bigger quench. You'll be fine on 93, mine is with 11.8, no timing retard. 91 might have the PCM pulling some timing at times. You do have knock sensors don't you? I used Felpro .041 MLS gaskets on my build, .036 quench cold with aluminum block. Iron block, you'll want more like .040.
As far as knock sensors yes we will be running them .
Builder wants me to run the MSD Atomic efi #2950 not sure about knock sensors and that system .
http://www.csgnetwork.com/compcalc.html
NA keep your quench tight it helps to control detonation. Last engine on 91 pump gas we ran .037 quench will 11.6 compression shifted at 7K never had any issues. Tuner will play a big roll.
NA keep your quench tight it helps to control detonation. Last engine on 91 pump gas we ran .037 quench will 11.6 compression shifted at 7K never had any issues. Tuner will play a big roll.
http://www.csgnetwork.com/compcalc.html
NA keep your quench tight it helps to control detonation. Last engine on 91 pump gas we ran .037 quench will 11.6 compression shifted at 7K never had any issues. Tuner will play a big roll.
NA keep your quench tight it helps to control detonation. Last engine on 91 pump gas we ran .037 quench will 11.6 compression shifted at 7K never had any issues. Tuner will play a big roll.
TECH Addict
Joined: Aug 2005
Posts: 2,305
Likes: 126
From: Monroe,NY
Bore: (diameter) 4.070
Stroke: 4
Cylinder Head Volume: 67
Effective Dome Volume:
Use (-) for Dome and (+) for Dish. -3
Deck Clearance: 0
Compressed Gasket Thickness: .051
Number of Cylinders: 8
CALCULATE
Compression Ratio : 12.38 : 1
Total Displacement (in.3) : 416.32
Stroke: 4
Cylinder Head Volume: 67
Effective Dome Volume:
Use (-) for Dome and (+) for Dish. -3
Deck Clearance: 0
Compressed Gasket Thickness: .051
Number of Cylinders: 8
CALCULATE
Compression Ratio : 12.38 : 1
Total Displacement (in.3) : 416.32
Staging Lane
Joined: Feb 2017
Posts: 63
Likes: 14
From: Leola PA
No, the piston has a slight domes to help offset the deep valve reliefs. If they hadn't put those slight domes on the piston it would probably be listed as a -6cc. It's listed as a -3cc piston because that's the NET negative volume, valve relief volume minus the smaller dome volume. You must use the +3 in your calculation. It's confusing because the piston has reliefs and slight "domes" to offset the deep reliefs. It's a net 3cc "hole" in your piston. I used the Wiseco -3cc pistons in my 427 and was surprised to see the little "domes" on a -3cc piston.