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#1
05-25-2017, 01:09 PM
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Compression Calculator
Anyone have an online compression calculator they TRUST? I am putting the same specs in at Wallace Racing and Butler Performance and getting two different results. Wallace Racing says 9.2:1 and Bulter says 8.5:1. Pretty big difference.
Here's the specs: Bore: 4.16 (400 +.040) Stroke: 3.75 Heads: 92cc Valve Relief: +10ccs (Icon Pistons) Gasket Thickness: .045 (crushed) Deck Clearance: 0.00
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Jimmy www.firebirdgallery.com 2002 WS6 M6 Trans Am - original owner 1973 Trans Am 455/4-speed 1971 Formula 400/400 1969 GTO 400/400 - 3rd owner 1968 Firebird Convertible 350/200R4 POCI - AAC |
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#2
05-25-2017, 01:14 PM
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I wrote the one on the old classicalpontiac website, made it back in the '90s. People have been using it for nearly 20 years now, so I'm pretty sure it is reasonably accurate.
EDIT: Yep, ONE of those calculators has a problem! EDIT #2: I agree with the Butler calculator, I get 8.46:1, which is what I'd expect with that size head.
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'73 T/A (sold). Low budget stock headed 8.3:1 455, 222/242 116lsa .443/.435 cam. FAST Sportsman EFI, 317rwhp/385rwtq on 87 octane. 13.12 @103.2, 1.91 60'. '67 Firebird [sold], ; 11.27 @ 119.61, 7.167 @ 96.07, with UD 280/280 (108LSA/ 109 ICL)solid cam. [1.537, 7.233 @93.61, 11.46 @ 115.4 w/ old UD 288/296 108 hydraulic cam] Feb '05 HPP, home-ported "16" D-ports, dished pistons (pump gas only), 3.42 gears, 275/60 DR's, 750DP, T2, full exhaust Last edited by Lee; 05-25-2017 at 01:22 PM. |
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#3
05-25-2017, 01:29 PM
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Ok, Wallace is the outlier. Classical, Butler and Summit all say 8.46:1. Wallace has the value 6.6 in by default for valve relief (guessing due to TRW pistons being +6.6). If I change that value to 10 then I get 9.2:1 on his calculator. If I use -10 I get something over 11:1 so 10 or +10 is right. Seems his calculator is just off for some reason.
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Jimmy www.firebirdgallery.com 2002 WS6 M6 Trans Am - original owner 1973 Trans Am 455/4-speed 1971 Formula 400/400 1969 GTO 400/400 - 3rd owner 1968 Firebird Convertible 350/200R4 POCI - AAC |
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#4
05-25-2017, 03:02 PM
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I get 8.41 on the Wallace Calculator. I assume the .045 gaskets have a 4.3 bore ?
Ran it twice @ 400 + .040, 92 cc heads, 10cc reliefs, .045 x 4.3 gaskets, .000 deck height. CR = 8.41 If I don't enter anything into the gasket bore box, I get 9.19 CR. So, what did you guys enter for the gasket bore size ? Last edited by ponyakr; 05-25-2017 at 03:11 PM. |
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