924Board.org

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The compression 924 calculator.

http://924.terribly.fun/compression/
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe

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[Beartooth]

Hmmm, it seems I, and some previous posters in this thread, have it wrong. I was under the impression that the NA piston was closer to a flat top and the NA head was more conventional (open combustion chamber). If the NA head is anywhere near zero volume, then using turbo pistons with a NA head would increase compression (reduced combustion chamber volume). Depending on the combination, there's a good chance you'd end up with more compression than you'd want, even for an all-out NA build. So I was probably right about it not being a combination that you'd want, but wrong about compression being too low.

Actually, looking at the 924 compression calculator, the US S2 pistons gives you a healthy compression for an aggressive NA build - 10.6:1 - I'd think that's still manageable on pump gas if you were careful. For a supercharged application, I don't think any combination of NA head and 931 pistons would be wise: the lowest you could get is 9.7:1. That would only work with racing gas or AVgas, or maybe water/methanol injection. Thicker head gaskets would only buy you a half point or so at most. One thing I do notice is a lot of deck clearance (6.5mm) on the NA engines. I can't say whether you'd run into valve or head clearance issues decking the block, or if one would be interested in going through the trouble, but depending on how much room you have to play with that, you might be able to dial any compression you'd want with a 931 head and NA pistons by decking the block. I'm just speculating of course; it could be you don't actually have that much piston-to-head or piston-to-valve clearance. You'd also have timing issues to sort through. But for someone looking to build a motor on the cheap with a good NA bottom end and 931 head, it might be a possibility.
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1980 931 diamond in the rough

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The NA pistons are pretty close to flat with small relives for valves, the head is compleatly flat.
The Turbo pistons have a small dome if I remember correctly, so I'm not sure they would be compatable with a NA head anyway.

I run 1 bar of boost with the stock 9.3:1 on euro 98 octane pump gas. If you run e85 I think you can boost until the pistons leave the block....
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe