924Board.org

[Fifty50Plus]

You can run 931 turbo pistons with an NA head but it becomes an interference assembly. It gives you a tad over 11:1 compression ratio but on an overrev, it will bend all your exhaust valves. Screw up a downshift and you are doing head work.
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car

[MikeJinCO]

I had close to 40 hours on my setup of 931S2 pistons on a NA head before I lost the car in a fire. Motor was pretty tired at that point due to initially running without an air filter. I cut the valve pockets on a CNC router and had made several practice runs on old NA pistons. I initially checked the clearance using clay and I don't recall if a head gasket or not and the pistons got close to the valves, hence the pockets which I probably cut too deep. According to several vintage race motor builder people the problem is with the exhaust valves on overlap and even with better valve springs the valves are bouncing off the seats at higher racing rpms.

That motor with 45mm Webers, header, and Elgin "hot rod" cam of shorter duration than the current D production cam was a whole new deal. Pulled hard from 4000 to my red line of 6500 due to having old stock rod bolts. Valve springs were some I had gotten from Britain ---, quite a bit more pressure than stock, and could never find anything close.
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Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)