924Board.org

[MikeJinCO]

If one wanted to consider putting a 931 head on a NA where would a person look for help with the mythical oddly shaped pistons required? Any suggestions on how to approach this idea? Has anyone actually done this?
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Mike


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

[ideola]

This is exactly the approach I am taking with the Cheap-A$$ long rod high compression stroker motor.

In short, there is no OEM 924 piston slug that is going to give you a desirable result in terms of CR. The 21.5cc of combustion chamber in the 931 head renders all of the OEM pistons useless for developing any serious CR. So you MUST go with custom pistons.

If you stay with stock rods, the only solution is to go with a custom piston that is modeled after the 931 piston (which is designed with appropriate compression height & quench ring), but with a much smaller dish in order to achieve a usable CR.

Once you've solved that problem, you then have to deal with the exhaust manifold. As it turns out, the aftermarket headers I sell on my site will work from a porting perspective, but will require enlarging and slotting the stud holes because the NA head uses an M8 stud that are spaced closer together than the M10 studs on the 931 head. Using the stock NA manifold can be done by modifying in the same fashion, but being cast iron is reportedly very difficult to work with.

I will have my modified crank, with off-the-shelf rods and pistons for my 931-on-a-NA setup within 7-10 days and will report back in the above linked thread as I continue to progress. The components I'm using are all stock off-the-shelf aftermarket parts. It will increase my displacement from 1984cc to just over 2100cc. It will provide ~11:1 CR with a quench distance of ~1mm (a little more than I'd like, but it was the best I could do without making custom slugs). The piston for my application has a very slight dish to it, with no valve reliefs.
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erstwhile owner of just about every 924 variant ever made

[leadfoot]

I suppose it depends on whether you are keeping it N/A or going to turbo/sc??
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...