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Shuie Shuie is offline
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Join Date: Feb 2003
Location: ....down Highway 61
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Iím not an expert on this stuff at all. Hopefully nothing Iím about to type is too far off base.


The 98mm pistons are available in two different piston profiles. We'll call them RSR and Wedge Domed, a.k.a 'Max Moritz' pistons. Each piston profile can have a high (10.3:1) and lower (9.8:1) compression ratio.

The 9.8:1 wedge domed pistons were made as a direct bolt on upgrade to the CIS SC engines. These are great for single plug, but not so great for hot cams.

Both versions of the RSR pistons should be used with twin ignition since the shape of the piston splits the combustion chamber into two sections on the compression stroke. These pistons have large valve pockets, so they donít require any extra machining for valve clearance with hotter cams.

Heres a pic of the wedge domed Max Moritz pistons on the left and rounded domed RSR pistons on the right


Your cam choice will be limited by your piston and induction choices. Cams with high valve lift and long durations will obviously not work with pistons that donít have the proper valve clearance. Cams with closer lobe centers (high overlap) will not work with a common plenum intakes like on CIS due to fuel reversion issues.

Don't focus on one component of the engine. You have to figure out what kind of motor you want/need and then pick an induction system, a piston, and a cam that will work well together.

Between the four different versions of the 98mm pistons, dozens of camshafts, and four different induction systems, there are tons of recipes for 3.2 short stroke engines. Here are my three favorites


CIS induction
98mm wedge domed (Max Moritz) pistons 9.8:1
964 or 20/21 cams
single plug

while you could certainly run a different induction system, you may not gain a lot of performance due to cams and pistons on this motor
substitute the high compression Max Moritz pistons and twin ignition for more performance, but do not try to run a hotter cam with CIS or without machining the pistons for valve clearance


40mm Webers, MFI, ITB
GE40 or S cams
98mm wedge domed (Max Moritz) pistons 9.8:1, relief cut for valve clearance with the cams
single plug

substitute high compression Max Moritz pistons and twin ignition for more performance. The limitation here is the pistons since they need to be machined for valve clearance with these cams. Forget about CIS with anything hotter than a 20/21 or 964 cam

Thai Hot:

MFI, 46mm Webers, or ITB
98mm RSR high compression pistons, 10.5:1
GE80, sprint, 906, or hotter cams
twin ignition

Last edited by Shuie; 12-31-2004 at 07:54 PM..
Old 12-31-2004, 07:18 PM
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