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jpnovak jpnovak is offline
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Join Date: Apr 2000
Location: Austin, TX
Posts: 7,984
Maximum cam on stock pistons

I am currently assembling a 3.3T motor for a friend. You have probably all seen the thread on the build. This is a full build with ported heads, twin plug, EFI conversion, 3.2 manifold, 993 HEs, K27-7200HFS (?) turbo, internal coatings, etc.

After finishing a dry build of the top-end I found I have a piston-valve clearance issue between the stock flat top 97mm pistons and the GT2-EVO cam. Not surprising. There are several ways to solve this problem. Each has advantages and disadvantages.

Shim the cylinders (bad for deckheight)
Valve pockets in the pistons (Crowns too thin)
New pistons with bigger valve pockets (JE)
3.4 98mm pistons and cylinders ($$$$)

and lastly...

Install less cam. Yes, this is the easiest solution but not necessarily the best for the intended application. The car will be used primarily for track events spending significant time on boost and at high rpm. The GT2 cam was chosen to meet these rpm demands and match the characteristic peak TQ and HP drop-off profile to the driving style. Absolute numbers of peak power are not a concern. The character of the engine is the concern. smooth power delivery is a major factor in the combination of components used for this build.

Having this much time and effort (and $$) in the build means it should be done correctly. This is why the attention to detail on clearance is so important.

The SC cam is not enough. Its high rpm characteristics are not what we are looking for.

the 964 cam is a possibility. Will it clear? Can someone tell me numbers of valve clearance complete with what deckheight and base shim height are required?

Has anyone had a custom grind? Maybe the lift of a SC cam with more duration? Maybe the duration of a 964 grind with a little more lift? Maybe the lift of a 964 with more duration? I am sure I could come up with a reasonable cam grind. Right now, changing cams might be the best way to solve the problem.

Please post specifics of your build if possible. The more information the better.
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Old 11-05-2010, 07:42 AM
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