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I would rather be driving
 
jpnovak's Avatar
 
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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Well that's depressing!

As an experiment, I was hoping to slide in the GT2 cams with my stock P&C's before I do a complete engine build, but it looks like what the internet sources stated is true. Thanks for posting the info.

As far as 964 cams go, I have them in my engine with stock partial fin P&C's and they work fine. Can't tell you what the deck height, base shim, or cam timing is because these were in the car when I bought it, The PO had just done a full engine rebuild with all new parts, and P&C's.

I can tell you from the receipts I have, they used stock valve springs, if that's worth anything.
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1986 911 Turbo
3.3L, K27HFS, Tial 46mm, TurboKraft Intercooler, 964 Cams, Monty Muffler, MS3Pro Evo, M&W Ignition, Zietronix WBO2 Data Logger, Wevo shifter, coupler and motor mounts.
Old 11-06-2010, 09:42 AM
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OFF THE BOOST PIPE NOW...
 
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Join Date: Oct 2003
Location: Mount Pleasant, South Carolina
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I have the 964 cams in my car with the fully finned 3.3 P&C's. No problems to date.
Old 11-06-2010, 09:53 AM
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Quote:
Originally Posted by A930Rocket View Post
I have the 964 cams in my car with the fully finned 3.3 P&C's. No problems to date.
A popular cam from the early builds with the lower lift profile and much longer duration are the group B cams that were designed for turbo racing because of the restrictions to the class rules regarding the combustion chambers at that time, may provide just what you are looking for if piston clearance and upper RPM performance is what you want. The group B ones in my car ( built by kremer in 1984 ) were the ones they chose. Terrible running below 3000 RPM but really preform nicely in the upper RPM. I have cars ( and have had them ) with 964, SC and numerous other grind spec and they do preform very nicly, depending upon the usage intended. However the only one that compares from my experience with the grinds available that I have had, for upper end grunt, is a special Andial 993 twin turbo race grind from the late 90s that is in a 3.2 liter single turbo, so lots of fuel mixture from a 3.6 liter designed cam being feed into the combustion chambers. Also went thru the Evo 2 ( a cam also designed for a larger liter motor) in a 3.4 twin turbo track car build, which is not finished yet so do not know how that will go, but but the 7.25 compression 3.4 JE pistons were also selected regarding clearence issues. We also went for the lower compression pistons as to adjust the effective compression ratio with the boost adjustment instead of a fixed higher compression ratio piston.
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Last edited by voitureltd; 11-07-2010 at 11:45 AM..
Old 11-06-2010, 11:56 AM
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Good idea on the Group B's.

I do not believe lift is really what lets these motor's breath unless the ports are opened up a bunch (like 39mm). It is more the cam timing and duration.

Has anyone else had the motor apart before?. You might CC the heads? In the past it was popularized in Bruce Anderson's book to take 1mm off the heads to bump the compression a half a point. This still worked with SC cams. If this was done it would be costing you some clearance.

Also, have you put better rods in it. The stock rods should not be used much past about 6700rpm.

After the Group B cams my vote would be for new light weight J&E turbo Pistons with a little more compression. This might help make up for some of a big cams loss of effective CR and allow a safer margin at RPM.

Not an expert, must my thoughts.

Old 11-07-2010, 08:19 AM
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