Shaving pistons?

WydRyd · 03-02-2006, 12:49 PM

I have a mate who's considering a turbo conversion on his 964 C2.

My suggestion to him was to just use the 964 pistons, but have them shaved down to reduce the compression ratio to a more "boost friendly" level, but he's nervous about doing this and would rather buy new JE's, or go with 993TT P&C's.

I think it's a waste of money. It looks to me like the stock 964 piston has a LOT of meat on the crown, so even machining the piston until the top is nearly flat leaves a thick enough crown to cope with some serious HP.

What do you guys think?

How much would a mod like this cost by a good machine shop?

cnavarro · 03-02-2006, 05:24 PM

Someone might want to chime in on the Mahle's, but I know that that JE recommends a .250" minimum crown thickness and they said it's possible to get away with a .200" crown thickness in a pinch. This is considering forced induction and a high-strength forging, and as long as someone thinks the factory forged pistons are just as strong, I don't see why those recommendations wouldn't apply. I might go as far as to say that you might want to thermal barrier coat the tops to reduce the crown temp and further increase the strength of the piston if you want to be extra safe.

Eagledriver · 03-03-2006, 08:05 PM

It seems to me the 964 had high compression (around 11 to 1) pistons. I think you'd have to shave off too much metal to get to a turbo compression ratio (9 to 1 or less).

-Andy

Steve@Rennsport · 03-03-2006, 08:41 PM

Merv:

The stock Mahle 100mm (3.6) pistons cannot be machined enough to get the CR down below 9.5:1 safely and I'd not use such a thing in a N/A or turbocharged application.

I'd advise your mate to get some Mahle TT P/C's (he'll need to machine his heads to fit) or some JE's.

WydRyd · 03-05-2006, 10:23 PM

Interesting responses. Thanks!

Anyone know how much a set of JE's would cost to make up or this app?

cnavarro · 03-06-2006, 06:44 AM

You might want to contact Spyder Automobiles; they are in your neck of the woods so to speak and I know they offer JEs. They may even have something like that in stock.

Henry Schmidt · Henry Schmidt's Garage

I am not trying to be disagreeable but we have machined many sets of 964 pistons for use in turbo engines. We have built 3.3, 3.5 and 3.6 turbos with these pistons and never had a problem, what so ever.
We have even machined valve pocket to accommodate CTR cams in race application.
Machined to 7.5 for 3.5 application we still have .280 " dome thickness.
The 964 Mahle piston is a solid forged piston very well suited for turbo charging.
The 993 na piston is another story. These pistons are cast and offer far less quality and strength.

The engine in this car is running shaved 964 pistons in a 3.2 based 3.5 engine with 935 style rod / crank configuration.

WydRyd · 03-06-2006, 11:40 PM

Hi Henry. Thanks for your input.

I'm told if they (964 pistons) are machined approx .100" from the deck, this will yield approx 8.5:1 C/R and will easily handle north of 600+ HP.

How much boost is that car above running with the shaved pistons? What Compression ratio?

Kemo · 03-07-2006, 11:38 AM

Henry,

How much does it cost to have the pistons profesionally shaved?

WydRyd · 03-08-2006, 03:42 AM

Not much in terms of machining cost I'd think. The main labour cost would be mocking up a dummy engine to measure the C/R desired and machining the domes as required

Maybe some of the more experienced engine builders here would chime in?

Porschephd · 03-08-2006, 06:57 AM

I am with Henry on this one. The 964 piston is fine to do this with. In fact most of the RUF converted cars used the same process and they typically never had any issues.

The 993 Piston is no, no.

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally posted by Kemo
Henry,

How much does it cost to have the pistons profesionally shaved?

$20-$25 per piston.
They should also be balanced after the dome is trimmed.
Valve pockets would be an extra $90.

Kemo · 03-08-2006, 09:35 AM

Quote:

Originally posted by Henry Schmidt
$20-$25 per piston.
They should also be balanced after the dome is trimmed.
Valve pockets would be an extra $90.

so about $150 bucks per piston

thats 600 bucks to get your engine ready for boost?

that sounds like the best bang for the buck...unless lower cr pistons are cheap to buy..but then you still have to have the matched to the cylinders right?

am i missing a part of the equation here. Seems VERY reasonable if my numbers are right.

Henry, Thanks for posting, I always enjoy the stuff you present here.

Henry Schmidt · Henry Schmidt's Garage

I guess once again my communications skills are lacking clarity.
The price is $20-25 per piston to deck or shave them.
Then there is a $90 per set charge for valve pockets.
The means the price is between $35-$40 per piston.
Or $210-$240 per set of six pistons.

WydRyd · 03-08-2006, 02:23 PM

That's an extremely viable solution, instead of forking out over $1k for JE's, or MORE for a set of 993TT P&C's

Kemo · 03-09-2006, 07:47 AM

Henry,

I may hold you to that price one day

$250 bucks is about what i spent in gas last month...

now i just need a 964 engine. how much HP do you lose if you are planning to go turbo, ONE DAY, and you have the pistons shaved and run the engine with no Turbo?

WydRyd · 03-09-2006, 02:25 PM

Don't know! You'll probably be at 180-200HP on a 3.6L? But you also have to make the necessary changes to the EFI programming to account for the drop in C/R and then re-program it once you have the forced induction system installed.

A twin turbo 3.6 would be awesome though

len911 · 03-09-2006, 03:58 PM

FWIW, March Panorama (page 116, 5th ad from top) has someone selling a twing plug 964 3.6 engine minus case, crank and fan. Paul Fusco, NJ 917-378-8309. Not associated with ad, just passing info along... Lou

WydRyd · 03-09-2006, 06:56 PM

Thanks... I've dnne too much to my engine to justify buying a 3.6 any time soon. Mine is already twin plugged with GT2 CAMs and currently under the knife having twin turbo's installed

If I do ever go 3.6L, I'll get a 964 crank, bore out my spare 95mm jugs to 98mm & flame ring the whole setup