Boring 97mm turbo cylinders to 98mm?

[911TT33]

Hi all,

Is it possible to bore out the Mahle 97mm cylinders to 98mm for 3.4L capacity?

Thinking of possibly doing this and slipping in some aftermarket 8.0:1 forged pistons.

This is for my twin turbo 3.3 EFI street car. I'd like to do a top end refresh and am thinking of bumping the C/R up to 8.0:1 to make it a bit more street friendly. Boost will be limited to 1.0bar.

Advice appreciated.

[JFairman]

It's physically possible but with the displacement increase and higher compression, heat in the heads and cylinders will increase.

The 3.3 cylinders have no cooling fins on the top side and boring makes the cylinder wall thinner so heat transfer is reduced. Depending on how hard you drive it they will run hotter or too hot and I've been told because of that they won't last long.

Thats why it's much better to buy a set of used fully finned 95mm bore 3.2 carrera cylinders and bore/plate/hone them to 98mm to increase displacement to 3.4 liters and install forged pistons.
You see matched used cylinders sell for around $400 a set on the classifieds and ebay once in a while and EBS, Used Porsche Parts : Porsche 911 : 924 : 928 : 944 : 964 : 993 : 996 : Porsche Engine Parts : EBS Racing Inc. tries to keep some used ones in stock for a little more.

Millenium Technologies, Cylinder Repair, Head Repair, Cylinder Replating Services by Millennium Technologies and US Chrome are the places to have the cylinders bored, nicasil plated, and honed to perfectly fit your pistons.
It's best to send one of your new pistons with the cylinders to the machine shop boring the cylinders so they can measure it and bore them to fit perfect.

[911TT33]

Thanks JFairman!

[mikedsilva]

Hope it's ok to rebirth this old thread- more info in one place.

If the turbo cylinders didn't have fins for cooling, why not? You'd think they would require more cooling than a naturally aspirated car?

Here's my situation: I have a 3.0SC with 97mm pistons and turbo cylinders already. The plan is to convert to turbo and efi.
Also, cos I like to rebuild engines, I'm contemplating upgrading to 98mm pistons and to use a 3.2 crankshaft that I already own, to get to 3.367 litres (or 3.4 as people call it).

Question - am I really better off to source cylinders from a 3.2 to complete this?
Goal is to use a lower boost setup to allow running higher compression, to aid off boost driving.

But back to the beginning of my post; I'd really like to understand why the turbo cylinders don't have fins on the top.. does the lack of fins provide less on an obnstruction and hence the air passes through the cylinders at a higher velocity which in turn, aids cooling?

People say the finned cylinders dissipate heat better... seems weird that Porsche would fit a "less effective" cooled cylinder for the turbo??

[Jeff Alton]

Mike,

Turbo cylinders were finned on the bottom/lower side only in an attempt to balance the temp across the cylinder. I can't offer any opinion on if it worked or not...

Cheers

[faapgar]

I am sure the engineers as Porsche are pretty smart.First these were for a boosted motor.Going to 98mm should not be a problem.Just use my new old piston design which locates the firing charge in the center of the piston and reduces heat to cylinder head gasket.Add the Supertec 906 style cylinder air baffles and life is good.The engineers designed them for one reason.To pull heat away from the heads.Pretty smart those guys.Also 97mm turbo cylinders are readily available and I have used them.I can not see the difference in engine life with a proper tune.Fred

[mikedsilva]

Originally Posted by faapgar

I am sure the engineers as Porsche are pretty smart.First these were for a boosted motor.Going to 98mm should not be a problem.Just use my new old piston design which locates the firing charge in the center of the piston and reduces heat to cylinder head gasket.Add the Supertec 906 style cylinder air baffles and life is good.The engineers designed them for one reason.To pull heat away from the heads.Pretty smart those guys.Also 97mm turbo cylinders are readily available and I have used them.I can not see the difference in engine life with a proper tune.Fred

Hi Fred
I'm still a novice in the scheme of things... I'm unsure about the sharpish looking edges on your pistons....

[Flat6pac]

The 3.3 C2 turbos were fully finned
The turbo cylinder dividers are slightly different hold cooling air longer around the bottom of the cylinder.
Bruce

[Ken911]

3.2 cylinders are actually pretty hard to get used that are still good. They tend to be oval and will not stay round unless cryo treated before boring. Sent mine to L&N engineering to get all that done several years back. The company that does the boring is millenial but L&N specs the finish different and you can get the cylinders individually match to the pistons for the same price by sending them to L&N. JE sells 98mm pistons in 8 to 1 compression.