Weston Dillard

Good questions gentlemen. My car runs 968 pistons with shorter Pauter rods to get down to 8:1. It has the 89 Turbo head that came with the car with modified cooling passages. If I had it to do over again, I'd probably go with the 968 turbo pistons and stock rod length, but live and learn. It is standard bore so that may still be in my future. It has a lightened crank and flywheel, no balance shafts and uses a stock barn door AFM. We run 16 PSI. We do most of our events at TWS which includes a very hard pull in 3rd, 4th and a bit of 5th every 2 minutes, frequently in a lot of heat. I break stuff with some regularity, but usually not the motor.

When we got it it was all shiney and looked just like the car in Jim's picture (minus the IC duct). Not any more.

West

hosrom_951

Problem is, the alusil blocks require very special equippment to bore and hone accordingly, and not many people do that. Infact, no one in my area does that at all.

Secondly, those oversize 100.50mm or 104mm and 104.5mm low compression turbo pistons are becoming more and more hard to find and expensive. I don't know if JE has any pistons that size for low compression and compatable with alusil blocks (and iron coated).

The new Cayenne V8's are manufactured by Kolbenschmit, the same company that made the blocks for the 924, 944, 968 and some 928 engines. And yes, the Cayenne uses alusil (that is what i figured out from KS)so it should be easier to find shops to hone alusil, since it is used on a mass production block now.

I had a slight, hairline score in cylinder 3 on my block, i have a complete 88NA block with perfect bores. If i did not have that NA block, i would probably look for someone who could install dry sleeves, since i know they have been used a lot in 944 turbos (rage2 for example) and survived a lot of beating without issues.

Unless someone could come up with alusil sleeves to bore and press into the 944 blocks, that would be nice

333pg333

What sort of things are you breaking West? Glad to hear it's not major components.

Weston Dillard

Clutch, CVs, wheels, rear wheel bearings, transmission, the occasional head gasket (we run pump gas). Nothing surprising on a car that sees so much track use.

West

lejams

Thanks West, very useful info. What is the problem with the shorter Pauters if the engine isn't breaking? You are exactly right about the 968 8:1 pistons, I just bought the last two sets from Andial. One for my new build and one for - I don't no - they were the last ones. Mahle will never make these again is what I do know.

lejams

Mercedes use Alusil widely, so anyone who does Mercedes blocks can do Alusil.

The pistons I just mentioned are the 104.5's. The 104.0's 8:1 already sold out too. That's the end of them.

Weston Dillard

It may be putting too fine a point on it for a motor that makes less than 400HP, but in general short rods cause bad rod angles and the fact that my pistons run lower in the bores means I give up any quench area in the cylinder.

The pistons you got are the ones to have, Jim! Proper compression ratio, rod length and combustion chamber shape.

West

lejams

Glad you're telling me I'm in the right direction. I'm using Carrillo's with the Mahles & my 89 head as you have. Turbo is the remaining selection

azmi951

the short rod exacerbates detonation issues because it has poor quench. the quench makes the charge turbulent and in turn mixes the charge. During that mixing hot spots are cooled and one region of the charge is not left to transfer heat from the chamber surface for as long.

Quench is good. I know on a 2 valve V8 there is a spec of much less than .100 for acceptable quench clearance. The clearance also widens as RPM increases because of rod and piston stretch.

Olli Snellman

Hos,
actually there is nothing special in that process. I have a machine shop located just 2 km’s from my house who can do it.

I have not yet received pistons, so I can’t say for sure. I will let my machine shop inspect the pistons and prepare block according to them

lejams

I didn't realize that shorter rods in West's case were contributing to reduce quality of combustion that much. Have read that properly designed squish is like free octane.

Chris White

These have been around for a while.

Chris White

Wossner recommends .08mm for clearence (at least on the 3 or 4 sets I have).

Chris White

BTW – I am driving a 944 with a new 2.7 MID sleeved engine (JE pistons) today – running it in and tuning it.
Runs fine, no leaks (!), it makes a slight piston noise for 60 seconds (normal at 50 degrees ambient temp) and runs like a top.

azmi951

I would think piston noise is nothing to be too concerned about in itself except it should be an indicator not to run it hard when cold because of the excessive skirt clearance. I have had forged pistons in my V8s and I know that I just have to let them warm up before flogging them.