reducing 944S2 compression ratio
06-19-2003, 01:44 PM
#1
Rennlist Member
Thread Starter
reducing 944S2 compression ratio
Looking for opinions from folks with direct experience of turbocharging 3 liter 16V engines (in this case, my 91 S2). This will NOT end being a 500 HP grenade engine, the goal will be an occasional DE event street car with a modest increase over typical 951 power, perhaps into the 300-350 HP range. This shouldn't be too tough with a 3 liter turbo.
In reducing the compression ratio, new piston$ are of course my (technically, not financially...) preferred route.
Main issue here: opinions seem to vary about how much wiggle room is available in the stock S2 pistons to dish/machine them down to a reasonable CR but with adequate piston thickness remaining.
Otherwise, this particular car seems to have a very solid bottom end; no running problems and the original rod bearings had virtually no wear. The head will be undergoing a full P&P/rebuild. Will address intake manifold & turbo plumbing and selection along the way, of course.
Other questions: who has the best (if any) wide-fire-style head gasket for this 3 liter application - Powerhaus, etc.?
Necessity of Carillo/Pauter etc. con rods?
Regards Al
In reducing the compression ratio, new piston$ are of course my (technically, not financially...) preferred route.
Main issue here: opinions seem to vary about how much wiggle room is available in the stock S2 pistons to dish/machine them down to a reasonable CR but with adequate piston thickness remaining.
Otherwise, this particular car seems to have a very solid bottom end; no running problems and the original rod bearings had virtually no wear. The head will be undergoing a full P&P/rebuild. Will address intake manifold & turbo plumbing and selection along the way, of course.
Other questions: who has the best (if any) wide-fire-style head gasket for this 3 liter application - Powerhaus, etc.?
Necessity of Carillo/Pauter etc. con rods?
Regards Al
06-19-2003, 03:25 PM
#2
Race Director
Aftermarket pistons will need to be iron-coated like the original Mahle ones so as to not gall on the aluminum bores.
Stock S2 pistons will be weakened if you mill enough out to reduce compression down to a reasonable level for forced induction. You'd want no more than 9.0-9.5:1 compression.
The simplest way that's been done in the past is to use shorter and stronger forged aftermarket rods (Carillo is way better than Pauter). Even the forged 968 rods aren't up to the task of turbocharging.
Stock S2 pistons will be weakened if you mill enough out to reduce compression down to a reasonable level for forced induction. You'd want no more than 9.0-9.5:1 compression.
The simplest way that's been done in the past is to use shorter and stronger forged aftermarket rods (Carillo is way better than Pauter). Even the forged 968 rods aren't up to the task of turbocharging.
06-19-2003, 06:18 PM
#3
Three Wheelin'
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Callaway machined some early 944 and 928 pistons for turbocharging. I would not recommend this. I just upgraded my pistons to JE Forged units. They are not plated, but my block is now Nikasil plated on the bores.
On the head gasket, I would think you will be just fine with the stock units, just control the detonation. I am using the stock Reinz units on my 928
I'd keep the comp as low as you can, like 8.5:1 or so. You could always look to the heads to increase the volume, but you might hose up the squish voulme in the head by machining.
Shorter rods will drop the compression for sure, but then the pistons will not deck out...that may be a big concern.
If you keep the boost modest, I'd bet you can run on a stock lower end without issue.
There are some guys running over 10 psig of boost on the stock 928 S4 bottom ends with a 10:1 compression. Go figure?
On the head gasket, I would think you will be just fine with the stock units, just control the detonation. I am using the stock Reinz units on my 928
I'd keep the comp as low as you can, like 8.5:1 or so. You could always look to the heads to increase the volume, but you might hose up the squish voulme in the head by machining.
Shorter rods will drop the compression for sure, but then the pistons will not deck out...that may be a big concern.
If you keep the boost modest, I'd bet you can run on a stock lower end without issue.
There are some guys running over 10 psig of boost on the stock 928 S4 bottom ends with a 10:1 compression. Go figure?
