3.0 turbo question
 

Hi engine rebuild guru's SmileWavy

If one were inclined to rebuild a 1979 911SC 3.0 motor for forced induction, what would be the best way to lower the C/R from 8.5:1 to around 8.0:1?

Would using the 0.5mm C2T head gaskets do the job?

Or would it be better to just slip in a set of 8.0:1 JE's?

Actually, would the earlier 930 3.0 Turbo 95mm P&C's slip in? I think they ran 6.5:1 C/R and allowed 0.8bar boost?

What about 930 97mm turbo P&C's?

Just looking for options :rolleyes:

Increasing the deck height by spacing the cylinders up seems to be a recipe for disaster- excessive deck can promote detonation in the stagnation area at the edge of the cylinder. Also, having a bump on the piston makes the flame front travel around corners-- something we tolerate in the n/a world but if you need lower compression, use a flat-top piston.

Youre going in two different directions here, I ll try to clarify.
You can lower the 8.5 with a specific base gasket. At 8.0 youre limited to the boost youre able to use, probably in the area of .5bar.
The 930, 3.0 are slip in as is the 3.3 cyl. The 3.3 requires opening from 22mm to 23mm the small end of the rod or the use of 3..2/3.3 crank and rods.

Bruce

OK, so the best way to lower the compression in a 3.0 SC motor is to slip in some aftermarket JE forged pistons made to whatever C/R desired?

If one was going to run 1 bar of boost, what C/R should one aim for? 8.0:1? 7.5:1?

This will eventually be an EFI and intercooled setup using a Carrera intake, aftermarket intercooler, programmable engine management, aftermarket fasteners etc. Probably won't be going twin plug though. Just aftermarket MSD CDI ignition with larger plug gaps.

I wouldn't lower the CR at all and 8.5:1 is totally ok, even with slightly domed pistons. You will get much better driveability with 8.5:1.
Since you're going EFI route too...that makes things much better in terms of control.

Lots of understandings are dated, there is no point in taking everything that factory did or did not for one and only truth. Time has changed and so has engine managements etc.
Personally I wouldn't bother building a turbo engine with CIS at all and convert it to EFI first but that's just me.

I think the simplest way to do this would be to use OEM Mahle 3.0 Turbo pistons and a 3.0 Turbo CIS system.

I just happen to have both on the shelf available for sale.

Let me know if you have any interest.

Sorry to disagree but installing and tuning EFI is much simper than pulling and opening the engine and retrofitting CIS. That way he still ends up with low compression pistons and crappy fuel management => laggy and slow 911.

Appreciate the advice guys.

We will initially run a 0.5bar bolt on system, and then down the track scrap the CIS and fit a carrera intake, intercooler and ECU. We are aiming for 1bar boost. Would it be safe to run 1bar at 8.5:1 CR using EFI ?

If we use 930 97mm turbo pistons and cylinders, what would be the end displacement on the SC motor?

Also, does the SC pistons have enough meat on the crown to machine them down to a lower C/R?

Edit: ok, a 97mm P&C set would yield 3.12 liters. Not sure about c/r though. Maybe still 7.0:1?

I would go with twin-plug when aiming to 1 bar of boost and 8.5:1 CR. With boosted engine it benefits the most and it is easy since you're going EFI route anyway finally.

Just for reference, we have been running a turbocharged VW bug on dragraces ad streets with 1600cc engine, boost at 1.4 bars and around 230-240 hp and it holds fine for the whole season. It has megasquirt efi and $hit engine internals compared to any 911 engine.

My 2.5 ss turbo is around 8:1 CR (66mm crank+RS 90 mm 8.5:1 P&C's) but I haven't run it yet. It has twin-plugged heads, S cams and VEMS EFI.

Good point. A twin plugged, EFI, intercooled, 8.5:1 turbo motor running 1.0bar would be a very nice and responsive setup indeed :cool:

Any guesses as to what HP he'd obtain? He's aiming for ~400hp. Not bad for a lightweight SC :D