ENGINE INTERNALS

I've been through a number of various engine build combinations.
Here is what makes up the latest build.

- 88mm VW Flat-top hypereutectic pistons with coated skirts
- Block bored & honed
- Cylinder-to-piston clearance set at 0.002"
- Deck skimmed to straighten ~0.003"
- Block hot tanked, primered & painted silver

- Crankshaft journals polished, left standard size
- New rod & main bearings

- Rods reconditioned
- Stock 24mm small end bushings replaced with 22mm bushings

- Balanced all rotating mass: crankshaft, rods (with bearing shell weight), pistons/pins/clips, flywheel, front crankshaft pulley and timing trigger pulley

- Head intake ports gasket matched
- Intake manifold ports gasket matched
- Head combustion chamber volume opened to 25cc
- Head combustion chamber polished
- New valve guide seals

- Raceware head studs
- All new gaskets / oil seals / crush rings
- New clutch / throw out bearing / guide tube / fork bearings
- Alloy 911SC pressure plate (7.5lbs lighter than stock)
- Flywheel surface turned
- Flywheel lightened by 2lbs

With all these goodies, I'm hoping to push the engine to a solid 250hp as a start. With a 2800lb curb weight, that should make for a fairly quick ride. After that, I'll start looking into upgrading the turbo and maybe playing with the exhaust and cam.

And here are all sorts of various pics of the fun.

Pulling some extra volume out of the stock 931 head.
The first pic shows an almost-done combustion chamber on #1. You can compare it to the unfinished chambers to see just how much was taken out.

Flat-top VW Pistons with coated skirts, yes, these really are only $80 for a set through AA Pistons. Just remember to replace their rings if you're runnig forced induction!

Awaiting bare block, and freshly polished crank

Lightweight alloy pressure plate with higher clamping force. Saves 5 lbs of rotating mass!

There are a couple versions of this pressure plate. There are two versions for the stock 931, and three versions that are 911-based. I'm using the alloy/iron version.

Sachs #3032.015.399-b2
Alloy casing / alloy friction surface
*requires modified 931 ring gear, or 911 ring gear
Weighs 6.5 lbs. with TO bearing

Sachs #3032.015.399
Alloy casing / iron friction surface
Works with stock 931 ring gear
Weighs 9lbs. with TO bearing

Sachs #3032.055.099
Iron casing / iron friction surface
Stock 931 pressure plate
Weighs 14lbs with TO bearing

Sachs #???
Iron casing / iron friction surface
For use with a rubber-center clutch on a 931
Weighs ??

The stock specs for the pressure plates are as follows-

7200-7900N for stock 931 pressure plate
7800-8500N for stock 911 pressure plate

Although the alloy/alloy b2 pressure plate felt like the pedal was much stiffer which would imply more clamping force. This could be a "sport" version.

Lightened flywheel

Clutch bolted up

Intake

During the reassembly, I also tapped the oil pan using a time-sert for an oil temperature sender to keep an eye on temperatures.

931 Performance

Porsche factory numbers are notoriously conservative. I haven't found many magazine tests, but I'll add them when I do.

US 1979-1980
156hp @ 5500rpm
157 ft-lbs @ 3500rpm
7.5:1 compression
0.43 - 0.47 bar (6.2 - 6.8 psi)
0-100 kph (~62mph) = 9.4 sec
1/4 mile = 16.3 sec
Top speed = 132mph [143mph magazine test]
Weight = 2779 lbs.

US 1981-1982
156hp @ 5750rpm
160ft-lbs @ 3500rpm
8.0:1 compression
0.43 - 0.47 bar (6.2 - 6.8 psi)
0-100 kph = 9.3 sec
1/4 mile = 16.2 sec
Top Speed = 134mph
Weight = 2779 lbs.

RoW 1979-1980
170hp @ 5500rpm
187 ft-lbs @ 3500rpm
7.5:1 compression
0.67 - 0.73 bar (9.7 - 10.6 psi)
0-100 kph = 7.8 sec
Top speed = 140mph
Weight = 2779 lbs.

RoW 1981-1982
177hp @ 5500rpm
190 ft-lbs @ 3500rpm
8.5:1 compression
0.62 - 0.68 bar (9 - 9.9 psi)
2596 lbs
0-100 kph = 7.7 sec [7.1 sec magazine test]
Top Speed = 143mph
Weight = 2779 lbs.

The Carrera GT, with hardly more than an added top-mount intercooler and an 0.75 bar boost pressure is reported as giving the following numbers:

1981 924 Carrera GT
210hp @ 6000rpm
213 ft-lbs
8.5:1 compression
0.75 bar (10.9 psi)
0-100kph = 6.5 secs
weight = 2600 lbs.

I'm shooting to approach 250hp at the rear wheels with this current build. Where that will put me on the performance map is hard to say. With an extra 180lbs and an extra 40hp over the GT, I think that 0-62mph in under 6.5 secs is a pretty safe bet.