Scenario:

1) 8.0:1 compression + .8 bar of boost

2) 7.5:1 compression + .8 *or* .9 bar boost

Merits of either? Fair comparison?

Any and all opinions - especially informed experiences wanted!
THX

 

I got this chart from A. Graham Bell's Forced Induction Performance Tuning. I think the 98 RON fuel is similar to our 93 octane. I settled on 7.7:1 with 1 bar boost although boost will be controlled by the ECU so I'll probably run less boost on the track.

 

You will always get more HP with increase boost compared to increase compression. Also you will be able to have less knock with higher boost lower compression. The only advantage of more compression is that the car runs better off boost thus feel quicker in street driving situations. However going EFI and new ignition can solve knock and thus get more compression and boost at the same time. This is the reason why the new 996TT can have over 9 compression.

 

ehhh...I run 1.2 bar and 7.5:1 compression, its fast as f*** and makes big flames. if it blows it blows...that'll just be my excuse to send it to stephen for a EFI upgrade, some new cams, etc.

 

"This is the reason why the new 996TT can have over 9 compression."

Part of the reason the 996TT can have over 9:1 compression is a highly sophisticated engine management system, but the other reason is a much more efficient cylinder head design and variable camshafts.

If you have EFI, I think that 8:0:1 compression is about optimal for this engine when running boost pressures at 1bar and under.

 

125/Geoffrey -

Scenario:
1) 8.0:1 compression + .8 bar of boost
2) 7.5:1 compression + .9 bar boost

Does scenario #2 make sense with IA fuel head, 964 cams, SC heads, K27 hiflow, twin plug, RUF IC?

 

I'm certainly not as experienced with this as Geoffrey, but I think either option is safe with your setup and actually a little conservative. Option 2 would allow more boost so I would personally lean toward that.

A problem I had when building my engine was even getting more compression. When I measured the 98mm Mahle pistons that were in my engine, I only had about 7.1 compression. The JE pistons I got put me at 7.7. To get more than that I think I would have had to get custom pistons or get the heads welded up. Do the SC heads have a smaller combustion volume?

 

I thought a major reason the 996tt can have high comp & boost was due to the watercooling.

The modded Ford Sierra Cosworths in the UK can run really high boost pressures compared to the 930.

 

Geoffery you're right about the better combustion chamber design. The point is that engine management is crucial when trying to run high boost and more compression.