Eric_Oz_S2

My S2 is running well with the supercharger and soon I will have a base dyno figure to work with.

It is running at around 6.5psi boost with the stock CR of 10.9:1 and intercooled. The ignition is retarded about 5 degrees at max boost from stock value. I can run the supercharger a little harder with a smaller pulley and probably get to 7.5psi max boost.

At what stage is it worth decompressing the engine so ignition can be further advanced. Would the power gain from the advance cancel out the loss of compression, especially considering that the boost is only about half the peak in the midrange?

Are there any rules of thumb for the "best" CR for a particular boost level?

It is easy to drop to 10.4:1 by using a 1.4mm head gasket. More than this requires decompression.

So is it worth replacing the HG with a 1.4mm one??

333pg333

Interesting question Eric. Not one I can answer and am hopeful some of the smart guys start to theorise about this one. I would also enter into the equation the nature of the tracks you run at. Short tight, vs long fast. From what I see Lakeside has a bit of both where as the Paperclip would probably be more suited to higher c/r-quick spool type characteristics. I can imagine gearing would also enter the sum. The upcoming chart should give a few clues.

fast951

As a fairly loose rule, each 1 point of compression equates to roughly 5% of power.
How much of a power increase a 1psi of boost is netting? It boils down to where you want the power.

Have you measured the intake temps? You may be able to raise the boost by 1psi, but I would consider adding water injection. It cools the intake charge and provides anti detonant (same as running higher octane fuel).
Pulling so much timing out, may cause the EGTs to climb. With WI you can put back a few more degrees of timing.

Jfrahm

If you go in I would consider stronger rods, which could be resized shorter to lower compression a bit.

Do you guys worry at all about the short piston skirts in the S2 and 968 motors? I'm a bit concerned about piston scuffing. I dimly recall a supercharged S2 build in the UK that failed with scuffed pistons.
The 951 pistons have more skirt area:

http://forums.rennlist.com/upload/pi...ghts_copy1.jpg

That oil squirt mod looked sensible also, where the oil flowing out from the rod bearings is directed towards the underside of the piston. The 968 has oil squirters which probably help but the S2 does not. I would be inclined to run an oil with some extra high pressure additives in it as well.

Cheers,
-Joel.

Dubai944

My 968 motor ran fine for a full years race season with stock CR and internals and 7psi. At that boost it was putting out 275whp on a dynodynamic (308 on a dynojet) and was tuned with a haltech piggyback interceptor. After changing rods (still with the stock CR pistons) and a retune to run 9psi it quickly developed a series of overheating problems, and eventually seized a piston. Cause was eventualy suspected to be a block off hose not put back on properly on the intake pipe after the MAF which was sucking air and causing it to run lean.

I rebuilt it with 9:1 pistons, got rid of the MAF and swapped to a Motec standalone and was still able to tune it to the same power level with the same 7psi boost. It has since been running at 11psi with no problem.

As long as you have control over your tuning I doubt you will have any problems running 7.5psi on the stock motor if it's in good shape. My motor was fine at stock CR and that boost level despite constant running at full boost and high rpm so in your occasional track driven road car, it should be fine.

Jfrahm

Could be, but the piston oil squirters in the 968 might make it a lot more forgiving.

Eric_Oz_S2

So it seems that about 7 psi is a reasonable limit for stock compression. If I drop 0.5 cr then I loose a few percent power down low bit perhaps gain some advance. It would be interesting to know if a 0.5 drop on cr would net any potential power increase and be kinder on the engine (Lower temps)