Highest safe compression ratio

[hobieboy]

Just wonder what is the consensus on highest safe compression ratio on a 930, converted to EFI with twin plug AND one that will be driven hard on track?

If we limit boost to 1 bar or below, would 8.5:1 be max? Or closer to 9:1 is still safe?

thanks...

[cstreit]

What octane gas are you thinking of running? For instance, you could run dynamic compressions much higher if you run 116 octane gas...

[camgrinder]

How good is the intercooler? how heavy is the car? there are a lot of variables to consider.

[hobieboy]

Here's a bit more details about my car & question...
last time I corner balance the car it is still 3100lb with me in it. I expect it to be still around 3000lb. I'm installing a full bay intercooler based upon a 12x19x3 core. The rest of my build includes SC cam (I'm seriously considering changing to 964 or John's EVO cam), K27HF and GHL headers. The head had been ported to accept 3.2 intake.
Fuel-wise, no - I want to targe using 94 (street) gas.
And, the reason for this consideration - I could get a set of almost new Mahle 98mm for next to nothing hencethese calculations.
Thanks...

[DonE]

In my opinion, still too broad of a question. For example, what kind of track work? Technical or is it a turbo track?

High compression ratio is good for tight, technical tracks. Lower CR (aka heat management) is great for long WOT tracks. For the ultimate track car, put a G50/50 tranny in the car, install your EVO cams, set the CR to 7.0:1, set the AFR to 11.8:1, timing to 18 - 20 degrees, equal length headers, ARP or raceware hardware, boost to 1.2 bar and have fun. Putting the car on a 500lb diet will help BIG time too. With that tranny and weight loss, the CR can be low and you can still get your butt out of a corner pretty well.

Otherwise, use the SC cams, tune the AFR to 11.3:1, timing to 22 degrees (above 4000), boost to 1 bar, CR at 8.5:1, install ARP or Raceware hardware and learn to carry speed through the tight turns.

I can show you dynos on the above two scenarios and you can decide for yourself where you want the power. And if you are using EFI, you can tune the car to run well on premium pump gas at either 8.5 or 7.0:1 CR.

PS - let me know if you ever want to sell the SC cams

[hobieboy]

Don, yes - would love to see the different dyno results. Have PM you.

[David]

I would be surprised if you could get 98mm Mahle pistons to much over 7.5 without significant head work. When I cc'd my set up with 98mm Mahle's it was about 7.2 so I changed to JE pistons. I got 7.7 with the JE's. This was using a small second spark plug and about 0.010" taken off the heads. To get much more compression I believe you would need to cut the heads more and then machine for the squish (which might cause cam chain problems) or weld up part of the heads to reduce volume.