Machining piston tops to lower C/R?
 

Does the 911SC 3.0 pistons have enough 'meat' on the crown to machine down to 8.0:1 C/R to be more 'boost friendly'?

I know this has been done on the 964 pistons many times before and still retains strength, but just wondering if it's possible to do it on the 911SC pistons?

Who offers this service?

Or is it cheaper to fit thicker base cylinder shims to decompress the motor? What thickness shims would one need to obtain 8.0:1 CR from a 8.5:1 SC motor?

You can calculate this. You know the displacement of your cylinders (bore and stroke). Knowing 8.5/1 allows you to calculate the volume above the piston needed for that displacement to give that ratio. You can then back calculate to see how much more volume is needed to get the lower CR. Knowing that volume, you can easily calculate what height cylinder equals that extra volume. That's your extra shim thickness.

Porsche uses 0.25mm base shims, with 0.50 reserved for rebuilds. I used 1.00mm shims (EBS sells them, as someone on this list kindly informed me) to get needed head clearance on a race motor. This required that I space out my chain boxes to get the cam seals to line up properly, though it didn't require anything special for the chains. Something to think about.

I've seen a spec for piston top thickness somewhere. Maybe Wayne's book (you do have that, don't you?). But the CIS pistons are a very irregular shape, so machining them seems like it would be tricky, and calculating how much to flatten the top of an irregular dome equally tricky short of a lot of cut and try on a junk piston and measuring with fluid volumes in a cylinder. I'd not want to reduce the depth to the first ring, especially on a blown motor.

How about calculating how much you would have to reduce the stroke to gain a bit more volume? Same calculation. I know that you can have wrist pin bushings offset bored, though there may be a prompt limit to that.

You'll likely get multiple opinions on this that may conflict, but I'd simply offer mine.

SC pistons are cast and I would not machine them for any boosted applications. I think that's looking for trouble. If you need a particular CR for a turbocharged engine, I would have either JE or CP make a set for you with a ring package that can withstand detonation far better than the OEM Goetze's do.

Hope this helps (and prevent problems).

There is no engine component (piston, rod etc) that would withstand detonation for sustained periods. Detonation happens due to poor tune or bad gas. And forged pistons can't cope with detonation until bad batch of gas is used, they will fail just like cast pistons.
400 hp on 6cylinder engine is nothing with cast pistons (only less than 70 hp per cylinder) but as with ANY part, it requires controlled environment.

I have run my completely stock euro 924 engine (with crappy cast pistons, crappy combustion chamber design etc) with 1 bar of boost, 9.3:1 CR and 250 hp/320 Nm for around 4 years and 30 000 km's without ANY issue and that includes track sessions top speed events etc. This is more than 60 hp per cylinder.
And I remember from somewhere that one cannot boost 924 engine, it will break immediately :)

My point is that you should leave the pistons as is.

We are considering this option. If we can get away with running 0.8 - 1.0 bar of boost on the 8.5:1 C/R motor with EFI and a full bay intercooler, we'd be more than happy.

8.5:1 would offer awesome pre-boost driveability / response, then when boost builds and kicks in, it should be a very fun street car. It won't be seeing any track events.

If we do end up cracking a piston or ring land, then we'll slip in some lower compression 8.0:1 aftermarket JE's, or source some good used 930 P&C's ;)

BTW, we're considering the BITZ EFI conversion kit for this SC motor.

I am running 9.5:1 with 0.6 bar and am seeing sub-audible detonation even with near winter temps, a full-bay inter-cooler, and meth injection at 15 degrees of timing. Now, I don't see any evidence of this on my spark-plugs and I only do freeway pulls so I am not doing repeated back to back boost runs or running sustained boost on the track. I suspect if I tried to run my car on the track I would quickly discover the limits of my heat dissipation systems, and pistons. I think 8.5:1 is reasonable in these cars with EFI and complete timing control but I don't know whether the CIS piston adversely affects your detonation risk. The lower you go on compression, the more of a safety margin you have.
The Bitz kit may not be the best starting point for boosting as you don't get timing control with it and you're stuck with smaller fueling tables. You really should think of an MSII or MSIII unit with EDIS wasted spark ignition if you want to go that route.

You might be OK with stock 8.5 pistons. If you actually measure the compression ratio with a deck height of 1mm, you'll probably find that you only have 8.0 CR anyway. It's very common for Mahle pistons to measure the full compression ratio with a deck height of zero.

-Andy

Good point about ignition control. I was thinking the BITZ EFI conversion in conjunction with the MSD 6ABTM ignition which retards ignition under boost.