ngoldrich

A couple of not so random thoughts on shims - I believe to be correct:

(1) Cylinder shims would improve the valve to piston clearance, but would lower the CR too much since it is affecting the whole cross section (diameter) of the cylinder versus a small indentation in the piston.

(2) In order to gain the needed clearance, the shims would be pretty thick - take a look at how deep the valve relief pockets are. That may also throw off the valve train geometry / chain lengths. Because you are actually spacing out the whole cam tower...

(3) Removing shims at the bottom of the barrels (if you have them) would raise CR and reduce valve to piston clearance.

Norm

Geoffrey

4) Removing the cylinder base gaskets reduces the DECK HEIGHT CLEARANCE and the piston may hit the head

5) Increasing the clearance by adding shims will cause combustion problems and make the engine prone to detonation if the deck height clearance is beyond about 1.2mm because the squish area is no longer effective.

ngoldrich

Geoffrey,

Both of your points make sense. My comments were really more theoretical, as I dont think shimming is agood way to adjust valve to piston clearance.

Unfortunately I left out tht important little tidbit in my note.

Thanks for the further clarification...

Norm

A930Rocket

I was going with 964 P&C's because they were fully finned. I can get them for $2700 vs. $3400 everyone else was paying for 3.4's.

Also, don't they require case work = more money?

If the head or cylinder top could be machined to compensate for the cc loss, would that work? But, then that's money that could go towards a set of 3.4's.

DonE

There are different sized shims - I said nothing of removing shims. You will need to do the calculations on what the resultant deck height will be, the correct CR and so on. You might find that you need a .25mm shim to bump the CR and keep an acceptable deck height. To do it right, have a machine shop measure everything and give you the recommendation. With my set up, my heads were altered by the previous owner, so I had to get a custom off-set on the pistons and 1 mm shim to give me a deck height of .040 and CR of 8.0:1.

Geoffrey

You CAN machine the heads, however, Porsche specifies that 2 cuts of .010 are the maximum allowed. The problem that you'll run into is that the lowest cooling fin which gives the head sealing surface strength will become too thin and weak and there will be leakage. You can weld the bottom two fins to strengthen them, however, now you are talking quite a bit of modification. Cutting the heads will gain you 2cc or so.

Machining the cylinder means you will effectively be pushing the piston higher up in the cylinder...see issue 4 above. You can machine the edge of the piston to match, but then you are placing the compression ring closer to the combustion and in a turbo engine that is not necessarily the best thing.

The right way is to have a piston with the correct pin height and net dome volume for the rod and head combination you have to meet your desired compression ratio. Back to the Mahle vs aftermarket piston discussion.