2.7 with RS p/c?s > how to raise comp ratio?

[farleyd]

Hi to all, just acquired new Mahle p/cs with the stock 8.5:1 comp ratio. Will be used with Webers, E-cams and headers.

The engine is apart, ready for head rebuild.

Is there a way i could raise the comp ratio a bit?
Does it worth it?

Thanks.

[Rob McKibbon]

I believe you can machine the block at the cyclinder ports to increase the compression. My mechanic did this to my block but not exactly sure how or what he did? It didn't seem like a big deal/expense for him.

Check with your/a mechanic if this is right?

He put in 90mm Mahle Carrera RS Pistons & Cylinders in. Didn't say what compression these were, but I know he machined the block. (2.7L)

[snowman]

Check out Bruce Andersons book " Porsche 911 Performance Handbook" for details on this very subject.

[911pcars]

I think Mr. Anderson recommends only 0.040" maximum should be taken off the cylinder head gasket surface/cylinder. Not sure of the compression ratio change, but you could compute this from well-known formulas.

However, the more material you remove, the closer the relationship between the cam and the crank. This results in additional chain slack. Furthermore, when an engine is rebuilt, the machine shop may have to line bore the case, remove material from the crankcase cylinder spigots to true this surface and even the individual cylinders to make them all the same height; that's more material.

Chain slack is normally taken up by the camshaft idler sprocket, but the extra slack may cause the idler sprocket and chain to run uncomfortably close (too close) to the side of the chain box, especially on the driver's side.

If you want to raise the compression ratio a significant amount, I recommend using different pistons.

Sherwood Lee
http://members.rennlist.org/911pcars