How does re-sizing a case affect compression?

[dweymer]

When a case is re-sized, doesn't that affect CR, and valve/piston clearance? What else does it have an influence on?

[dtw]

Hey Donnie,
The chain 'length' can become an issue depending upon how much narrower the engine becomes. If you wish to rebuild back to stock specifications, you can simply shim the cylinders with an appropriately-sized copper base gasket. The gaskets are readily available from Porsche or well-stocked builders.

On my 2.4, we shaved the heads a bit for compression. The chains became 'longer', but this did not require use of a larger chain idler sprocket. I did, however, have to shave down the mating surfaces of my chain cases a bit. My cam seals would have failed if I didn't.

Still, none of this matters if you want to stay stock - just shim your cylinders out.

[911pcars]

IMHO, the important spec to identify is the deck height. The ideal deck height provides the minimum squish area around the periphery of the piston @TDC while maintaining adequate clearance between piston and cyl. head. Spacing the cylinder changes the relationship between piston and cyl. head.

In addition, machining operations to square up the crank and crankcase, renew cylinder and cylinder head contact surfaces, equalizing cylinder heights, etc. all add up to decrease the deck height. As mentioned, additional or thicker cylinder base gaskets can restore the proper deck height, but I wouldn't use this method to remove slack from the timing chain. The "stack" height is what it is due to the material removed in the above operations.

Also, the total reduced stack height is the amount to remove from the chain box-to-crankcase gasket surface in order to re-center the camshaft in the chain box, otherwise an oil leak will develop due to the off-center cam.

If all cylinder components are equal and square with proper clearances and the timing chain slack increases after all is said and done, then deal with the longish chain at the idler arm/sprocket end. I would recommend a larger idler sprocket in this case.

And to answer the initial question, yes. If the machining operations affect the cylinder/combustion chamber volume, it will affect the compression ratio.

Comments?

Sherwood

[dweymer]

Not much to comment Sherwood, very concise and readable. So it would, but stacking the cylinders to restore deck height, and shaving the chain boxes to match any "left-over" height not restored by the stack will return the squish area, thus CR to before resize numbers.

[911pcars]

"but stacking the cylinders to restore deck height, and shaving the chain boxes to match any "left-over" height not restored by the stack will return the squish area, thus CR to before resize numbers."

Donnie,
Spacing the cylinders to restore deck height is one operation.

Shaving the chain boxes compensates for the new stack height, net material added/removed from cylinder head, cylinders and crankcase. This is a result of but independent from CR changes.

"Returning to the previous compression ratio."
If the machining operations reduce the volume of the combustion chamber and/or cylinder, the CR will change. The compression ratio is the ratio between the cylinder+combustion chamber volume at BDC and the volume remaining at TDC..

Hope this helps,
Sherwood

[dweymer]

got it, thanks!