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Join Date: Oct 2003
Location: Bremerton, WA
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cut heads and increase CR?

If you were to cut 0.25 mm off your head and keep the 0.25 mm base gasket how much would you raise compression?
If you were starting with a stock 8.5:1 83 SC motor would you even see the increase?
Old 06-20-2005, 06:38 PM
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Early 80's SC's have a stated stock CR of 9.3:1. If you do the actual measurements, stock CR usually measures about .25-.50 lower. Nonetheless, if you start with the stated 9.3:1 and remove .25 from the heads, this will drop your head volume from ~90cc's to ~87.5, increasing the compression ratio to about 9.66:1. Remember also, not shimming the cylinders will bring your piston/head and piston/valve clearances .25 closer as well. Do your measurements to make sure all is well.

The above was calculated assuming bore-95mm, stroke-70.4mm, deck height-1mm, cylinder head volume-87.5cc, and piston dome volume-37cc. Itís probably in your best interest to do these measurements and calculate your actual CR.
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Old 06-21-2005, 04:29 PM
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To add; anytime the cam becomes closer to the crankcase (via head surfacing, cylinder trimming, crankcase-to-cylinder surfacing etc.), it no longer remains centered in the chain box opening. To restore the correct relationship (oil leakage normally follows despite an O-ring), mill the chain box-to-crankcase-mounting surface an equal amount that was removed from the cylinder/head stack.

You may also find the chain a bit too long after these operations. This may result in the idler arm sprocket (especially the left side) rotating precariously close to the wall of the chain box. Instead of removing a link, the suggested solution to maintain the correct geometry is to install a slightly larger idler sprocket. A Porsche-savvy machine shop should be able to supply one.

Sherwood
Old 06-22-2005, 04:21 PM
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People think of machining down the heads "while you're in there" as basically free horsepower. As Sherwood points out though, it isn't really free.
If I was going to mess around with changing dimensions I'd at least want to get a more favorable deck height out of it.
-Chris
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Old 06-22-2005, 06:08 PM
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Quote:
To add; anytime the cam becomes closer to the crankcase (via head surfacing, cylinder trimming, crankcase-to-cylinder surfacing etc.), it no longer remains centered in the chain box opening
Good point to add. Not an issue an issue unless the these 2 dimensions differ .5mm.
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Old 06-22-2005, 07:30 PM
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