Deck height, compression ratio, and valve relief flat top pistons, In a 2.0 |
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Deck height, compression ratio, and valve relief flat top pistons, In a 2.0 |
emerygt350 |
Jul 13 2023, 11:41 AM
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#1
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Advanced Member Group: Members Posts: 2,132 Joined: 20-July 21 From: Upstate, NY Member No.: 25,740 Region Association: North East States |
Ok, my question of the day:
Put my new 96mm pistons and cylinders on and measured deck height, .033 all the way around. Let's say .032 to be conservative. I will not be running head gaskets. The calculator gives me 8.8 CR at .033 8.6 CR at .043 8.4 CR at .053 The pistons have a valve relief (pretty large) cut. I am shooting for 8.4 CR. Heads are in the shop so I can't check the CC yet. Everything I read says .04 - .06 for deck height. With the valve relief do I need to worry about going lower? If I had dishes would you measure from the edge of the piston or the dish? |
technicalninja |
Jul 22 2023, 01:46 PM
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#2
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Senior Member Group: Members Posts: 1,290 Joined: 31-January 23 From: Granbury Texas Member No.: 27,135 Region Association: Southwest Region |
.028 deck height on all four would make me want a base shim of .010-.014 with hypereutectic pistons (Keith Black).
With forged pistons I'd want a base shim of .015-.025 depending on the base material. 4032 would get .015-.018 2618 would get .020-.025 I'm a believer in NOT using anything between the cylinder and the cylinder head. I prefer the hyper pistons OVER the forged pistons for a street car. Want to scare yourself? Check deck height with a forged piston with no rings on it at 90 degrees from the pin bore. Then rock the piston on the pin bore. You will see a total range of .020 change in deck height between low and high. Forged pistons can rock in the bore SO MUCH more than you expect... Thank God that variation doesn't actually occur during operation. There is so much resistance generated by the quench areas on the head that the pistons tend to stay flat during operation. If they didn't, we'd need double the quench clearance that we use now, and the quench action would be greatly diminished. I do have one question. Why do you need shims to check the combustion chamber CCs? Figured it out! poor reading skills on my part... How are you checking this now? I've always used a plexiglass plate (round plate at the correct diameter for VW air-cooled stuff) with a couple of holes drilled in it and either a cattle syringe (60 cc) or a burette to measure volume. The reason for two holes is it is a shitload easier to clear the bubbles with a vent hole. I use Vaseline to form an airtight seal to the cylinder head. |
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