im looking to purchase a set of forged pistons for an engine im slowly building. 4.125 bore, 3.75 crank, 6" rod. this engine will be turbocharged. so my question is what are the main factors i should take into consideration when purchasing pistons for a turbocharger? ex: im shooting for around 8.5:1 cr should i go with dished or d-dished pistons? when it comes to compression height is more better or is less better? should i look for a piston with the top ring closer to the top of the piston or further away? ive done a little math and if im correct a piston with a 1.125 compression height with a 6" rod and 3.75 crank should put me right at 9". is there an ideal quench area ( i think thats what its called) for a turbo setup? thanks for any and all advise!
Squish is said to be not quite as critical in a blown application because the mixture is homogenized by the blower on the way into the cylinder. I, however, would still build with a 0.035" to 0.045" squish.
Depending on boost levels a tight squish can create more problems than it solves- some purpose built turbo race engines run have .100" squish to slow down the mixture.
It's that old "if some is good then more is better" way of thinking that can sometimes lead to trouble.
With 8,5:1 compression I assume he'll be running at least 15psi.
D-dish rather than full dish. Ring placement lower on the piston(to get it away from the heat). Thicker piston top is also better(more resistant to heat). As Tech posted, quench not as important with forced induction.
heres what i came up with JE pistons part #242885-8, 6" rod, 3.75 crank,4.125 bore...... if my math is correct i can deck my block to 9.015, use a 72cc head, use a 4.125/.030 (compressed) head gasket...this should give me real close to 8.5:1 c/r with a squish area of .045. does this sound correct? also are these pistons a good choice for a turbocharged motor?
In my opinion, there is no better blower piston on the planet. Your plan sounds bulletproof. Make sure, ahead of time, that the rods will clear the cam lobes. You may or may not have to do a little grinding at the pan rail for the big end of the rods. A miss is as good as a mile.
Depends on the boost level, for a mild street engine .045" should be fine. Anything over 15psi and is be opening it up some. At 15psi your charge will be twice as dense. That has an effect.
Your saying 4.125" bore...I'm assuming you are buying a new Dart or similar block??
Hasn't been a used stock 400 block around for 20 years or more that will still be good at standard bore, unless you got extremely lucky and stumbled onto one.
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