Lower compression is better for boosted cars, I don't know why it just is. I think you would be able to run higher boost more eficiently. The stock bore on the 4g63 is 85mm if you bore it to .020 the new cylinder bore would be 85.5mm. So .020 would be an increase of .5 mm and .040 would be 1mm. So the bore will be 86mm on the 4g63.

.020= .5mm
.040= 1mm
.060= 1.5mm

What CR is better is up to your abilities, goals, and uses for the vehicle. A higher CR will squeeze the air more, raising cylinder pressures. A lower CR will squeeze the air less, resulting in lower cylinder pressures. A higher CR will always make more power, but is less safe to boost on. Forced induction raises the pressure of the incoming air charge, before the pistons get a chance to squeeze it even further.

Many people boost with a 9.5:1 CR. Matter of fact, the factory 420a bottom end can handle about 8psi safely, at it's 9.6:1 CR. Lowering the CR opens a larger tuning window, and generally allows for more boost. Most guys building their 420a for boost go with a lower CR.

When your block is being machined, the machinist will determine if boring is required to correct tapers and scratches. This will make your cylinder larger, requiring a larger piston to fit correctly. Do not purchase pistons until your machinist has prepared the block. He will tell you what size pistons you will need.

I'm boosting 16 psi on a t3/t4e 50 trim and got 8.8:1 CR psitons (JE). Yes, this is with the 420A.

Jyden said:

I am in time going to convert it to FI

Click to expand...

Your going to convert your Talon to Fuel Injection Last I checked our cars are already EFI....

God, I hope"FI" does not mean "Fuel injection" because if that's the case he definitely shouldn't be asking questions about pistons then.

Yeah, I didn't mean that. I'll just edit it out as I seem to have gotten abbreviations wrong and because it really isn't needed. Thanks so far.

Lol, I read "FI" as forced induction.... But thats just me.