I've been wondering if there is a substantial difference of cylinder compression tests between a forged piston and a cast OEM piston. I've noticed that many people in the past have posted very high compression test numbers with stock pistons/rings in relatively stock motors. But I've seen and performed compression tests on a few forged piston motors that have fairy high compression (12:1) but don't register as high on the tests.

For example, I've seen JDM ITR motors have 260+ psi but a 12:1 ITR motor with JE's only put out 250psi. Is this mainly due to the difference in piston to wall clearance? Or are the motors with forged pistons I've seen just not built correctly. Engine builders and anyone else please tell me what you think. TIA

 

You'll need to also consider what primary lobes and valve lash cam phasing the motor has when trying to compare compression numbers across different motors.

 

also.....you will need to use the same guage on each of these engines...guages read differently....up to 100 psi difference (at least the 2 i have do)

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Rocket &raquo;</TD></TR><TR><TD CLASS="quote">You'll need to also consider what primary lobes and valve lash cam phasing the motor has when trying to compare compression numbers across different motors.</TD></TR></TABLE>

I totally agree with Rocket. Your camshaft specs and valve lash will determine your dynamic compression.

 

I know that bigger cams tend to drop compression ratio. But given everything equal except pistons and rings, will it make a difference? I did a compression check yesterday on my motor and it was about 245 psi across the board. This is with 12:1 CR and JE pistons/rings. I am suspect to rings because I added a little bit of oil and the compression shot over 300 with much less strokes. But still wondering if the greater piston/wall clearance will cause a bigger difference in dynamic compression?