6G Celicas Forums

Theres a guy that has a sick ST215 down here...As the mod list started to grow and the PSI started to climb..first thing to go were the rods...pretty much blew a hole through the block...

But didn't we already confirm Gen 3 and 4 use same pistons?

I read in the MR2 power primer also that depending on which head you get, you'll have less cc's in a gen 2 and 4 head than a gen 3 head. due to the 8.8 and 9.0 CR vs 8.5 and that compression was totally based upon the starting average head chamber capacities whether or not you've had the head skimmed, they say you may lose up to 5cc in each head chambers with a head skim/resurface

then they have big graphs showing what kind of CR to expect.

Why you guys still arguing this?

initially i was just stating that taller pistons will affect the compression ratio w/o affecting the total volumetric displacement of the engine (1998cc) assuming both heads are the same. now that we know the rods and pistons are the same, then the difference is definitely in the head design. i believe Smaay's post about how head design having no effect on the CR is incorrect. by altering the head chamber, it will also alter the volume of the head chamber at TDC which also alters the CR. just to clarify, factors that affect the CR are the pistons, rods, crank (which alters the stroke), and even head gasket thickness. however, the total volumetric displacement of the engine (1998cc) remains unchanged as long as the bore and stroke remain unchanged.

delusionz, you're absolutely right in that you can have different cc's when using different generation heads. but just to be clear, the primer is referring to cc's as the volume in the head chamber, not the total volumetric displacement (1998 cc).

Yeah you're actually right about that,

moving the so called top of the deck into the head area vs the new (slightly higher) piston BDC level will keep 1998cc if you look at it that way.

but yeah pistons are one way to raise compression as is a head skim, either way the pistons shouldn't be sticking out (not talking about the crown) above the top of the deck in factory form regardless of the generation

This post has been edited by delusionz: May 8, 2011 - 5:16 AM

you are right.

the volume of the air/fuel when the piston is on TDC is called "death volume" and its remains compressed on the head.

CV:cilinder volume 499.5562141cc
DV:death volume

CR= 1 + CV/DV

-for 3rd gen CR=8.5

8.5=1+ 499.55/DV ---> DV=66.6076cc

-for 4th gen CR= 9.0

9.0=1+499.55/DV' ----> DV'=62.4446cc

so death volume on heads are different.

While browsing the MR2 forums, there was also interesting discussion about how the GEN 4/5 oil pumps are higher flowing. Even more so than the 5sfe pumps.
Just my .02 cents. lol

This post has been edited by 3WayStunna: Jun 9, 2011 - 11:57 PM