Austarion

g'day all!

just a quick question, for those of you running vr4 pistons with SOHC head. what compression test results (warm) are you getting?

cheers :beer

I remember reading compression goes down to 6.8 or something like that so I think you should be getting 120psi or therabouts on a fresh build.

cool. i knew that they comp ratio went pretty low but i didn't know that low! 20psi here i come :pimp

mines got 115psi in all 4.

cheers! :D

it goes to 6.8 with a 2.4 head, 7.5 with a 2.0 jet valve type head, havnt measured a T head yet. somewhere around 120 ish would be good i'd say for a 2.0 head.

orange car with VR4 type JE's and 2.4 head had 100psi comp on a fresh motor, and hasnt changed in 4 years. starts fine, power comes on at 3000 rpm :)

shouldn't camshaft choice be mentioned ? or no difference to comp test readings ?

yes it will make a difference but its pretty rare for people over here to change cams..

camshaft will only make a difference to the number that the gauge reads.. but all cylinders being roughly the same reading is the more important thing isn't it?

quest wrote:shouldn't camshaft choice be mentioned ? or no difference to comp test readings ?

Yes, intake closing degree. A more accurate test is a leak down test rather then comp test.

I should mention a cr of 6.8 will always make less power then the same motor with a cr of 8.5 so if you can avoid it, please do so.

yes, but they're comparing compression gauge readings, relative to compression ratio. A longer duration cam will lower/skew that.

pretty rare for people over here to change cams..

From reading here, u seem to have at least 2 excellent low priced proven regrids from Wade & Camtech. Why would one who pursues performance, go as deep as pistons but stick with the stock cam ? that is odd.

usually pistons are a necessity, cams are a power thing, usually not done until you start really throwing money at em :)

cams are easy to change at any time really

quest wrote:Why would one who pursues performance, go as deep as pistons but stick with the stock cam ? that is odd.

Usually anything above 15psi will kill our stock pistons due to bad ring land design.

a good reasonable priced cam seem like a very wise addition for such a minimal cash outlay. You won't have to run as much boost to make the same power with a cam.
With boost built up in the intake, that "restriction" heating things up, surely not doing those ring lands any favours

reason i went to forged vr4 pistons is because at 18psi my cast hypertec pistons didn't really like it too much...... ended up being able to pull chunks of ring lands out of 1 piston and the other had a few cracks in the lands. 7,500km old motor too!

i do also have a ported head, i think with some combustion chamber smoothing, and roller rockers with a billet cam from RPW. :) which would affect readings, but i was mainly after a vague indication.

i was mainly doing the comp test to see if cylinders were even, but i was a little worried when comp came out so low. but not now :D

My ported & polished & cc'd head now resideing in sunny downtown Rocky had 50.4 cc chambers before it was recoed and a skim to ensure a true surface .You can do the math if you have the negative volume of the pistons +the hg volume