Bart

I have a 7,2c/r cosworth engine and want to raise the c/r so i was thinking to use 133,5mm rods and skim pistons 5mm.
They are genuine cosworth pistons. Would this be possible to skim them that much and what would the c/r be?

xr2wishy

iTrader: (2)

5mm is a lot.
Better to deck the block a bit, but not that much.
Best to sell your pistons and get some new ones to the right spec.

Adam-M

How much wall thickness will be left at the top of the piston once 5mm has been taken off? I doubt there will be much left.

Bart

They have a bowl so only skimming the sides of the pistons. Then it needs new valve cut outs so maybe thats not possible.

stevieturbo

Presumably you have the pistons there to measure and see how they are constructed ? SO you would be best placed to decide.

But you can be sure removing 5mm from the crown of almost any piston is going to leave minimal top ring land, which on a boosted setup is never good.

turbotrev

Out of interest why do you want to raise it?

Cheers Paul

Mark Shead

My guess is it dont need to be that low for any reason other than 95oct fuel.

Mark

turbotrev

Would it matter if it wasn't on that fuel?

Cheers Paul

Mark Shead

If you have your intake temp under control there is no need to lower comp other than to have a wider window of ign for when it goes in to intake retard so it doesnt loose as much hp as a higher comp engine.

Mark

turbotrev

Interesting, nice one

Cheers Paul

Bart

Yes, i want more c/r for more response and power @ the same boost.
Running a KMS ecu so closed loop fueling and good ignition control.
I will buy new pistons because 5mm will be way to much to skim + valve pockets.
Just was thinking to save some good pistons.
Any other ideas to raise it, would like to be around 8/1.

turbotrev

How does a near standard compression ratio give you more response when compared to the lower ratio you have now?

Is it in regards to the timing as Mark mentioned?

Cheers Paul

Mark Shead

Custom pistons are easy to get hold of.

Mark

Mark Shead

Higher comp engines are more efficient this is fact you only drop the comp ratio on turbo engines due to exhaust back pressure and all the problems that come with it.

Mark

Bart

I know but these pistons only did a few trackdays but will change them for customs ones.
Will be another engine with higher c/r and twin scroll 7064 efr.

turbotrev

What kind of problems?

Can you not control the back pressure with different sized exhaust wheels/housings?

Cheers Paul

SHaDoWLanDZ

My guess also...?


Bart, welk achterhuis ga je zetten?

Lekker bezig man!

Bart

T4 twin scroll .92 on the 7064.
I also doing an engine with 6758 wich also is t4 twin scroll .80
Will be on the dyno soon so will post results later.
I did a gt3071 i while ago on a .63 housing and made 432bhp@1.5 bar.

vroooom ptssssh

If your old pistons are going in the bin i'll have them