compression ratio with 18cc pistons
Turbo Mini Forums

So I got some 18cc pistons yes I know they ain't the best but at 230 quid couldn't say no.
First question roughly what should I add to the equation for the head gasket when working out the compression ratio?
Secondly I know I need to skim the head to get decent compression but how do I work out how much by?
Third question would it be more effective to skim the block slightly so the piston are out of the block as the vizard bible talks about?
The extra dish would give me a bit extra clearance between piston and valve?

I want about 8.5 for the ratio on a 1293 if anyone just feels like doing the math for me :p

3.8cc if you using a bk450

you cant do the math without knowing what the chamber sizes are its worth doing anyway to get a starting point

Edited by wolfie on 22nd Sep, 2010.

Doing it backwards to work out what the chamber size should be for 8.5 I meen.
Then need to work out how much to skim of head and maybe block.

THe AFRacing spreadsheet does it all "backwards" easy enough but you need to quote your current deck volume (how far down the bores are your pistons at the moment?) and the ring land volume to give an accurate answer for the required chamber size.

Schrödinger's cat - so which one am I ???

What pistons are they? If accralites, they sit 0.8mm down the bore.

In the 13's at last!.. Just

Omega

Skim the block so that pistons are 0,5mm down the block. This way you get a decent squish area and material for another skim later.

Mental note made

If something is worth doing, it's worth doing half of.

Schrödinger's cat - so which one am I ???

i would do as V said above.
Got the chart Rod thanks worked it out using that

so the final question how do i calculate how much to skim by?
i meen i know how many CCs to cut down by but is it a case of a little at a time then messure and repeat?

For the block it's just ((pi * d^2 /4) * skim) to get the cc.

For the head, if you can estimate the CSA of the chamber shape at the face its just CSA * skim.

A quick and nasty way to do the CSA is a bit of transparant metric graph paper, trace the outline and count the squares....

Schrödinger's cat - so which one am I ???

You need to decide what size chambers you want, set the head up level, fill one chamber with the required volume of parrafin & using a depth mic measure from the head face to the top of the liquid in the chamber. This will tell you how much to skim off.
There may be other ways but this is what I do.

Edited by Turbo Phil on 23rd Sep, 2010.

WWW.TURBO-MINI.COM

In my experience on a std A head. 0.3mm equals approx 1cc. On a modified turbo head obviously, this is different.

Well i worked out that a solid 20cc chamber allows me to have 8.3:1 ratio with the 18cc dish :)

that might be a ittle low, my old 1355cc was 8.3:1 and it would have been a nicer drive a little higher

Edited by Brett on 23rd Sep, 2010.

Yes i moved to the darkside

Instagram @jdm_brett

Just guna skim the block down to 0.5 brett
Just realised Il have to enlarge the chambers anyway will I no going to be running
31 exhaust and 36 inlet so wont have a decent idea of size until then

dont just take 0.5 off... take just enough off to ensure its flat and measure whats left, calculating it in.. makes more scence in the long run,

Yes i moved to the darkside

Instagram @jdm_brett