Dont think im working out cr correctly?
Turbo Mini Forums

So I was over on turbo-mini foloowing this guide on cr:

C is the volume above the piston at TDC. So this includes the head, head gasket, piston dish, piston ring land and any space btween the top of the piston & the top of the block if the piston doesn't come flush/level. You need to add all these volumes together to give C
The standard head is 21.4cc
Turbo head Gasket 3.8cc
Turbo Piston dish 12cc
Ring land 0.8cc

You then need to measure the distance from the top of the piston to top of the block when the piston is at TDC using a DTI.
Unless the piston comes completely flush there will be quite a bit of volume here.
The following chart will give you the figures for a standard 1275 bore, +0.020" & +0.040" overbores.


BORE 0.010" 0.020"
1275 0.99cc 1.98
+0.020" 1.01 2.02
+0.040" 1.02 2.04


I worked out that a 1310cc with omega 10cc dished pistons would have a higher compression ratio than a standard 1275 turbo

I ended up with: 9.85 (rounded up) on the calculator which I presume to be 9.85:1

What am I doing wrong here guys because there is no way it can be higher?

Im using standard head etc and presuming 0.010" as there was a very gap between the piston and top of the block?

Cheers

Craig

You'll probably find the cast Omegas sit around 0.040" down the bore, this gives around another 4cc, so about 14cc inc the dish.

WWW.TURBO-MINI.COM

I hope so I'd like to run more than 5psi of boost =/

looks like your calcs are right,

overboring will raise compretion ratio, assuming every thing else is the same.

also std Deck height is about 20-30 thou IIRC

with the pistons sitting at 0.040 I get a ratio of 9.1875:1 so should be good for about 10psi then?

edit: how would I get it slightly lower then? use deeper cc pistons? get the head machined out a bit as well? Any other ways?

Edited by Craigie-B on 21st Jul, 2011.

yeah that will be ok for 10 PSI,

you can get pistons machined out a bit more (10 each piston) or get the chambers taken out a bit.

the only other option is decopmression plates bit thats a nasty bodge IMO

oh and what Phil said about deck height.