Anyone running 12.5/1 ish compression ratio?

Blimey colin, can you run unleaded on that kind of compression ?

only if it's about 115 RON

that's some compression. i'm lucky if iget any at all with mine
tom

Hows the rebuild tom ?

will you make stoneleigh ?

Well I'm currently running 18:1 compression and 1 bar of boost from a turbo......































... it's my Audi A4 TDi!!

[Edited on 2/2/06 by bimbleuk]

Colin,

My intruder pistons are 11.5:1ish but i'm running a 1.9mm headgasket which brings it down to about 11:1 (standard is 10.5:1)

There is a guy called fat arnie with a vauxhall caterham who is running near that CR if memory serves correectly, maybe worth a quick search on blatchat.

Ned.

bike engines?
stock yamaha r1 is 12.4 (on the current engines) and if you swap the head gasket for the thinner racing one it goes up to around 13.5 but that requires the head chambers to be polished to stop detonation on pump fuel and the water temps to be kept low.

Touch of methanol in the juice damps down the detonation.

Also helps to keep any moisture that gets in the system at bay. Not that you would want to put water in the system.

You've got t be careful with methanol, it'll eat away your fuel system if it hasnt been designedto take it.

Thanks for the comments.

The reason for asking is that I'm considering fitting forged pistons and the options range up to 13/1.

I'll probably opt for the 11.6/1 option to keep my fuel options open though.

Cheers,

Colin

Millers CVL in Super Plus should be ok

expensive if you use the car alot though

when i had my celica GT4 import it was still mapped for jap fuel, covering 200 miles a week cost me 45 quid a week using super unleaded and octane additive.


The thing with compression is it changes with the RPM and varys on the camshaft timing.

static it will be 12.5:1 but dynamic compression changes as the rpm increases. not so much on car engines as they do not rev high enough but bike engines show this more because of their higher RPM limits.

with a lairy cam with a long intake duration the air velocity at low RPM is slower and the mixture inside the cylinder can be forced back out as the piston is on its compression stroke, this lowers the compression as there is not as much inside the cylinder to compress.
at high RPM the velocity of the air entering the engine is that much higher that as the air enters the cylinder it creates a vacuum behind itself drawing more air in thus increasing the compression as there is more to compress.

this is why race cams idle so poorly.

what you need to find out is how the cams you are using will effect the compression. bike engines using very lairy cams require domed pistons to increase the compression so at idle it will stay running.

Jag V12 H.E. (high efficiency) engine runs 12.5:1 CR.