Whats the optimum Compression Ratio?

[rysmee]

hi there - i know its a general question as it greatly depends on engine work carried out etc etc but whats the best comp ratio to aim on a 1600 8v without the risk of it detonating?

[gorilla1q]

When I rebuilt my 1400 8V engine a year or so ago I was aimimg for around 11.5:1 compression ratio.This though was a competition engine and the high ration was required to extract its potential power. But standard your looking around 10:1.
HTH

[rysmee]

yeah thats cool. it will be comp spec when finished but trying to take material out of the pistons and then compensate to give a desired comp ratio isn't the easiest thing to do!

[meritlover]

why are you removing material from the pistons?

[Stuart]

depends on the revs you plan on going to and the cam profile (amongst a million and one other things)

10-10.5:1 tends to be "sensible" though, much above that and you usually have to back out the ign so much to stop it deting that you start burning exhaust valves out (with primative dizzy ign that is)

[meritlover]

depends on the revs you plan on going to and the cam profile (amongst a million and one other things)

10-10.5:1 tends to be "sensible" though, much above that and you usually have to back out the ign so much to stop it deting that you start burning exhaust valves out (with primative dizzy ign that is)

as said, depends on loads of factors, biggest of which being VE and octane of fuel.
best gains with mappable ignition. but even small increases to comp ratio are noticeable as a crisper engine. We took 40 thou off a 1.2 ported and cutback valve stem carb head and the difference was extremerly satisfying, we were even able to advance the timing a few deg over book figures without any det or even pinking.

[rysmee]

ok - well i'm running omex on boddies and taking material out of pistons to allow clearance to valves for 308 degree cam. so naturally the ratio is going to be affected by that straight out. the guy who is doing the work for me asked what sort of comp ratio we were aiming for so i said i know just the place to ask!.....being mapable and with a vernier should give the widest facility to work around timing / deting issues but without a ratio figure to aim for i wasnt sure where to go next.

[meritlover]

i wouldnt jump straight into a high CR, i would aim for 10.5:1 then see how it goes at the end of the day, its only £25 a skim and £8 for a gasket if you wan to increase it further. i dont think youl get much bother with det as the cam has poor ve until high revs, so youl have to use lots of adv at low rpm then see how it goes around the high load/RPM sites.
having a high overlap should pull a lot of charge straight through the cylinder removing a lot of heat, which will help reduce chances of det.
im sure you will have no problems. you have a nice engine.