CR / bore / PSI ?
Turbo Mini Forums

Hi all, im sorry if this has been asked before or sounds like a dumb question but i have attempted to do some reading on CR's and use Alex's compression ratio calc on the AF racing site.. but i still dont really understand it or what i need to achieve.

i would like to know if my bore size of 71.12 and head chamber volume of 30cc will give me a 'good' / 'ideal' CR.. (think this is a shade under 8.1.1)

and what psi with an intercooler i can sensibly run?

im a novice when it comes to this so not sure if i have provided the correct info for you

hope you can help .

cheers, Mark.

Edited by MarkGTT on 18th Nov, 2006.

what pistons? std metro turbo?

assuming std turbo pistons and an engine in good condition (or rebuilt) you should be ok at 10 to 12 psi boost with a decent size intercooler with an applicable needle and dizzy.

as an example Alex B has run 15? psi on std metro pistons with the bowl machined out to give about 8.5:1 But this is with a rebuilt engine with a 2wd cossie intercooler.

Hi, thanks for the reply.

it’s a rebuilt engine and box, the pistons are +0.02 not sure on the make but should give me about 1293 Engine Capacity i think, and I have a MED cylinder head with a chamber volume of 30cc over the standard metro one that’s 21.4cc. I will also be running a Renault 5 Turbo intercooler. Thought i would of been able to run a bit more than 10 psi but would be limmited by the gear box... but there will be others out there that know loads more than me!

Edited by MarkGTT on 19th Nov, 2006.

in that case you should be able to run more assuming the 8:1 is correct, do you have any idea of the piston dish size?

what is the gearbox, like you say this could well be the limiting factor, if its std i'd be worried at 10psi.

No idea on the dish size.. sorry. and as far as i know the box is standard. If i can afford it in the future this will be uprated.

Sound like i should keep it to 10psi just to be safe.

cheers.

yeah, i'd advise doing something with the diff as this is the weakest point,

idealy a 4pin diff ( this can then be put in a SC box when funds permit) but failing that a hardened diff pin, if the engines out its not hard to do,

also just a bit of machanical sypathy goes a long way, a std box may well survive with 140 turbo bhp and a sypathetic driver, the same box, may go bang on a 998 with a complete hoolagan behind the wheel.

what really kills a box are wheel spins and burnouts (unsuprisingly) basicly heavy shock loads and any situation where one wheel is turning much faster than the other.

a mate of mine thought he was well clever making his 1100 mini go in tight circles with the hand brake up.... until mr diff pin decided to go on holiday via the back of the casing.LOL

anecdotes aside get it on the rollers and adjust the boost for 120 brake tops but thats just me, you don't want to be widing the boost up with an unknown needle.

turbo phils site is a very good place to refer to, it covers all the basics and has some great advanced stuff.

http://www.turbo-mini.com/43348/9737.html

Cheers for the sound advice, im past the wheel spins etc now, but want to give it some stick on the track at some point. So i will take your advice, 120bhp and sensible driving :) until funds permit for me to push it harder.

thanks again.

enjoy!!

obviously a decent box is on the shopping list, after this i'd recomend ditching the dizzy and going megajolt, will make the car generally nicer, well worth the money.

funny you should say that i have been reading up on megajolt over the past few weeks, looks like a great option. How difficult is it to get the timing right with the megajolt system? because its currently a pain with the dizzy.

The main advantage to the megajolt is that you can retard the timing as bost pressure increases. It is difficult to achieve this with a dizzy.

yes, what Tom says, also being 3d (jolt is 3d right? I've only used megasquirt) part throttle can be advanced up making cruising much more efficient.