what C/R should i be aiming for on my 998 turbo? urgent-ish!!!
Turbo Mini Forums

i have a A+ 998 block that will have to be bored out +20 or +40 depending what pistons are available

with this i'll be using a small bore cooper 12g295 head which as standard has a 28cc chambers
my head like many may have had a skim

mg metro cam,standard pushrods & rockers

what i want to know is, i've been advised to stay with flat-top pistons rather than dished

what do you think? i'm a lil unsure

what would be an ideal C/R & what measures should i take to achieve it the parts are to machined soon

Edited by clubminiflip on 12th Feb, 2007.

SKYLINES ARE LIKE CLITS, EVERY FLANGE SEEMS TO HAVE ONE: see this sticker on my silvia

the Search-section on this forum is blummin superb & simple to use, unlike some other forums, without the search-section & the help of this bunch of chaps on here i'll never be able to !!!Start the day with a 998cc turbo!!! :) i'm getting closer though!

Tom Fenton says: I have it all, make me an offer

First thing to do I would have though would be to measure the chamber size of the head. That way you will know where you are working from.

As for ideal cr, it depends on what you want the car for, how much boost you are running, etc. My safe bet would be around 9:1, but that is because I am aiming for that on my road 998 (+20).

Don't worry too much about pistons. The mega pistons are available (although Wil tells me they are not always that available) and we have had a 6cc dish machined in them no problem. They are yet to be run and tested though in our engine.

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A man without a project is like a like a woman without a shopping list.

I'll endorse what Ben says, - 9:1.

That can be achieved with flat topped pistons on a standard 12g295. You can check if it has been skimmed by measuring the thickness - it should be 2.75" if untouched.

With 9:1 and a good intercooler I'm hoping to be able to run 12 psi boost and get about 120hp - theorectically - yet to be tested.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I'm going down the same route for my road 998. Only planning on 80 to 90 bhp at 7 or 8psi though.

The TT has a CR of around 8.5:1 and we run upwards of 15psi.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

Mine has just a shade under 28cc after a light skim, on a std bore setup, should give about 8.8:1 iirc

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"

whats the calculation for doing this? was'nt there a lil pc program to do this for various specifcations of motor

i'm going to be using a 295 head, very minimal skim, just to be sure the face is flat

& probobly standard flat top pistons at standard bore size

any rough ideas what i'll end up with?

i'll post more detail when i can get it if it helps

Edited by clubminiflip on 14th Feb, 2007.

SKYLINES ARE LIKE CLITS, EVERY FLANGE SEEMS TO HAVE ONE: see this sticker on my silvia

the Search-section on this forum is blummin superb & simple to use, unlike some other forums, without the search-section & the help of this bunch of chaps on here i'll never be able to !!!Start the day with a 998cc turbo!!! :) i'm getting closer though!

Tom Fenton says: I have it all, make me an offer

You need to be sure of the head chamber size.

Most 12g295s have been heavily skimmed as they were originally made for use with raised dome pistons.

Start by measuring the thickness of the head.

If it is still 2.750" it should be OK.

Edited by Paul S on 14th Feb, 2007.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

well the head is in for recon/mods & machining, i measuresd it with a loverly presision plastic shatterproof whsmith ruler before it went & it seemed to be 2.750, thats as exact as i could be with the quality tool i have hehe!,

i suppose the best option is measure the cc's when i get it back, they have been informed to just take the most minute amount from the head face just to be sure its flat & it'll be unleaded to make life easier it'll have some other mods too a small amount could be taked out of the pistom tops could'nt it, a light dish to get a few ccs back

SKYLINES ARE LIKE CLITS, EVERY FLANGE SEEMS TO HAVE ONE: see this sticker on my silvia

the Search-section on this forum is blummin superb & simple to use, unlike some other forums, without the search-section & the help of this bunch of chaps on here i'll never be able to !!!Start the day with a 998cc turbo!!! :) i'm getting closer though!

Tom Fenton says: I have it all, make me an offer

thats what i had, just a quick flash to check its flat and give a good seal.

The lad who did mine said it would only take a couple of thou at most.

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"