few compression questions?
Turbo Mini Forums

what is the ideal compression for a 998 turbo?

im going with the 12g295 head which im pretty sure has got 28.3cc chambers

i would prefer to skim the head and the block so i know ive got a perfect seal. although the surfaces do seem flat with a metal rule. can the machinist check the serface is flat with something a little more accurate than a ruler.

how much should i skim and would i want flat top or dished pistons ive got both. they are the standard mini items.

Edited by 8ballmini on 12th Jun, 2007.

i was in th same position 8ballmini!

i have gone with a a+998 block, 295 head & i've bit the bullit,

i dont know how much morespeeds is skimming the head by but its a very minimal amount & the machinist i use for block work has taken just 5thou off the top of my block as it was needed for the headgasket to seal properly!

i think it is the right choice in my case, at the end of the day a turbo blowing air in the cylinders is putting the joints under a bit more strain than a NA engine in my view!

i also have gone for dished pistons but have the chance to use flat top pistons! but i wont know what type to go for until my head comes back from machining as i'll check the chamber volume when it gets back! i'll make my choice (usually with advice from the chaps on here) from there!

i'm not sure on the ideal compression but i think most aim for around 8.5-1ish!

hope this helps

dave

Edited by clubminiflip on 12th Jun, 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 decide first of all what you want from the car and the hp level you are aiming at.

Once you know the hp, you can calculate the boost required to achieve it.

Once you know the boost, you can decide the compression ratio.

You then calculate the cylinder head and piston dish volume required to achieve the compression ratio.

If you make too many guesses along the way it will end in tears.

As a guide, my own combination is 998 +0.040" giving 1030 cc.

I'm planning on approx 120 hp for which I need about 12 psi boost.

With a good intercooler and mapped ignition, a compression ratio of 9:1 should be OK.

To get 9:1, I'm using an unskimmed 12G295 and flat topped pistons.

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."

skimming of the head & block plays its part though does'nt it!

axel as i'm a novice to this lark! what C/R will my setup give me, its a +0.020

either dished or flat-top pistons

295 head with a minor skim

i'll be using megajolt & a series 2 rs turbo intercooler

sorry to hi-jack the thread, were all on the same page though i reckon

my head is being worked on so i think the chamber volume will still be 28.3cc's

cheers axel

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

By unskimmed I mean a head that has not had a lot of material removed to raise the CR. A few light skims is OK.

You can work these things out for yourself:

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=124316

and:

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=130839

Edited by Paul S on 12th Jun, 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."

thats helped a fair bit that axel, i feel safer now :) i'm not too far wide of the 8.5-9 mark!

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