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Home > 998cc > 998 Comp Ratio Question

burnz24

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Member #: 9779
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Hello everyone, only just joined the forum but i have been browsing it for a very long time (about a year)
I am currently toying with the idea of turbo-charging the 998 in my city e while i restore it..

I have been using a compression ratio table posted by Paul S back in 2007 and a boost vs compression ratio graph also posted on this forum but in 2006 by someone (its amazing what you find whilst trawling forums!).

I understand to reach my approx boost figure (12-15 psi) i need approx 30cc with cylinder head and dish volume, this is according to the table i have which assumes a 4cc volume for head-gasket etc and the 30cc does not include that.. i believe (if Paul s is reading this he may remember the table and confirm/correct me)

Trouble is i cant find any definite figures for the head volumes of 998s or what they can be 'taken out to' or the dish cc of many 998 pistons can anyone help?

Also could someone explain the castings for 998 heads i get confused with the various numbers, for instance what is the casting on a standard head, 998 cooper?

Edit; forgot to mention the engine will be +20 so 1014

Thanks in advance
Burns

Edited by burnz24 on 28th Jan, 2012.


jdisel

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126 Posts
Member #: 9515
Advanced Member

ireland

hi,
Heres a link to the various castings and there chamber cc.
http://www.minimania.com/web/SCatagory/CYL...26/ArticleV.cfm

A 12g295 head is probally your best bet, its got 28.3cc chambers as standard, thats what im planning to use myself on a simular project. And then get the remainder 2cc machined out of a decent set of flat tops, (+20 could be hard to find though and not 100% sure whats the most that could be machined into them),the dish on the standard pistons might be too much . The head would probally benifit with larger valves though as it sounds like your after a good bit of power, i think alot of people on here are using 33mm inlets with 29mm exhausts with good results.
I no expert as this is all new to me so dont take it as gospel but im sure the gurus on here will lead you in the right direction.

But more details on your planned engine spec and bhp would help.

Edited by jdisel on 29th Jan, 2012.

A job worth doing is a job worth over doing


Paul S

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8120 Posts
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Formerly Axel

Podland

This thread?

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

The spreadsheat works out what you need, then takes away 4cc for gasket etc.

As jdisel said, a 12G295 is good and will give arond 9:1 with flat topped pistons.

Your target CR depends on your planned boost and if you are using an intercooler.

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."


burnz24

13 Posts
Member #: 9779
Member

aye paul that be the one, would running dished pistons with a 295 (from what jdisel said) not allow even more boost? or would that mean the engine would be really lumpy and slow off boost?

Edited by burnz24 on 29th Jan, 2012.


Paul S

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Formerly Axel

Podland

Yes, although not lumpy. A good intercooler would be a better way to allow more boost.

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."


burnz24

13 Posts
Member #: 9779
Member

so a standard 295 would do the job? port and polished? or should it be bored out?
flat tops would allow me to run about 12 psi of boost with a decent intercooler? and the car wouldnt feel too slow off boost? afterall its going to be a fast road car so i suppose shooting for 'pub' figures is pointless..


Paul S

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Formerly Axel

Podland

I run a standard 12G295 with flat tops on 12psi with an intercooler and make 120hp. Lots of torque off boost but then I'm useing long runners and EFi.

Edited by Paul S on 30th Jan, 2012.

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."


wil_h

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8502 Posts
Member #: 123
Post Whore

Betwix Harrogate and York

I run a similar spec to Paul, and you never get caught off boost, it just drives like a 60bhp mini off boost.

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

Prescott Modified Series Production upto 1400cc course record holder
Oliver's Mount Modified Series Production upto 1400cc Hill record holder
Thoresby Park Modified Series Production upto 1400cc course record holder

www.twin-turbo.co.uk

On 2nd Jan, 2013 fastcarl said:

the design shows a distinct lack or imagination,
talk about starting off with a clean sheet of paper, then not bothering to fucking draw on it,lol

On 20th Apr, 2012 Paul S said:
I'm mainly concerned about swirl in the runners caused by the tangential entry.


Advantage

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Near Paris - France

9:1 CR, 295 head and MG cam (?) is 60 bhp ?
Or is it mostly in the runners and EFi ?

Rusty by nature

On 23rd Jun, 2008 paul wiginton said:

They said "That sounds rough mate." I said "Cheers it cost me a fortune to make it sound like that!"

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