12G940 Head on 998 with decomp plate?
Turbo Mini Forums

Hello all, great forum

I have been putting together a turbo kit for a 998, I am only really doing this for fun on an old mini for my lad, and dont want to spend £££ on new pistons etc as my other car is what i like to spend on (kit car with metro turbo challenge engine). its gone through a set of sc gears and a set of drops in the last 3 months so money is tight


I have been lurking for a while, using the search function in this section of the site to guide me on this 998cc project, but I have a couple of questions if anyone has a minute

1) anyone know or rough guess of the CR if using an unskimmed 12G940 head on 998 with the block pocketed.

2) If the above CR is too high, would using a 9cc decomp plate and two head gaskets increase the deck height sufficiently so as not to need to pocket the block? I know decomp plates are frowned upon on this forum but i've used them in the past with no problems so far

3) Any ideas for a needle to start with? i understand it will likely need filing but would like a starting point. the carb i am using is a HIF44 off my old maestro turbo

i think its the same as the metro unit. I have removed the electronic choke and fitted a manual one, of course it has the benefit of a plenum chamber with the intercooler takeoff already, which is great as i plan to use a landrover td5 intercooler i have here.

Any help would be appreciated.

Kev

Why not just work the chambers on the 940 & get a suitable compression that way ? No need for the cecomp plate & two gaskets then.

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3) The maestro carb is as you point out identical to the metro one, just the auto choke and the needle.

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

I've said it before and I, no doubt, will say it again.

The 12G295 head will give 9:1 compresion, perfect for a low boost turbo and no need to pocket the block.

You'll not notice the performance difference unless you are using high revs.

They are readily available at shows and eBay, although they are getting pricey.

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."

decomp plates are a bodge as you know and you WILL end up turning up the boost as it is so easy and so tempting, the 295 head may be more money but will save costs on pocketing and the decomp plate

thanks for the replies. food for thought :)

Hello again

having a dig through the garage and found a 12G202 head. not sure but think it was off a 1100 midget.

I have measured it and come out with the following

26.2, 26.1, 26.15, 26.2,

theres quite a beak in the chamber, and the exhaust valve is very shrouded, but looks to be lots of material that can be removed.

Assuming that this head will mate up to an A+ 998 block, I think it may well drop the compression down a bit


I don't have a standard 998cc head to hand, does anyone know the chamber measurement of an unskimmed A+ 998 head?

The 202 would be a good solution. particularly if you open it out a couple of cc by unshrouding the valves.

It will fit the block, no problem.

Cc of the standard head is 24.5cc. That gives around 10:1 compression on flat topped pistons, I believe.

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 just did the math (which prompted me to come on this forum to search for 998 turbo options).

Here are all the figures I used:
21.8cc head (I measured it--12G940 metro head that's been skimmed at rebuild once)
5.8cc piston dish
.25cc (yes, pretty small) for pockets so far. largest I've found others make was .6cc
2.8cc (quoted not measured) for copper head gasket
.26415cc for the ring land volume (piston is 2.525" at top and ring is .225" down from top of bore)
.018" piston to deck height so 1.4981cc

Unswept volume is 32.4123cc

Swept volume is 249.692cc

Unswept + swept / unswept is 8.7036:1

Hope this helps!

Off to search the forum for cam and turbo options!

edit: Double post, sorry. Board has horrible lag :)

Edited by Spank on 21st Jul, 2009.