What Cylinder Head / turbo?
Turbo Mini Forums

Hi All

I have been having a good read of the forum and the 998 section in specific for a while now. Today I finally brought myself a new 998 block with the intention to turbo it. I have a standard 998 at present but have just brought a block that has been bored +30 and also has pockets for a 12g940 head the plan is to use this along with parts from the original 998 to build a turbo motor.

A couple of questions I have:

Should I stick with a standard 12G940 head or should I be looking for a modified 12g940 or something along the lines of one of turbo Phils modified turbo heads as I see these use the 12g940 as a base?

Also I have seen most people are using a T2 on a mirage manifold for a 998 but don’t really want to go to the expense of a mirage manifold. What sort of results are people getting with the standard metro T3? I assume there will be more lag compared to a T2.

Any advice would be appreciated, as I will be looking to build up the parts needed over the coming months before attacking my engine.

Edited by dan_w on 21st Jan, 2009.

Well with a 12G940 head you will at least need modifications to get the desired CR for your level of boost. They have a very small combustion chamber to start off with (21.4cc). I done my own head work initially but then when I discovered that it was cracked I got pissed off and then went with one of our resident experts. You won't be disappointed if you spend money getting a cylinder head prepped professionally

As for manifolds, you could always make an adapter plate to run a T2 on a Metro manifold. Or you could make a manifold from scratch

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

I am guessing that this is for a road car? If so I would not recomend the T3, it does work and we made 140 bananas with one, but it was horrible to drive. T2s, GT15, GT17 are all options and can be easily fitted to the standard metro manifold with an adaptor plate.

As for the head, again we ran 140 bananas with an almist standard 12G940 head. The only modification was to the combustion chamber to lower compression.

What cam and pistons are you planning on using?

http://www.twin-turbo.co.uk
http://www.hillclimbandsprint.co.uk/default.asp

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

Thanks for the responses guys. I will most probably look at getting a modified head once I know the correct CR needed for my engine then.

Ben, the car will be a mainly a road car with t&t but wont be a daily drive it will be more of a weekend toy / track car so I am not to bothered about drivability around town etc, just want a nice fun car to drive when the sun is out:).

The block came with a crossdrilled, wedged crank and powermax pistons on lightened and balanced rods. As for a cam what can you recommend? I have read that the standard metro cam or kent 500 are a good place to start. With the standard metro cam this would be a 998 cam I assume?

As for the adaptor plates for the T2 I had a search and see a member was selling these. does anyone know if they are still available.

Again any advice is really appreciated and any suggested specs would really help.

Thanks, Dan

Well if driveablilty isn't an issue then I'm guessing you are looking for HP as a priority. In that case, a ported 12G940 will give you the best HP whilst compromising torque.

Again, cam choice is very subjective. Wil and Ben have had greatest success with the Metro NA cam. They did try an Avonbar phase 2 but that made the car too undriveable with nothing low down. I will be using an SW5-07 with my 1098 as I'm going for torque.

Get searching and you will find any answers about head and cam choice you need

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

Hi Spanner...
I wouldnt say drivability isnt an issue, i just wont be using the car around town so dont need it to be nippy that low down.

Again I am not after monster HP if it just dosent have the torque to deliver throughout the rev's.

Again i am new to this so dont shoot me down in flames if i am talkin S*** but is it possible to use the T3 with a ported head to deliver the HP but use a cam that would even this out a bit with some torque lower down.

Thanks again for the advice.

Don't worry, I'm not shooting you down :) I'm a novice to turbos too. I'm just a sad fuck that spends too much time on here.

Unfortunately I think a T3 seems to be a lost cause on a small bore. The 1 litre engine capacity just doesn't seem to create enough airflow to spool the turbine regardless of cam choice.

As Ben said, the 3 turbos you should be concentrating on are the T2, the GT15 and the GT1752. GT are just the new generation whereas T are older stuff. There are obviously manufacturers other than Garrett but there is a wealth of knowledge about these turbos here. So us inexperienced shouln't get too adveturous with turbo choice.

I suppose budget would be the biggest factor as to what sort of 12G940 you use but a standard head will produce good results.

As for cams, again stick with what is known. Either the NA Metro cam or SW5 would be a good choice. Alot of people have used the Metro cam on small bores here and made no sacrafices with driveablility. Nobody has used an SW5 with a small bore yet but the general consensus is that it won't produce as much HP as the Metro but it will produce a bit more torque. We will find out once my turbo is up and running.

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

Well, I have been doing hours and hours or resarch on here and other sites and think i have finally decideed on a spec for my engine build. As before I dont really have a set BHP or anything in mind but looking for a nice car to drive on the odd sunny weekend we have here in England and take it to a few track days.

Any way let me know what you think. Anything I should also be looking to do or not do.

998 block bored +30 and pocketed for 12G940 head (already have)
Wedged and cross drilled crank (already have)

Standard (or very lightly modified) 12G940 head
Standard Metro N/A cam
Metro turbo carb
Dump valve
Intercooler
Boost valve
Uprated oil pump
Alloy fuel tank with return
Fuel pump
Braided fuel pipes
T2 turbo (just brought one that needs a rebuild)
Standard metro exhaust manifold with adaptor plate for T2
Standard metro turbo Fuel pressure regulator
Oil cooler

Questionable extras any advice / comments more than welcome.

Looking to remove my heater and fitting a electric fan to replace fan on the engine at present.
Fit megajolt
Have centre main machined for my block
Fit a X-pin diff

Thanks

What about pistons.

Std flat topped pistons will give too high compression ratio.

I suggest some 20773s.

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."

A modded 202 will lower CR quite a bit without much problems and will flow in the same range as a 940 head with some mods ( not full of course).

The weird pocket you get from the overhanging valves when using a 940 head just seems wrong..

you could minimise those by going up to 1061cc bore

That sir, is not rust, it is the progressive mass reduction system

http://aseriesmodifications.wordpress.com/

http://www.twin-turbo.co.uk
http://www.hillclimbandsprint.co.uk/default.asp

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

Your spec looks good.

If you want to fit a steel backed oil pump to a small bore block then you will need to either make up a spacer or sufficently shorten the drive. PaulH on here sells steel backed oil pumps to suite small bores if you don't want to do it yourself.

Also, the bolts supplied with centre main straps are the lowest quality "high tensile" bolts available. You should probably get a hold of higher quality alternatives. There is a thread on here somewhere talking about alternative suppliers of these bolts.

Alot of people on here don't see the need for oil coolers. I believe even Jimster's car doesn't run one. Maybe just fit oil pressure and temp gauges to monitor things

Megajolt would definately be a sensible addition. There are absolutely no disadvantages with megajolt when compared to a dizzy.

A cross pin diff is a good idea. You can basically fit and forget whereas you would always be questioning a standard diff with so much torque

Edited by GaryOS on 11th Feb, 2009.

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

Thanks for the response guys.

Paul...
I have the pistons that came with the block they are powermax pistons. Don’t know a lot about them apart from the fact they are +30 thou, offset and have flat tops. I cant seem to find out a lot about them. As for the Compression Ratio this is something I still need to do a lot more research about to get my head fully around and hopefully once a have got myself a cylinder head I will be able to work things out. What sort of CR should I be aiming for if I want do be running reasonable boost?

Gary...
Thanks for the advice like you say I may save myself a few pennies and scrap the idea of an oil cooler and just see if there is a need for it. The cross pin diff is something I really want to fit, as you say it is just a worry of my hands. All depends on spare cash and if my gearbox needs a rebuild. Again something I need to look into a little further.

Thanks for the advice about the centre main strap, I had no idea about the bolts I will have a good search a bit later.

Wow so much to take in I am just starting to get my head around it all. This is such a helpful site I never seem to need to be posting anything.

Cheers guys

That head and piston combination will give you a compression ratio higher than 10:1. You should be aiming for less than 9:1.

Either the head or pistons will have to be changed.

With the block at +0.030" your piston choices are limited.

I would fit a 12G295 head with 28cc chambers as standard.

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."

As long as the head hasn't been skimmed

Ah, but for the price of a decent, unskimmed 295, you can get a suitable 940 with the chambers out to 28cc (ish). that is if you can even find an unskimmed 295.

http://www.twin-turbo.co.uk
http://www.hillclimbandsprint.co.uk/default.asp

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

I think we have a few hiding away in safe keeping :)

EDIT: hmm i know one has that gone on Paul S's engine but pretty sure we have a few other unskimmed ones.

Edited by sturgeo on 12th Feb, 2009.

Actually a 28cc chambered 940 on a 998 is a pain because the chamber will have to extend over the bore to achieve it.

Best to keep the chamber shape within the bore other than at the pocket area.

You should be able to get a few ccs taken out of the pistons. If you can get 5 or 6 then all you need is a 24cc chamber to get around 8.5:1 which will let you run lots of boost

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

www.twin-turbo.co.uk

im just planning my engine build at the moment and this is one of the issues ive got, because im a tite arse and dont want to fork out for some new dished pistons, i was thinking of putting a thicker copper head gasket in to raise the CR (as well as some head work to enlarge the chambers). is this a totally stupid idea?lol
but like wil h said i dont realy want to extend the chambers over the bores when i enlarge the chambers on the head.

have people done this method before using a thicker gasket?

You can buy decomp plates for the 998.

Look at the pisons thwy may be able to be machined, mine cost around £35 to be done iirc

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

www.twin-turbo.co.uk

has anyone done this using a decomp plate?

about how much do machine shops charge for enlarging the combustion chamber on a head?

cheers

danboy if you want the chambers enlarging please PM me

Thanks for the info guys. In relation to having the pistons machined would I be able to get this done by a normal machine shop or someone more specialised in cylinder heads.

Benross, is this something you can do along with a cylinder head once I have one?

I am slowly getting my head round all this. Received my turbo the other day and just starting to work out what everything is on it and what connect to where.

Thanks again

i have a set of brand new std size 20773s with rings if you like, i ended up going for full skirt flat tops rather than skim my 295!

was just gonna keep them in stock, but if theyre any use, let me know

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

"did you know you can toast potato waffles?"

Thanks for the offer Joe, But I think I will have a look in to getting the pistons machined to acheve the CR needed.

Cheers