32 replies to this topic

[lewistranter]

Right then, due to the soaring prices of 1275's i've decided to go down the turbo route with a 998 :)
i know this has been done a few times and i've seen it produces good results!
So i bought a 998 complete with all ancillaries and gearbox on here for £80.... not bad
Also, i've read that the turbo 998's can produce good horses, yet still be very economical and easy to drive around city streets, you cannot get this on a 1380.
I'm planing on the following spec, feel free to correct me with certain parts, but i've done a lot of research and looked all over the yellow bible to get this right...

-12g295 cooper head- ported, polished, bigger valves etc
-standard pistons
-maybe +30 thou overbore (depending on bore condition)
-electronic ignition
-hif6 carb
-lightened/ balanced bottom end
-t2 turbo
-mirage motorsport manifolds
-intercooler
-MG metro cam
-running around 7psi
-1.5 rockers?
Thats the engine spec, obviously on top of that i will have to consider oil parts, fuel pump, fuel return etc.

I've done my research on the pistons, the standard ones are more than up to the job, also with the 295 head i was hoping that the large chambers will keep the CR low enough for a turbo so it doesn't blow the head off... and hopefully the CR will be high enough at low revs to help with lag.

This is the beast here... ;) just a standard 998 as you can see, done around 70,000 miles

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And then the gearbox will be fully rebuilt and i will add a racing clutch, cross pin diff, and possibly straight cut gears to cope with the added stress.
So for now i will be looking for some good advice on the project... for now though the engine is getting a full rebuild.
peace
x

[holmesy]

im thinking of building a turbo 998 for when mine gives out so will be watching this closely :)

[Wil_h]

You're pretty much there. For a road car I'd forget the 1.5 rockers, they'll increase the boost threshold and not make any more power.

For a 7psi target, any psitons will do the job. You'll need a set to take to the machine shop if you are planning a re-bore, not sure what +30s are available, +20 or +40 is more typical.

If you are using a carb, the easiest way is to use the HIF44 metro turbo one with the metro turbo plenum. You could use something else, but it's a pain.

[lewistranter]

You're pretty much there. For a road car I'd forget the 1.5 rockers, they'll increase the boost threshold and not make any more power.

For a 7psi target, any psitons will do the job. You'll need a set to take to the machine shop if you are planning a re-bore, not sure what +30s are available, +20 or +40 is more typical.

If you are using a carb, the easiest way is to use the HIF44 metro turbo one with the metro turbo plenum. You could use something else, but it's a pain.

Thats great cheers, i'll definitely follow your advice, you seem to know a lot about fast turbos! so standard pistons with a 295 head will give a good all round CR?

and yeah sorry, i meant it'll be +40 so that gives 1030cc doesn't it?
Metro carb it is!

so do you think this perfectly reasonable for a reliable, nippy car? i reckon i should easily be able to get around 80bhp?

[lewistranter]

^{I had some time earlier so partly stripped the engine..}
I stripped off all ancillaries and then started on the head...
The head was actually really good, no dirt it was very clean and the rockers looked nice too!
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Then the Rockers came off

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It was so clean!

Then Pushrods...
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then the head came off,
There is a lip so it will need boring out, but thats not a problem!
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[Wil_h]

A 295 has 28cc chambers, if you stick that on a 998 with flat top pistons you'll get around 9.3:1 CR, which is fine for the psi you are planning. With the extra cc, with the over bore the CR will increase slightly, but should still be ok. Personally I's just give it a +20 overbore as you won't gain anything with the extra ccs.

[lewistranter]

A 295 has 28cc chambers, if you stick that on a 998 with flat top pistons you'll get around 9.3:1 CR, which is fine for the psi you are planning. With the extra cc, with the over bore the CR will increase slightly, but should still be ok. Personally I's just give it a +20 overbore as you won't gain anything with the extra ccs.

yeah thats a good point, i could always buy some dished pistons seeing as i'll have to buy some new overbore ones anyway, the price difference shouldn't be that much,
cheers

[Lyford01]

Sounds good I wish you luck with it all

[danny79]

i could be wrong but with standard dished pistons and a 12g295 head the CR ration is 8.3:1.

[Brams96]

What about the same spec engine but with a 12G940 head? How would that affect t CR?

[Wil_h]

In all engine builds the CR is a critical factor. It would be a better idea for you to spend 1/2 an hour reading about it then working out what is what, rather than me spoon feeding you the answer. It's a very simple sum.

[lewistranter]

Update, i've just bought a 12g295 head off ebay, i'll check the chamber volumes when i get it as im not too sure if its been skimmed, looks okay though :)

[coopdog]

Shouldn't this be in the engine build section?

[lewistranter]

I debated this myself...
But I decided that I'd do it here, this is where most of the turbo boys look.
You're doing a build yourself at the moment aren't you?

[lewistranter]

This is the 295 head, i'll get it cleaned up and painted a proper colour, yellow ;)
Also, does anyone know the standard 998 headgasket cc? just for CR calculations
Just found the head has a crack where the stud goes in, looks like someones bottomed it out and tried to tighten it too much, but its only little, i think i'll just leave it there as the crack isnt too bad and it would be much more work to remove it, any comments?
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I may be wrong, but it looks like it has been ported a little bit more than factory spec, the middle exhaust port is also larger in comparison to the other two