998 turbo engine spec help
Turbo Mini Forums

Hiya i am planing on turboing a 998 or maybe a 1098 but not sure if i want such a long stroke. Question one i am planning on using a 12g940 and will open out the chamber as required not a problem i know with the 1275 turbo engines i dee beaked the chambers to stop pre ignition. i was thinking that the block pockets create edges. Will i have problems here with heat i know some use this head.
The next thing is what diff ratio should i aim for its not gona be too over boosted and will be a road car with a bit of fun too.

Thanks for your time
James

My small bore turbo project http://www.turbominis.co.uk/forums/index.php?p=vt&tid=473901

Surely removing the beak will be detrimental to swirl in the combustion chamber infact possibly leading to pre ignition?.........A 3.44 or a 3.7 should be suitable for a fun boosted road car. But i do like my high reving engines. Possibly a 3.1?

What are you trying to achieve? Are you building from scratch? Why the 1275 head? The sharp edges can cause detonation from the pockets needed in the block. That said if your careful you don't need to pocket the block at all. Just sink the valves slightly into the head so at normal lift the valves clear the deck.

But the big question is why the 940 in the first instance? Why not the 295 or 202? They were designed for the small bore, and require far less work to fit and get right.

The diff I run I'd the normal 3.44 which is fine with a 998 turbo. On the 1293 I'll be going 3.1, as there'll be plenty more torque to make up what's lost on the taller gears.

cheers for your posts. Weather im going to build an engine from scratch or not is what im trying to decide. I have a 1098 engine that i pocketed for the 12g940 head. This is a good engine that i just checked mains on. Or i have 998 that will need full rebuild. I have a few 12g940 heads and a couple of 202 heads to play with but as usual the 202's i have all have shagged exhaust seats so need a fair amount of money thrown there way. I will have a think i got time anyway im fabricating manifolds at the moment. Thanks james

My small bore turbo project http://www.turbominis.co.uk/forums/index.php?p=vt&tid=473901

Stick with the 1098, getting the compression ratio to decent figures shouldnt be a problem with a 940 casting.