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Hi, I'm considering doing adding a turbo to my Mini next year but need some advice.
Currently I've got a Morspeed build 1330cc (+60) engine with about 30k on it. It's got a Morspeed Phase 2 cam, standard 12G940 head, 9.75:1, MegaJolt, HIF44, LCB. Last time I had it RR'd it was about 75bhp flywheel.
My plan would be rather than a max-BHP monster, more of a torquey reliable everyday driver. Mini gets used all weathers and for all sorts of stuff - daily commute sometimes, holidays sometimes, shows, etc. At the moment I'm getting 38MPG town driving, 45MPG long distance A roads, and 40MPG motorways @ 80mph. I think I've got a 3.4 diff. I would like a higher geared diff to make motorway cruising and MPG nicer.
Anyway, these are my plans which I need some thoughts on:
Pistons - Pistons are +60 8.4cc 21251 pistons. Someone told me they are the cheapest, and really for performance I should swap to 21253 or forged. I think forged are probably OTT for my build, but should I keep 21251, or upgrade to 21253?
Deck Height - Currently my pistons sit 65 thou down in the block. Is this a good or a bad thing? When I got into tuning my old Triumph years ago, I was advised to go as high as possible with pistons - 10 thou above block was recommended with composite head gasket. With the Triumphs, this gives a better burn, and reduces pinking. I was told the head chamber was designed for combustion, so you want as much of the mixture in there rather than at the top of the block above the piston. So, should I have the block decked so pistons are 5/10thou over?
Head - Currently using standard 12G940 with 20.4cc chambers to get compression to 9.75:1. I have a good condition MG Metro head (actually came from Maestro) with 35.7mm inlets. I'm planning to have this converted to unleaded, and use Vizard's guide to remove some of the restrictions. Are the standard exhaust valves an ok size, or would I benefit from larger?
Compression ratio - Currently at 9.75:1. I'm planning the turbo engine to be a nice torquey, fast spooling, economical engine - so, I was thinking 9.5:1 compression? I know higher than most but I'm not looking for massive boost.
Turbo - I've been thinking about using the largest non-variable GT15 and using up to about 7psi with an intercooler. Possibly at a later date I might add a Water Injection kit and push it to 10psi. It looks like most use GT17's - is that because the GT15 is too small, or just because they are looking for more boost than me?
Gearbox/diff - gearbox will be rebuild (Roger Bayliss) and diff will be swapped out for a higher geared one - I'm thinking 2.76 MPi diff. I'm hoping with a small turbo, with high compression it should spool up fast enough to pull this fine.
Carb/Ignition/Exhaust - The plan will be Metro turbo carb/plenum, MAP based Megajolt (already fitted), and custom made exhaust manifold/downpipe.
These plans are based on a bit of research I've been doing, but I'm definitely no turbo expert, so please any suggestions welcome!
1994 Mini Mayfair, 1330, Mspd Ph2 cam, Benross turbo head, 9:1, GT1752, TT Intercooler, MegaJolt, BMW M3 leather seats.
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