more power!!.....DIY supercharge

[73GT]

I’ve done some restoration to a ’73 MGB GT, just the normal MGB stuff and now it’s my daily drive. I’ve also been slowly modifying it to my personal taste, to make handle the way I like and make it and easier to live with. I’ve done things like CSI electronic distributor, headlight/spot light relays, ¾” front anti roll bar, polybushed, parabolic rear springs and telescopic dampers, DIY panhard rod, MGF/MGB hybrid front seats, 15” alloys and 185/70 tyres (maybe a bit over tyred!).

But now I’m thinking more power…

I’ve considered conventional head/cam swaps, big bore B series, V8 conversion, modern 4 cylinder twin cam engine swaps but I’m finding these things a bit uninspiring, expensive or changing the character of the car (which I happen to like). What’s caught my imagination is the Moss supercharger kit (at about £3K) and an article in PPC about a DIY supercharge of a ford kent crossflow using the BMW mini eaton M45.

Budget is important so all in £1000 for the first phase of getting a SC on the engine and working safely and giving a bit more go. From there I can think about head/cam and engine block mods and fuelling as and when funds permit – I understand that this will limit the boost that I can use at first, but the engine is in good condition having been rebuilt about 3,000 miles ago and figure that Moss do it so why can’t I.

I’ve got a full workshop so machining adaptor blocks, plenum, brackets etc is going to be OK and being a frustrated / desk bound engineer in my day job, is one of the things that appeals to me about the conversion.
I’m a complete novice to supercharging, so there are some “known unknowns” and even more “unknown unknowns”, I’m thinking of the basic scheme as follows…
1) Supercharger – secondhand ebay special…Eaton M45 or MP45 – ex BMW mini or Mercedes, I think the mini is the one to go for as mounting the carb looks easier, the Mercedes seems to be cheaper (1/2 the price) and more available but has a curved inlet that would put the carb either pointing at the rear of engine or hitting it, from pictures it doesn’t look easy to modify either.
2) I’ve no idea about what sort of boost pressure I should be looking at with the standard engine, anyone?
3) Bypass valve – vacuum operated either the separate mini item with adaptation or I think the MP45 has this integral with the SC body – that’s if I can find one at sensible money?
4) Fuelling – 2” SU carb – I’m not against EFI but the additional complication and cost means this would need to come later.
5) Manifold/Plenum – machined from billet aluminium, to adapt the SC port to the standard twin SU manifold, I’m sure the volume of the plenum will matter, can anyone tell me what this should be? Does the internal shape matter?
6) Backfire/pressure relief valve – machined into the plenum, the magazine article used a valve spring, custom made piston sealed with a dowty washer and adjustable, set at 15psi so I’m thinking something along the same lines..
7) Drive belt and Pulleys – don’t know what’s best here, I’m thinking of retaining the standard water pump/alternator V belt and running a separate one for the SC. Options; multi v/serpentine belt; toothed belt; conventional V belt (single or doubled up). I’m OK with machining pulleys and broaching keyways so can custom make to a certain extent, I’m edging towards conventional V maybe doubled up as the an easier option as alignment, tension required and more complex machining for toothed for multi V would add complication – any advice?? Will the crank main pulley need harmonic damping??
8) Ignition – I’m currently using a CSI electronic distributor with selectable curves, I’m hoping that’ll do (to start with at least), I’m not against distributor-less ignition but again cost will be a factor initially.

if anyone's got any knowledge on this that they're willing to share that would be great..