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As above really. Currently running a B20 on a Webber 5 speed,lovely sound but missing that last 25% to make it fun.
My question is this: should I go all out with an M52 B28, or stick an M20 B25 in it?
It's crying out for an LSD already so will need to factor that in too.
Thinking a healthy 170bhp should be enough, and will probably be a cheaper option...but how to get round the LSD conundrum?
In the words of Penelope Pitstop "Heylp! Heylp!"

Assuming RHD UK...

M20B20 or M10B20? If M20 then the M20B25 is a doddle, bolts in up to everything. If you wanted the 325i management then you'd have to splice in the electrics which is joining a few wires, you'll want to upgrade the exhaust to at least E21 323i or minimum. This is assuming your car as an M20.

M52 or M50 is a ballache in comparison. Custom sump, custom exhaust, custom radiator, electric fan, M52 needs the ECU flashing to rid of EWS and the need to see ABS speed signal (engine will be in limp mode otherwise). If you have an M20 already then use that gearbox, prop, gear linkage.

The rear axle as standard is borderline with an M20B25, M50/M52 are pretty brutal on the driveshafts.

If you have a M10 E21 then you'll be starting from scratch needing the E21 M20 front subframe, engine mounts, gear box, prop, linkage.

I had an 323i with an M50B25 and it was awesome, such a revvy brutal engine, rapid, and the sound... wow! I'm into drifting and E21s aren't really suited for it without extensive modifications so I broke it up, sold the engine and I still have the shell sitting around. My 316 E21 is getting an M42B18 318is engine when I get around to it.

Not much to add to that, thanks!

It's an M20 B20. So pretty much a swap and then wire in the Motronic.
Thanks for the in depth reply.
Did you end up welding the diff in order to drift?
I'd much prefer the LSD route.

That would be the most simple option then, by far. Welded diff is only useful if you use the car for drifting only. Regular track or road use calls out for a LSD. On the road I wouldn't go beyond 25%. These babies are rare but they are out there. May need a rebuild though.

Sounds like a plan is hatching. I do like the M20 howl, and the 2.5's remind me of my apprenticeship days and "borrowing" my dad's various BM's
Just the LSD mod to overcome.

M52 conversions are awesome but not cheap. Added to which the diff won't last and you'll be wanting better brakes. An m20/2.5 and LSD is a nice set-up and a doddle to do. Si's car is a warmed over 2.5 and it goes and sounds really well. If you tire of 170 brake maybe think m52 then? LSD's do come up or ship one over from the states for about 400 quid all in.

Cheers Mark.
Think I'll stick with an M20. Do you have any links for the LSD's I'll google it now, just being lazy

Even with 170 BHP you want a brake upgrade. Enough threads on that topic in the Top Tech Topics section.

Agreed! They're not up to it now let alone with more power.

...and the seats have got to go!

Best place to find a USA LSD is e21 Legion on FaceBook.

on a budget the m20b25 with a nice cam and a bit of software will be more than enough for fun. make sure you get the flywheel lightened too that really helps.

In regards to LSD, they do come up but if you want something that is brand new and works brilliantly then I would suggest giving kevin birds (hartge) a ring and getting a brand spanking new quaife unit fitted. it costs about a grand but then it never needs a rebuild, uses normal oil etc.. a second hand ropey old e21 lsd and then rebuilding it costs the same anyway.

If you want bigger brakes the wildwood brackets im going to get made soon I have found an engineering place that doesn't mind doing the odd bit here and there, most engineering places want an order for 50 odd a time!

Nas

Wise words on the diff front Nas. Nothing worse than buying second hand only to find its toot.
I've got a set of Lotus Elise calipers, I'm going see if these will fit with brackets.
Thanks for all inputs so far, saving a lot of ifs and buts.

Having just done the M20B25 swap, it is pretty easy. Always a few niggles as with every swap.

I've driven it a few times (then the alternator failed) but I can certainly say it's plenty quick enough.