E30 Zone

Hi all i am looking to do a m50 conversion including 5 speed box,i've looked at a few sites and a few have suggested using e30 m3 diff 3.25. I use the motorway everyday for work so would a 2.93 or 2.79 be better and where would i sorce them in the uk and roughly how much should i pay

3.25 from a e28 or a 3.46 but most people like the 3.25

"Conversion" factor to E30 diff ratio is 0.81, so 3.46 = 4.27 and 3.25 = 4.01. Both too high for road use with a M50, IMHO.
2.93 = 3.61, which is what I use on mine, and is about right.
2.77 = 3.44, which is getting a bit too low for acceleration.

Brian's right

i ve just fitted a an ETA diff think it was 3.07 , and its fine for motor ways , Belive its 70 at 2000 rpm or similar , + gained 2 to 3 mpg on my work route too

but it depends what u want accel or mpg, quietness on m/ways

Things aren't quite as simple as higher diff ratio = more acceleration.
When I first ran my M50 E30 it had whatever diff a 318 auto touring has as standard. First gear was virtually useless, and it became effectively a four speed box with no top gear.

Use an M40 318i box and you don't have to swap the diff. It bolts straight on and is an overdrive unit. It'll be strong enough as long as you don't make a habit of doing burnouts. Make sure your engine didn't come from an H plate 525i with the early M50/260 Getrag overdrive box already fitted. Use an M20 flywheel and clutch.

My C1 2.3 had a Griffin built 2.7 and I fitted a standard 320i 240 box (same as 318i but different bellhousing bolt pattern) to replace the shagged dogleg box and it was fine. The most power I've seen being put through a Getrag 240 is 300 bhp on a modified 1974 2002 Turbo with a T3, Weber Marelli (Sierra Cosworth) management, big intercooler which was built by AVA in Glasgow. That used a five speed E21 316 box.

Not to worried about acceleration at moment cos the m50 will still give me a better response if set up right would be a big waste of time to end up with something that drives more or less the same if you get me drift