which diff?

[bosers]

Hi there, Im new on here. Im really enjoying my first e30, i have an H reg touring 320 which i put a 325 engine into for more grunt. it goes really well but was wondering if anyone had any advice on differentials? I think i have a 4.27 or 4.45 ratio and believe the 325 Touring had a 3.91 ratio. I understand that the higher no means lower gearing but wondered if anyone had any thoughts as to whether its worth changing or not? I dont do a lot of motorway driving.

[Grrrmachine]

Hi, and welcome

Putting a lower diff will certainly help your fuel economy without blunting your acceleration too much, so it's definitely worthwhile. When you fitted the 325 lump (M20 engine), did you use a 325i gearbox as well, or keep your 320i one? That'll make an impact on what diff to recommend, as it'll affect how 1st, 2nd and 3rd gear feel around town.

If you want more info on diffs and ratios, there's a whole page in the Wiki

http://www.e30zone.net/e30zonewiki/inde ... fferential

[bosers]

Thanks for that, interesting stuff. When i changed the engine i kept the original 320i gearbox. Any info or tips gratefully received! Nice one

[Grrrmachine]

I've done exactly what you did - 320i Touring to a 325i engine with the original gearbox. I ended up fitting a 3.64 diff, as it means I have decent town acceleration with good motorway economy. The 320i gearbox has "sportier" ratios for the first three gears, so there's no need to go for such a short ratio diff. 4th and 5th are the same on both gearboxes so having the taller diff is better for fuel consumption.

Generally, 3.64 = economy, 3.91 = performance, and 3.73 is a compromise of the two.

[bosers]

Many thanks, that sounds like a good plan, i'll look out for one of those. Cheers