e30 differential suitable for turbo

[megz2000]

 i have e30 m42 turbo with stock Getarg transmission and 3.91 differential unfortunately my top speed is 188 km i want change it to reach more speed  but i dont know what differential code will be suitable for me .
does any one can help me or have the same issue ?   
attached picture for 5th gear rpm

[alex230ro]

Hello

I don't think you have stock 318is E30 transmission

https://e30goodies.com/gearing-calculator/

based on the data given by you i think you have a E36 Getrag 220 transmission coupled with a 3.91 will give a top speed that you mentioned

https://www.e30zone.net/e30wiki/index.php/Differential  here you have all the diff ratios just play with the numbers personally i would go with 3.25 or even a lower ratio 2.93

Antother alternative is to get the stock E30 Getrag 260 / Getrag 240

[bmwman91]

Yeah, it definitely sounds like ratios are not stock in there somewhere. I have done ~210km/h with the G240 transmission and a 4.10 rear diff.

I do not know much about the G220, but the G240 is actually pretty good in that it is relatively lightweight and can handle a surprising amount of torque. Some guys with built NA M30 engines run the G240 and it seems to hold up well, so a turbo M42 should be fine too as long as you are not dumping the clutch too mcuh!

[DesktopDave]

+1 on the Getrag 220. It was the stock 5-speed on later M42 & M44 motors, the ratios are significantly shorter. I believe that the 220 used a direct drive 5th gear. The 240 is an overdrive 'box with an 0.8:1 5th gear ratio. A Getrag 240 with the M42 pattern bellhousing should be cheap, and they can handle a fair amount of torque in the lightweight E30.

With the wider 240 gears, I'd probably go with a 3.15 or 3.23, whatever was cheapest. You could also run larger circumference wheels, they'll drop your final drive ratio.

I actually happen to have both a 240 and a 220 in my garage, PM me if you wanted any firsthand specs.