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My race car is simply running an E30 m20/2.5 & gearbox with E21 LSD.

If I recall, the E30 box is a bit longer than the E21 box. Is that correct? If so, do I need to chop the 323 prop or can I use the E30 one?

Do I use E21 or E30 engine mounts?

Do I use the E30 gearlever & linkage or do I need to retain the 323 one?

Are there any other issues I need to take into consideration?

My understanding was that the e21 box was longer as it has the mechanical speedo drive whereas the e30 was electronic so didnt have the speedo drive.

I found this.

Notes about E30 M20 gearboxes (By Arne from the late graymarket site)
There is an alternative to those hard to find six cylinder E21 gearboxes: use the five speed from a 528e or E30 325e/i. The ZF transmissions in those cars are easy to find (translate as more affordable), often have lower miles, they will bolt up to our small six engines, and the gear ratios are almost identical to the E21 overdrive box. But there are some issues to deal with, which is the point of this article. Let's look at those issues one at a time.
The E30 trannies don't have a speedometer drive on them. To have a working speedo when you're done, you need to use an electric speedo from another model (I've been told the early 528e or 325e units fit well, but need to be glued in place) with a 'chopper wheel' kit to provide the electric signal for it. (Metric Mechanic has supplied the chopper wheel kits for people I've heard from). I am not sure how accurate these setups will be, as I don't know how the later cars with electric speedos compensate for the rear axle ratios. It may be that speedometers from some models are more accurate than others, or maybe the chopper wheels are different.
The E30 transmission has a pair of rubber mounts at the rear rather than the single one of the E21. The solution to this one is simple - use the rear transmission crossmember from the E30 that goes with the transmission. You will need to slightly enlarge the mounting holes in the crossmember to bolt it up.
Shifter - use the shift plate that came with the E30 transmission. Assuming you are replacing a worn E21 5 speed, use the E21 shift lever as it gives a shorter throw. You can use the E30 lever if you don't have the E21 lever.
Driveshaft - The E30 transmission is a tad shorter than the E21 unit. This means the E21 driveshaft is too short. Or is it? There are multiple options for this one.
Option one - install your six cylinder E21 5 speed driveshaft, and loosen the 4 bolts that attach the differential to the rear crossmember. Place a jack under the middle of the crossmember to take a tiny bit of sag out of it, then pry the differential forward about 1/2" - 5/8". Then tighten it up.
Another choice is to use the slip-yoke driveshaft out of an '84 E30 318i. This driveshaft's center bearing mounts are in a different location though, and a bit of welding on the underside of the car will be required.
If you have a six cylinder E21 four speed driveshaft and the flex disc mounts are the same (6 hole vs. 8 hole), you can have a driveshaft shop shorten it by 3.1 inches. This could be done with four cylinder E21 shafts also, but I don't know how much shorter it would need to be.

e21-Mark wrote:My race car is simply running an E30 m20/2.5 & gearbox with E21 LSD.

If I recall, the E30 box is a bit longer than the E21 box. Is that correct? If so, do I need to chop the 323 prop or can I use the E30 one?

Do I use E21 or E30 engine mounts?

Do I use the E30 gearlever & linkage or do I need to retain the 323 one?

Are there any other issues I need to take into consideration?

Haven't read the other posts but this is what worked for me:

E21 or E30 engine mounts with E21 Alloy engine mount arms.
E21 323i 5 speed standard propshaft.
E30 325i 1986 standard gearbox.
E30 325i 1986 rear gearbox standard mounts and modified crossmember, crossmember holes to chassis need to be elongated to match the e21.
E30 325i standard clutch complete.
E30 325i gear linkage with Z3 gearlever.
E30 rear linkage mount bolted to e21 shell. Bracket was massaged to position.



This shows the rear bracket arrangement, ignore the modified bar this is for the E36 M3 gearbox:


Hope this helps

Ben

Actually just noticed there are also 3mm spacers between the crossmember and the shell in the above pic.

Ben

anyone know what fuel pump you need to fit to get an m20b25 running in a 316 e21??