Should I keep my M10B20 or swap for an M20B20? I have dual weber 45s for the M10, but no exhaust system. Also, my M10 has a 4-speed. For the M20, I have a Holley 4160 conversion, and a full exhaust. It's also mated to a 5-speed. I have all the nessessary swap components (subframe, driveshaft, all mounts and ancillaries) but I'm still undecided if I want to give up double webers. The advantage of the M10 besides webers is lightweight, of course, and I think the engine is less tired. However, the M20 is much less common in E21s, especially B20s, and the sound of the M20B20 is unreal. I prefer it 10 to 1 over the M10, even with webers/headers. The M20 is so silky smooth and the M10 is almost...industrial? This is going to be a weekend/autoX/second car, so I could care less about HP numbers.

tl;dr. M10 or M20?

for weekend auto cross, the m10 would definitely be the better choice as far as weight distribution is concerned. I've never auto crossed my 320i, and I'm not sure how how the gearing is in the 4-speed, but you might have to find a way to get some more down low grunt to make sure you can keep it in 2nd without down shifting through tight turns.

The M20 would be great if it was a DD or a track car, but in the sharp carved corners of an auto cross track, I'd say go with the m10. I've read on here that there's a "big" difference in handling between the 320 and the 323 because of the heavier engine.

I've driven an E21 with a 2.0 M20, and I actually liked the "planted" feeling of the M20. Or maybe it was the front spoiler.

That planted feeling turns into understeer unless you're sideways. :)

That planted feeling turns into understeer unless you're sideways. :)

i assume your talking about the m20, mate ive never had any understeer in the my 323i

i know. but surely in tight auto X corners, the 4 would be better, if you didnt come in hot with liftoff to keep it tidy. thats what i do on downhill mountain roads to keep the front in.

im not sure
ive only driven the m20 once tho

well the m20 does handle pretty well if you got some good tyres up front, i remember with the old 13's and old shock/spring setup i had it was still pretty well balanced.

M30

If you're going to be using the car mainly for competition, where you'll be on the pipe most of the time, then stick w/ the M10 for the better weight distribution. The M20 having more available torque is a better all around street w/ the occasional track. You can practically drive in 4th gear all day long and the M20 will lug the car just about everywhere. So you don't need to do a whole lot of shifting w/ the M20. :clap

As far as the throttle lift off oversteer, I had more of this problem on my 320i than I do on my 323i. THe 323i comes w/ stiffer sway bars both front and back. So if you were to do an M20 swap, definitely look at how the 323i was setup from BMW and duplicate the heavier sway bars.

well, i've driven my is and a mates 323, and several other e21's... if my m10 had more torque i'd never change it for auto-x, ever. the m10 with a bit more torque and maybe a few more horses would be your best bet in a 320i weekend/auto-x'er. it light, i haven't experienced any under steer (lots of sideways if your not careful) and its a bitchin motor in a 320. period.

now the m20, good for tracks where you don't really want to do a lot of shifting, and a DD, i wish i had a m20 DD that;d be nice.

I have a stroked, 2.7liter M20 with 10.5:1 compression, head work, cams, header, Motronic system (about 220HP). It is a bit of a beast and I believe overkill for autox, which I don't do, as I like track days a lot better. So, it works for me and suits my needs.
The M10 is a great engine and can be made to work quite well and still be street-able. Good head work and cams will go a long way, increasing the bore will also help and I would certainly keep things simple and swap in a pair of DCOE carbs. You can get 150HP quite easily with this setup.

In the end, it is all about what you want to do with the car. We can all chip in with what we know about these engines, but you need to figure out what you want and which engine fits your needs.

The idea I had was to take my old 320/6 which I'm parting out and move all the mechanicals and euro bits to my 320i, which is rust free (the road is visible from many places inside the 320/6) I really enjoyed the sound and "guts" of the M20 on the street, and the M10 seems impotent compared to the M20. Out of all the BMWs I've had, I miss the 320/6 the most. I've also had it the longest, and I just can't let it go. So this is the best idea I can come up with to resurrect my favorite car. I guess it's just sentimentality, but I really want the M20. I also want this to be my DD (keep the miles off of my perfect E30) and something I can toss around the track and not worry about. I'm not competitive in autoX, I only run street tires if that gives you any indication.

m30 13:1 high comp with forged bits head work, titanium things. with 45 Dcoe's.
that would be so awesome.

Bit overkill for auto-X tho. You would have to drift into every corner to stop it ploughing. but it would be a shitload of fun. :)