2.9 Stroker Rear end gears—recommendations sought

[WAKman]

Hi all,

When I bought my E30, it had a tired m20 with 245k miles, and for some reason the previous owner had equipped it with 3.15 rear gears (I think— may have been 3.25). It was . . . slow. So I installed a 4.10 LSD rear as a stopgap until I could rebuild the engine.

Fast forward to today, and I have a healthy (205 at the wheels) 2.9 stroker that I’ve built and installed with a MS2 pnp. The 4.10s are now way too short, with first disappearing too fast, and then way too much noise on the highway with the engine turning at 3500rpms at 75-80mph. The car is also way too busy in regular traffic driving—sort of like an S2000. On the flip side, I took it to Pacific Raceways and the 4.10s were great for that. But I didn’t build it to be a track car; I just built it to be a good back road car.

So I need some recommendations here. Does anyone have a similar build to mine, and have you installed a gear set that works well? I know I can install the stock 3.73 gears, but I wonder if the car would be happier with something taller.

Thanks!

[MrBurgundy]

3.73 all day. Anything taller will kill the feel that you have with 205whp.

[bangn]

3.73 is still peppy with a getrag 260, 1st gear is still pretty short, 2nd is perfect. I find on the highway the RPM's still sit a bit high in 5th and driving a decent distance at 110kmhr makes me feel a bit uneasy of how high it sits. 3.73 is easy to live however with if you don't do much highway driving.

I will be going a 3.46 I think next year personally but that's mainly because mines turbocharged and need the extra load.

[AWDBOB]

I did a 3.46 in my 2.7L stroker, and I think it's too long.

If you really want to split hairs, the Z1 got a 3.64 diff.

I'd say a 3.64 is perfect, but they're hard to find, in which case a 3.73 is perfectly suitable.

I do a TON of highway driving in my car, so the 3.46 will stay, but would swap to a 3.64 if I could find one.

[packratbimmer]

3.64 is the perfect gear for both worlds

[WAKman]

3.73 is still peppy with a getrag 260, 1st gear is still pretty short, 2nd is perfect. I find on the highway the RPM's still sit a bit high in 5th and driving a decent distance at 110kmhr makes me feel a bit uneasy of how high it sits. 3.73 is easy to live however with if you don't do much highway driving.

I will be going a 3.46 I think next year personally but that's mainly because mines turbocharged and need the extra load.

Yeah, I realize turbos are different in terms of gearing, but this is good info nonetheless.

[packratbimmer]

I have 3.46 gears right now and like them in a few ways (and I'm stock tune so stock power). 1) 3rd winds out way past 85, maybe 90 mph. 2) First and second actually feel like useful gears. 3) It's useful to do a fifth to third shift on the highway without feeling like third is too short to do that. 4) On country roads, forth and fifth feel like good cruising gears - fourth does not feel too short. 5) It's just a nice relaxed ride overall! I still like 3.64 better, especially when I have my 225/50R15 tires - they are considerably heavier than stock 14" - need more torque.

[PEZ2]

It depends on cars expected use and your personal expectations. I currently am running a 2.7l with a 3.7 as above good go, limited on the highway. I too have a 2.9l ( on the floor waiting) looking at either a 3.25 with the expectation of e36 M3 like feel ( use to own a 96) reasonable go and highway manors, or a a 3.46. Already have a 3.25lsd so the 3.46 will have to wait.

[ZeKahr]

Heh, my E30 came with a 4.10 (two of them) from the factory, it gets quite loud on the interstate at 70-80 too. Although I have an auto, so I’m turning at around 2800-3500 depending on the speed. My solution is to bump up the radio volume and cruise at 70 rather than 80. I would prefer a 2.79 for ultra-low cruising RPMs, but for that you need an Eta motor (ideally one stroked up to 3.0L with no other changes made in order to make maximum low-speed torque and accommodate for uber-tall gearing)

[digger]

you'd want to be doing a lot of cruising to go below a 3.64 IMO (unfortunately there is a decent step to a 3.46) , and why build a performance 2.9L or other stroker and kill it with tall gears. you probably want to be able to get to 60mph at the top of 2nd not sure what that translates to with the standard G260

[Digitalwave]

3.73 or 3.91 unless all you do is highway miles.

[digger]

6 speed swap

[kyleconstantine]

I have a 2.9 m20 myself. I do a lot of distance driving in the summer and I toss in a 2.93 for the highway but I have a 3.73 that I use for normal driving and track days. I did a couple track days this summer with the 2.93 because I was driving from NJ to SC the same week and it wasn't too bad. The 2.9 has enough torque to handle the taller gears. It's really not a ton of work to swap them back and forth if you've got a long distance drive.

[varg]

you probably want to be able to get to 60mph at the top of 2nd not sure what that translates to with the standard G260

There's a gearing calculator spreadsheet out there with bmw transmission ratios in it. I don't remember where I got it but I have it saved and from that:

24" tire, 60mph in 2nd gear
4.10 - 7600rpm
3.91 - 7250rpm
3.73 - 6900rpm
3.64 - 6750rpm
3.46 - 6400rpm
3.25 - 6000rpm
3.15 - 5800rpm
3.07 - 5600rpm
2.93 - 5400rpm

MPH at 3,000rpm in 5th
4.10 - 64
3.91 - 68
3.73 - 71
3.64 - 73
3.46 - 76
3.25 - 81
3.15 - 84
3.07 - 86
2.93 - 90

[packratbimmer]

Thanks for the chart! I went on a road trip the other day and the 76 MPH at 3K was nice with the 3.46 gear set. Definitely not going to win a drag race, but 20-90 is still very respectable. I'm not sure I would be as satisfied in the mountains though.