E46 6SPD gearbox opinion

[Breeze1]

That's something I have been considering doing for a while now but did not really get to it because the cost doesn't seem justified compared to the benefits, or so I think anyway. It just so happens that I found what seems a pretty good deal on a 6spd manual gearbox with matching flywheel and adaptation required to fit on an M52TUB28, and thought asking here for inputs.

Is the manual 6 speed gearbox found on E46's really a 6 speed or more like a 5+1 intended to improve gas mileage? I'm under the impression that it would depend more on the diff it is coupled to, which is in my case is a 3.07 that came stock with my 323i, but I have no idea how good they would go together, or what diff I should get with it to have an improvement on performance while keeping a reasonably low noise in 6th, for highway purposes (if the 3.07 is not a good fit). The diff I have now, with its 5spd gbx is doing great in that department: a bit shorter than the 2.93 diff that comes on stock 328i's, but no too short.

In a nutshell, would performance benefit from it? I don't really care about gas mileage since I drive less than 10000miles/ year anyway and that wouldn't make much of a difference at the end of the day.

It seems like my stuttering issues are - fingers crossed - coming to an end since I unmounted and fixed a few issues on my Intake manifold today and I would like to forget about all that hassle by doing something UNNECESSARY . I still have a bit of hesitation at very low revs when I touch the TP at first, but it's either due to a small exhaust leak upstream of the O2 sensors (suspecting the EM's gasket) or to a failing MAF, which are both relatively easy fixes anway.

Chris, like I said in my pm the injectors have arrived and are scheduled for replacement as soon as the stuttering is 100% fixed, because I want to be able to feel the difference with the ones I have now

[pbonsalb]

The 6 speeds typically have similar 1-5 with an OD 6th. They do have higher torque capacity, but you would not need that. To improve performance, you could swap in the 6 speed with OD 6th and then shorten your diff to something like a 3.64.

[Breeze1]

The 6 speeds typically have similar 1-5 with an OD 6th. They do have higher torque capacity, but you would not need that. To improve performance, you could swap in the 6 speed with OD 6th and then shorten your diff to something like a 3.64.

Thanks for your reply. So basically a 5spd with a 3.64 diff would be the same as a 6spd from 1 to 5 but would lack top speed compared to the 6spd. And I imagine it would also be very noisy at highway speeds...It would be interesting to know how much that shorter diff is worth "hp wise". I'll see if I can find a chart about it.

[pbonsalb]

www.diffsonline.com

Under the technical menu choose gearing calculator. Plug in the gear ratios and tire sizes.

[Breeze1]

Thanks pbonsalb!

[Breeze1]

You were spot on with the 3.64 diff: engine speed in 6th ends up being the same as with a 3.07 diff and a 5spd gearbox. I'm not sure how this can be translated in terms of "horsepower gain" though.
As they say, shorter diffs are the "supercharger of the poor", but how much does that equal to, I don't know yet. It might not even be possible to calculate it.

[pbonsalb]

It’s not horsepower gain. It’s a gain in performance in some speed ranges. Your 0-60 and 1/4 mile may not improve, but your 40-70 might. It’s torque multiplication through gearing. I ran a 3.64 witha ZF320 5 speed and 7000 rpm limiter plus Schrick cams and M50 intale and full exhaust so the engine made good high rpm power. I loved the combination except on the highway, where I wished I had a 6th gear. Later I turbocharged the motor and it makes so much torque I am happy with a 3.15 and would consider a 2.93.

[bmwdirtracer]

Yep. It's all about the torque band of your particular engine. When you're in that torque band, your acceleration is at its best. When you go to a shorter rear end ratio, each gear will run out of revs sooner than it would with a numerically lower diff ratio.

My M5, for instance, has a 3.15 diff. I'd never change it to a shorter ratio, because OVERALL, I don't see any advantage. I can pretty much just choose the appropriate gear for any given speed and situation, and achieve all the acceleration that the engine has to offer. I will say this: when I'm cruising at, say 65 or 70 mph on the highway, but waiting for an opportunity to advance through traffic to my more normal ~80+, I'll usually be in 5th, at the very bottom of the torque band. For max acceleration, 3rd or 4th would be better....but annoying, over any distance. When I KNOW I'm going to have no opportunity to pass, I shift to 6th between 65 and 70, and the world slows down, and it becomes a luxury car.

As pbonsalb says, torque fixes everything

[Galapolis]

If you are looking for an overall performance increase, you'd probably be better off just getting a bigger engine. I have the 6 speed in my 330i coupled with the 2.93 diff. The biggest advantage to me is being able to cruise at high speeds while maintaining relatively low RPM's. The 3.07 also came standard with the 330i ZHP, which also had the 6 speed, so you would be running a stock combo (minus the LSD) but with a weaker engine.