[OldCrow7xx]

Hey guys why do the automatics seem to get a taller rear end gear always?

Why wouldn't it be the same?

Is it something with the math of having 6 manual gears vs 8 automatic gears?

I always want the shortest gear possible for peak acceleration and dont care about top speeds. I noticed the 8A has like a 2.5:1 or 2.8:1 or something crazy instead of 3.25-3.75 most cars seem to have.

Thanks in advance for all info!

[XutvJet]

There's more to gearing than the rear end ratio. You need to look at the final ratio which is the calculation of the rear end ratio with the each gear ratio. The 8AT has WAY shorter gearing than the 6MT.

My M235 6MT with the stock 3.08 rear gear does 41mph in 1st, 72mph in 2nd, 109mph in 3rd, 142mph in 4th, and a limited 155mph in 5th/6th. The M235 8AT max speeds in each gear are WAY SHORTER than that. I don't know the exact mphs, but they are much shorter in each gear. By the time I'm out of 1st, the 8AT is deep into 2nd. By the time I grab 3rd, the 8AT is nearing 4th.

The 8AT makes better use of the power band than the 6MT. The 8AT is rapid through the gears and the 6MT more long legged.

Since these motors make a ton of torque, they aren't remotely as dependent on gearing to maximize acceleration. Adding more gear to an 8AT, especially one with mods, will not make it accelerate faster. It might feel that way from the seat of the pants, but it won't be quicker/faster. It is possible to over gear a car.

[OldCrow7xx]

Originally Posted by XutvJet

There's more to gearing than the rear end ratio. You need to look at the final ratio which is the calculation of the rear end ratio with the each gear ratio. The 8AT has WAY shorter gearing than the 6MT.

My M235 6MT with the stock 3.08 rear gear does 41mph in 1st, 72mph in 2nd, 109mph in 3rd, 142mph in 4th, and a limited 155mph in 5th/6th. The M235 8AT max speeds in each gear are WAY SHORTER than that. I don't know the exact mphs, but they are much shorter in each gear. By the time I'm out of 1st, the 8AT is deep into 2nd. By the time I grab 3rd, the 8AT is nearing 4th.

The 8AT makes better use of the power band than the 6MT. The 8AT is rapid through the gears and the 6MT more long legged.

Since these motors make a ton of torque, they aren't remotely as dependent on gearing to maximize acceleration. Adding more gear to an 8AT, especially one with mods, will not make it accelerate faster. It might feel that way from the seat of the pants, but it won't be quicker/faster. It is possible to over gear a car.

So the rear end gear is different to make sense when compared to each of the eight gears in the box?

So that you reach the same terminal value, just by a different combination?

[XutvJet]

Transmission and rear gears (final drive) are torque multiplication devices. An engine would struggle to accelerate a car from a stop if it were direct drive (i.e., 1:1 or one engine rotation to one wheel rotation).

1st gear in your 8AT is 4.7 and the final drive is 3.08. https://www.bimmerpost.com/goodiesfo...oupe-specs.pdf That means torque from the engine is getting multiplied by a factor of 14.5. Yes, that means that theoretical torque to the wheels in 1st gear is massive. For each successive gear, the torque multiplication goes down because the motor doesn't need to work as hard to accelerate the car and the longer gears allows the motor to operate in it's powerband longer.

Cars with low torque need more torque multiplication through gearing to maximize acceleration. The turbo BMWs aren't as reliant on gear ratios to maximize acceleration because the motors themselves make a ton of torque across a very wide powerband thus less need to add deeper gears. The stock gearing on the 8AT is ideal for maximum acceleration. The 6MTs could use a little deeper gearing, but the downside is reduced fuel economy in 6th. The stock 6MT gearing already suffers from poor mpg compared to the 8AT.

The 6MT M2, M3, and M4 run deeper 3.46 final drives compared to the 3.08s in the 6MT/8AT M235, BUT the M2/3/4 run over 1" taller tires which reduce the gearing advantage a bit and making it closer to a 3.20 gear when compared to the M235 assuming similar height tires.

And don't go thinking you'll swap the 3.46 into your 8AT. For one, it will blow the tires off in 1st and 2nd and be useless and annoying. And two, the rear differential in the M2/3/4 is electronically controlled and won't work in these cars.

Based on you posts over the past few months, I think you're expecting a bit too much out of the M235. With the typical Stage 2 93 octane mods, it's 370-400whp/390-420wtq car. That's quite strong for a street car and good for lower to mid 12s at 114-116mph or in the realm of a stock M2C/M3/M4. If you're wanting more power, I'd suggest stopping now and moving on to the M2C/M3/M4. It will be a hell of a lot cheaper (and reliable) to go fast assuming you lock down the S55's crank hub.

If you think there's a issue with your car's power, go get it dynoed or always drive in Sport+ or DSC Off to get a majority or all of the power. Running in Sport only or Comfort can reduce power to the wheels in full throttle situations.

[OldCrow7xx]

Originally Posted by XutvJet

Transmission and rear gears (final drive) are torque multiplication devices. An engine would struggle to accelerate a car from a stop if it were direct drive (i.e., 1:1 or one engine rotation to one wheel rotation).

1st gear in your 8AT is 4.7 and the final drive is 3.08. https://www.bimmerpost.com/goodiesfo...oupe-specs.pdf That means torque from the engine is getting multiplied by a factor of 14.5. Yes, that means that theoretical torque to the wheels in 1st gear is massive. For each successive gear, the torque multiplication goes down because the motor doesn't need to work as hard to accelerate the car and the longer gears allows the motor to operate in it's powerband longer.

Cars with low torque need more torque multiplication through gearing to maximize acceleration. The turbo BMWs aren't as reliant on gear ratios to maximize acceleration because the motors themselves make a ton of torque across a very wide powerband thus less need to add deeper gears. The stock gearing on the 8AT is ideal for maximum acceleration. The 6MTs could use a little deeper gearing, but the downside is reduced fuel economy in 6th. The stock 6MT gearing already suffers from poor mpg compared to the 8AT.

The 6MT M2, M3, and M4 run deeper 3.46 final drives compared to the 3.08s in the 6MT/8AT M235, BUT the M2/3/4 run over 1" taller tires which reduce the gearing advantage a bit and making it closer to a 3.20 gear when compared to the M235 assuming similar height tires.

And don't go thinking you'll swap the 3.46 into your 8AT. For one, it will blow the tires off in 1st and 2nd and be useless and annoying. And two, the rear differential in the M2/3/4 is electronically controlled and won't work in these cars.

Based on you posts over the past few months, I think you're expecting a bit too much out of the M235. With the typical Stage 2 93 octane mods, it's 370-400whp/390-420wtq car. That's quite strong for a street car and good for lower to mid 12s at 114-116mph or in the realm of a stock M2C/M3/M4. If you're wanting more power, I'd suggest stopping now and moving on to the M2C/M3/M4. It will be a hell of a lot cheaper (and reliable) to go fast assuming you lock down the S55's crank hub.

If you think there's a issue with your car's power, go get it dynoed or always drive in Sport+ or DSC Off to get a majority or all of the power. Running in Sport only or Comfort can reduce power to the wheels in full throttle situations.

I thought the m235i was an electronic differential too? I know there has been a lot of debate and bmw white-papers posted around that.

[OldCrow7xx]

Originally Posted by XutvJet

Transmission and rear gears (final drive) are torque multiplication devices. An engine would struggle to accelerate a car from a stop if it were direct drive (i.e., 1:1 or one engine rotation to one wheel rotation).

1st gear in your 8AT is 4.7 and the final drive is 3.08. https://www.bimmerpost.com/goodiesfo...oupe-specs.pdf That means torque from the engine is getting multiplied by a factor of 14.5. Yes, that means that theoretical torque to the wheels in 1st gear is massive. For each successive gear, the torque multiplication goes down because the motor doesn't need to work as hard to accelerate the car and the longer gears allows the motor to operate in it's powerband longer.

Cars with low torque need more torque multiplication through gearing to maximize acceleration. The turbo BMWs aren't as reliant on gear ratios to maximize acceleration because the motors themselves make a ton of torque across a very wide powerband thus less need to add deeper gears. The stock gearing on the 8AT is ideal for maximum acceleration. The 6MTs could use a little deeper gearing, but the downside is reduced fuel economy in 6th. The stock 6MT gearing already suffers from poor mpg compared to the 8AT.

The 6MT M2, M3, and M4 run deeper 3.46 final drives compared to the 3.08s in the 6MT/8AT M235, BUT the M2/3/4 run over 1" taller tires which reduce the gearing advantage a bit and making it closer to a 3.20 gear when compared to the M235 assuming similar height tires.

And don't go thinking you'll swap the 3.46 into your 8AT. For one, it will blow the tires off in 1st and 2nd and be useless and annoying. And two, the rear differential in the M2/3/4 is electronically controlled and won't work in these cars.

Based on you posts over the past few months, I think you're expecting a bit too much out of the M235. With the typical Stage 2 93 octane mods, it's 370-400whp/390-420wtq car. That's quite strong for a street car and good for lower to mid 12s at 114-116mph or in the realm of a stock M2C/M3/M4. If you're wanting more power, I'd suggest stopping now and moving on to the M2C/M3/M4. It will be a hell of a lot cheaper (and reliable) to go fast assuming you lock down the S55's crank hub.

If you think there's a issue with your car's power, go get it dynoed or always drive in Sport+ or DSC Off to get a majority or all of the power. Running in Sport only or Comfort can reduce power to the wheels in full throttle situations.

Also where are you getting a multiple of 14?

Is "final drive ratio" the combination of the rear end gear and individual transmission gear together

Or

Is "final drive ratio" the gear ratio of the rear end gear device itself alone?

[XutvJet]

Originally Posted by OldCrow7xx

I thought the m235i was an electronic differential too? I know there has been a lot of debate and bmw white-papers posted around that.

The M235 has an open differential that uses the ABS system to control wheelspin when the car is in DSC Off. This system is often referred to as the "eLSD". It's marginally effective but most don't know that it only works in DSC Off.

The M2/3/4 come with a true clutch based limited slip differential that is electronally controlled which can vary clutch lock up.

The M235 has an optional clutch based limited slip differential that isn't electronically controlled but does work with the eLSD system.

[XutvJet]

Originally Posted by OldCrow7xx

Also where are you getting a multiple of 14?

Is "final drive ratio" the combination of the rear end gear and individual transmission gear together

Or

Is "final drive ratio" the gear ratio of the rear end gear device itself alone?

4.7x3.08 = 14.2

The term final drive often refers to the rear end gear ratio.

Watch this: https://youtu.be/lLGPHl1o0bc

[OldCrow7xx]

Originally Posted by XutvJet

The M235 has an open differential that uses the ABS system to control wheelspin when the car is in DSC Off. This system is often referred to as the "eLSD". It's marginally effective but most don't know that it only works in DSC Off.

The M2/3/4 come with a true clutch based limited slip differential that is electronally controlled which can vary clutch lock up.

The M235 has an optional clutch based limited slip differential that isn't electronically controlled but does work with the eLSD system.

That would only come on RWD cars correct?

[XutvJet]

Yes, BMW only offers the M Performance LSD for the RWD cars; however, there are xdrive owners on the forum that have added them without issue.

With the xdrive, you cannot change final drive ratio as the xdrive system will flake out and would destroy the transfer case within a few miles. With the RWD cars, you can change the final drive slightly and not have the DME, cruise control, and/or transmission ECU (8AT cars only) flake out and throw codes.