[IMS-340C]

HI All,

Sorry if this was discussed before But I searched and couldn't find anything.

I own a 2018 340i MPPSK with 8 speed automatic transmission.

I am thinking of changing my final drive gear from 2.81 to 3.23 found in the manual transmission.

Is it possible ? Has anybody done it before ?

Thanks guys.

[mbanks21]

Manual diff will not fit on auto car and vice versa, at least for the 335 it won't

[B58togo]

Even if it mechanically fits, you’re going to end up with errors when the TCU doesn’t see the expected RPM for a given speed. This can probably be recoded however.

[mbanks21]

Originally Posted by B58togo

Even if it mechanically fits, you’re going to end up with errors when the TCU doesn’t see the expected RPM for a given speed. This can probably be recoded however.

Unfortunately it doesn't even mechanically fit. Myself and a few others have tried

[B58togo]

So the entire pumpkin won’t fit?

There’s still the option of just changing the ring and pinion gears and then recoding. I’d check with these guys.

https://diffsonline.com/bmw-f3x-340-...l#.XTC8bxYpD7o

[tobefair-idontcare]

If this did fit, or found a gear that would at the same ratio, what would the result be? Better gas mileage or more power in 8th gear?

[NISFAN]

It does fit. Others have done it on B58 F22 platform. Car needs some kind of coding to avoid fault codes and possible drivetrain CEL.

The ratio (at least here in Europe) is 3.08 for manual and 2.81 for auto.

[mbanks21]

Originally Posted by NISFAN

It does fit. Others have done it on B58 F22 platform. Car needs some kind of coding to avoid fault codes and possible drivetrain CEL.

The ratio (at least here in Europe) is 3.08 for manual and 2.81 for auto.

I'm going off the F30 335 manual vs auto diff, 100% does NOT fit. Are you saying with 100% certainty the 340 manual diff fits the 340 auto?

[NISFAN]

Originally Posted by mbanks21

I'm going off the F30 335 manual vs auto diff, 100% does NOT fit. Are you saying with 100% certainty the 340 manual diff fits the 340 auto?

Sorry, I guess I was addressing the OP (B58). I don't know about N55. Although I find it somewhat strange that the N55 isn't the same. After all it uses the same part number rear subframe, and I'm pretty sure manual B58 diff is same part number as manual N55 diff.

On B58, the only potential difference is the way the prop-shaft joins the diff. Some cars use a Guibo, whilst others use a 50mm prop nut. However the diff is the same, you can fit guibo flange to the other, or alternatively remove guibo flange and revert to prop nut. (The guibo flange fits to the same 50mm prop nut).

I have a spare of both in my garage, they are identical externally.

[IMS-340C]

Actually it’s more torque > acceleration to the wheels in all gears but lower top speed and more fuel consumption as it would be running higher RPM.

Originally Posted by tobefair-idontcare

If this did fit, or found a gear that would at the same ratio, what would the result be? Better gas mileage or more power in 8th gear?

[IMS-340C]

According to the attachment below it seems BMW used to offer different diff ratios for Automatic transmission but not anymore.

[Wires]

Would you actually get more torque once if you need to code so that it doesn't toss errors? DME does a lot of torque management and throttle cuts. It might software intervene circumventing the gains from the mechanical changes.

I know in "yesteryears" it was what was done, just not sure since the car runs torque targets for power. You might find that it just dials back the boost to keep the torque targets in line.

[IMS-340C]

Yes these engines are torque based, I noticed in warm weather or when switching the AC it runs higher boost to compensate for the lost power.

As for the diff ratio I am not sure as you are not changing the torque of the engine, we are changing the torque going to the wheels.

Originally Posted by Wires

Would you actually get more torque once if you need to code so that it doesn't toss errors? DME does a lot of torque management and throttle cuts. It might software intervene circumventing the gains from the mechanical changes.

I know in "yesteryears" it was what was done, just not sure since the car runs torque targets for power. You might find that it just dials back the boost to keep the torque targets in line.

[kern417]

The torque is measured/estimated based on engine monitors. It won't put out less torque because you change the final drive. Just like if you change to smaller/larger diameter tires, lighter wheels, etc. that will make the car accelerate faster.

[neonnblack]

Originally Posted by NISFAN

Sorry, I guess I was addressing the OP (B58). I don't know about N55. Although I find it somewhat strange that the N55 isn't the same. After all it uses the same part number rear subframe, and I'm pretty sure manual B58 diff is same part number as manual N55 diff.

On B58, the only potential difference is the way the prop-shaft joins the diff. Some cars use a Guibo, whilst others use a 50mm prop nut. However the diff is the same, you can fit guibo flange to the other, or alternatively remove guibo flange and revert to prop nut. (The guibo flange fits to the same 50mm prop nut).

I have a spare of both in my garage, they are identical externally.

The axle splines are different on the 335 between auto and manual cars.

[NISFAN]

Originally Posted by neonnblack

The axle splines are different on the 335 between auto and manual cars.

May well be, but this thread is about B58 engined cars. So I'm not sure why people keep butting in with why it doesn't work on a 335i ???

B58 use same output shafts regardless of gearbox, and it happens to be the same shafts as was used on manual gearbox cars.

[MissionPerformance]

Once you get differential to fit mechanically, you need to address adaptation and flash. You will have to either flash your car to appropriate file on both TCU and DME (as long as the ratio you went with was offered as an option for that chassis), or have your DME and you tuned and your gear ratio written in.

After the flash, you will have to erase your learned values and have the car relearn them once again.

[Wires]

Originally Posted by kern417

The torque is measured/estimated based on engine monitors. It won't put out less torque because you change the final drive. Just like if you change to smaller/larger diameter tires, lighter wheels, etc. that will make the car accelerate faster.

I get that normally, just wondering if you need to code for the different drive, does the engine power scale accordingly (so that a manual car has the same power to the ground as an automatic).

Doing an aftermarket tune obviously bypasses this, just curious for a stock tuned car.

[kern417]

Originally Posted by Wires

I get that normally, just wondering if you need to code for the different drive, does the engine power scale accordingly (so that a manual car has the same power to the ground as an automatic).

Doing an aftermarket tune obviously bypasses this, just curious for a stock tuned car.

The only thing controled by the ecu is the engine. that's it. it has no control over what happens after the flywheel. That's why auto vs manual, awd vs rwd, all have different power figures to the wheels. But hp at the crank is (relatively) the same.

[IMS-340C]

Agreed with the exception of shift points in AT and traction control.

Originally Posted by kern417

The only thing controled by the ecu is the engine. that's it. it has no control over what happens after the flywheel. That's why auto vs manual, awd vs rwd, all have different power figures to the wheels. But hp at the crank is (relatively) the same.

[kern417]

Originally Posted by IMS-340MP

Agreed with the exception of shift points in AT and traction control.

TCU controls shift points
ABS controls traction control

They communicate to the ECU but they only make corrections based on your inputs. They don't limit power based on mods/drivetrain differences.