[iaknown]

Ok guys, I am having an lsd built for my D and they are offering me a lower 2.56 (DCT) ratio instead of the stock 2.81 ratio. I am wondering if this would be advantageous or more of a hassle. What do you guys think? Any technical input?

[Mark M]

Better fuel mileage with a 2.56, but in theory will equate to slightly slower launch.

However that said, during a launch 1st gear is rather useless since the car winds up so quickly then shifts to second. Perhaps a 2.56 would allow it to make better use of first gear.

Ideally you'd want to find an online calculator and overlay the 335d's power / torque curve against the transmission and differential ratios. You'll want to insure that with the new diff ratio you'll still be in the optimum part of the curve during all gear changes.

[iaknown]

Good points, I was also wondering if it would allow use of 1st gear.

Where might I find a calculator like you mentioned?

[Hoooper]

You would probably need a transmission tune to clear up shifting issues. Speedo would be off by quite a bit also. Cruise RPM would drop about 150 rpm at 70 mph. I doubt you will notice much more use from first gear, it will only get you 3 more mph at 5000 rpm in 1st

[iaknown]

Thanks for the additional info. Sounds like more of a hassle than its worth.

On a side note, why did they put a 1st gear in these cars anyway????

[BB_cuda]

New 535D also has lower diff ratio. Might go compare to it on BMW site. I also recall that final overdrive ratio on 8 speed is same as 6 speed automatic is. You could sort of end up where the 5 is that if it is same final drive number. The 5 has larger diameter tires though so it will turn lower rpm at cruse speed on freeway. My thought is you will miss the faster acceleration and the fuel mileage at cruse is going to be excellent either way.

I evoke the KISS principle.

[iaknown]

Yea, if I was in Germany it might be another story. But sounds like more headache to get the trans sorted out. We can't even get a tuner to fix our horrible manual mode programming, I doubt I'll find someone to reprogram the shifting too.....

[BB_cuda]

US spec 535D final drive ratio is 2.6:1

[GreekboyD]

Originally Posted by iaknown

Ok guys, I am having an lsd built for my D and they are offering me a lower 2.56 (DCT) ratio instead of the stock 2.81 ratio. I am wondering if this would be advantageous or more of a hassle. What do you guys think? Any technical input?

You buying from MFactory?

[iaknown]

Originally Posted by GreekboyD

You buying from MFactory?

Yea, but they're telling me the 2.81 is a lot farther out than originally planned so they are offering me either the dct 2.56 or a bolted 2.81 using my existing ring gear. I'm going to opt for the 2.81.

[GreekboyD]

Originally Posted by iaknown

Yea, but they're telling me the 2.81 is a lot farther out than originally planned so they are offering me either the dct 2.56 or a bolted 2.81 using my existing ring gear. I'm going to opt for the 2.81.

Good move. Will the bolted version be cheaper for you too. Any real differences when installing versus our welded ones?

[iaknown]

Originally Posted by GreekboyD

Good move. Will the bolted version be cheaper for you too. Any real differences when installing versus our welded ones?

Not sure about price just yet. Should know more details next month including install arrangement/procedure.

On another note, I feel the need to thank you for your entertainment in another thread lol

[GreekboyD]

Originally Posted by iaknown

Not sure about price just yet. Should know more details next month including install arrangement/procedure.

On another note, I feel the need to thank you for your entertainment in another thread lol

Are you sending your core to them for fitment into a bolted version?

Other topic? *scratches head*

[iaknown]

Originally Posted by GreekboyD

Are you sending your core to them for fitment into a bolted version?

Other topic? *scratches head*

Had an extra that I sent them to develop a lsd for us. So I'll be the guinea pig and we'll see how it goes. Hopefully I'll have some feedback soon...

[GreekboyD]

Originally Posted by iaknown

Had an extra that I sent them to develop a lsd for us. So I'll be the guinea pig and we'll see how it goes. Hopefully I'll have some feedback soon...

Wow...this definitely changes the game now.

[Hoooper]

FWIW it looks like the 535d has an 8th gear of .669, our 6th is .69. Not much of a difference, only about 50 RPM at 70 based on a 2.81 diff. The diff ratio change makes a bigger difference.

[iaknown]

Originally Posted by Hoooper

FWIW it looks like the 535d has an 8th gear of .669, our 6th is .69. Not much of a difference, only about 50 RPM at 70 based on a 2.81 diff. The diff ratio change makes a bigger difference.

Interesting. I was hoping it would have more of an effect on 1st but like you mentioned it wouldn't be much. Bummer....

[BB_cuda]

Hooper is correct. The 535D has a slightly deeper over drive at 0.67:1 vs our 0.69:1. I did all the gear ratio math while at kid's appointment.

Our std car ( using 255/35R18 rear tires)
During 70 mph turns 1822 rpm

535D using 275/35R19 rear tires
70 mph turns 1542 rpm (15.4% slower rpm)

If Iaknown went to 2.56 rear diff and assumed all else the same as std 335D,
70 mph turns 1660 rpm (8.9% slower rpm)

[WreckerX5d]

I don't believe the slight RPM drop at the same speed is worth much more in fuel savings. I don't have any idea on how much fuel burn per RPM at a given speed is anyway.

[GottaBeKD]

In turbo diesels rate of fuel burn is more related to load as per my understanding.

Turning at a slighter higher RPM with a lower load due to gearing will probably not yield a measurable difference in fuel economy.

[floydarogers]

Originally Posted by Hoooper

... Speedo would be off by quite a bit also. ...

I don't believe the speedo would be off (any more than it already is ), since the reading is generated off wheel-speed (ABS sensors) rather than driveshaft rotations.

[GottaBeKD]

I'm hoping the speedo stays on