DCT diff change?

[NoGuru]

With xHP out we have lots of new options for the DCT. With that in mind, changing the diff in the past we were pretty limited.
There is an option for "Rear-Axle ratio in xHP. Does this mean we can put any diff in out cars now and just adjust the ration or is there more to it?
Has anyone done this yet?

This is from the xHP manual,

6.6. Rear-Axle Ratio Gear-Change and Throttle-Blip calculation of the DCT needs to precisly know all Ratios in the Driveline. If they are not setup correctly, all target RPM calculations are offsetted, which leads to spongy shifting and incorrect blipping on downshifts. Adjust this option ONLY if you have fitted a different RearDifferential. NEVER change this on a stock vehicle

 

[aus335iguy]

You can put any diff you like and change the ratio in xhp and it will shift normally. There’s more to changing final drive than that though. Our engines don’t rev like an M3 unless we use built motors etc so make sure you know what youe getting into before you change

 

[NoGuru]

You can put any diff you like and change the ratio in xhp and it will shift normally. There’s more to changing final drive than that though. Our engines don’t rev like an M3 unless we use built motors etc so make sure you know what youe getting into before you change

I don't know what I am doing, that is why I posted this thread
I guess what I want to know is there more options now for getting an LSD?

 

[aus335iguy]

I don't know what I am doing, that is why I posted this thread
I guess what I want to know is there more options now for getting an LSD?

Yes ! But, you have to factor in to your thinking that the Most of the other final drives (from M3s and 335i’s are much shorter (higher numerically) and that changes how the car drives and performs. For example it will accelerate faster but only if you can maintain traction. It will use more fuel and will likely limit your top speed. It depends on how you use or want to use your car?

 

[NoGuru]

Yeah I found an M3 DCT 3.15 diff. I don't know how much that will be a change from my 2.56 diff. I am sure I will loose some top end but I just did a half mile event and I think I was crossing a 150mph in 5th gear so I am sure the lower gearing won't hut me to much.

 

[Torgus]

Yeah I found an M3 DCT 3.15 diff. I don't know how much that will be a change from my 2.56 diff. I am sure I will loose some top end but I just did a half mile event and I think I was crossing a 150mph in 5th gear so I am sure the lower gearing won't hut me to much.

I have always been under the understanding that ideally you would reach your top speed desired at the end of your last gear and at redline. Why would you ever want/need acceleration past your desired top speed? This would allow for maximum acceleration. Unfortunately, that is not always the best choice as you can end up with VERY short gears, etc.

 

[NoGuru]

I have always been under the understanding that ideally you would reach your top speed desired at the end of your last gear and at redline. Why would you ever want/need acceleration past your desired top speed? This would allow for maximum acceleration. Unfortunately, that is not always the best choice as you can end up with VERY short gears, etc.

7th gear in my car tops around 170ish. If I continue to race in half mile events I need to get closer to my top speed (150mph) quicker. I know I am not going to hit 170mph at the HP I have so if I can target 160, I can try and get to that target faster.
At least that is my train of thought, but I am not sure its the best way to reach that goal.

 

[LMB335is]

Yeah I found an M3 DCT 3.15 diff. I don't know how much that will be a change from my 2.56 diff. I am sure I will loose some top end but I just did a half mile event and I think I was crossing a 150mph in 5th gear so I am sure the lower gearing won't hut me to much.

You'd be very surprised then. I have a 2.81 and I'm banging 6k rpm in 6th at the end of the 1/2 mile. You don't want to hit 7th.

 

[derekgates]

You'd be very surprised then. I have a 2.81 and I'm banging 6k rpm in 6th at the end of the 1/2 mile. You don't want to hit 7th.

Once I have cash to rebuild diff with 2.81 I will be doing so. 3.15 is SHORT! Makes the car fun to rip through gears but you run out of gearing rather quickly, even on the interstate.

 

[NoGuru]

You'd be very surprised then. I have a 2.81 and I'm banging 6k rpm in 6th at the end of the 1/2 mile. You don't want to hit 7th.

2.81 seems like a sweet spot. Is your LSD? What did it come out of?

 

[NoGuru]

Here is my car with the 2.81 and the 2.56 gear ratio

2.81
Tyre Circumference = 2077.22 mm
1st Gear = 60.31 km/h - 37.39 mph
2nd Gear = 94.34 km/h - 58.49 mph
3rd Gear = 133.9 km/h - 83.02 mph
4th Gear = 171.81 km/h - 106.52 mph
5th Gear = 207.41 km/h - 128.59 mph
6th Gear = 239.65 km/h - 148.58 mph
7th Gear = 262.09 km/h - 162.49 mph


2.56
1st Gear = 66.2 km/h - 41.05 mph
2nd Gear = 103.55 km/h - 64.2 mph
3rd Gear = 146.98 km/h - 91.13 mph
4th Gear = 188.59 km/h - 116.92 mph
5th Gear = 227.66 km/h - 141.15 mph
6th Gear = 263.05 km/h - 163.09 mph
7th Gear = 287.68 km/h - 178.36 mph

 

[LMB335is]

2.81 seems like a sweet spot. Is your LSD? What did it come out of?

I have a M3 diff and used a 210mm 2.81 R&P from a 850i rear end. It's a direct swap other than the ring bolts need sleeves for 12mm to 14mm. I went through realoem.com for days researching the various ratios available in the late 80's-early 90's V8 BMWs. They all used the same size, 210mm, as the M3 diff uses and are interchangeable. There's a 2.65, 2.81, and 2.93. The 2.81 is the one that works with M3 software or 335is software, and is lower without being too low and leaves a decent top end.

 

[naschine]

Could you please explain some more what you did and what parts are needed?

I'm trying my best to understand it, but since english isn't my first language I can't wrap my head around it.

Would it be possible to swap an 3.08 diff (MT N54 for example) in without to much hassle? Maybe anyone tried that already?

Thanks!

 

[aus335iguy]

Xhp now flashes DCTs and diff ratio changes can be done as a part of that.

 

[naschine]

I'm aware of that, the software part is now the easy part.

What I'm missing are infos what hardware is easily swapped to replace the stock 2.56 diff. Eg like you would need to shorten your drive shaft for an E92 M3 diff and so on.

 

[derekgates]

I'm aware of that, the software part is now the easy part.

What I'm missing are infos what hardware is easily swapped to replace the stock 2.56 diff. Eg like you would need to shorten your drive shaft for an E92 M3 diff and so on.

Here are most of my notes on the swap: https://www.evernote.com/shard/s19/...b70174775249/2c9e5b15e70b1eae68faac72cf7c8c25

 

[naschine]

Thanks @derekgates for your notes!

If I understand @LMB335is post correctly, you already did the m3 diff swap and then changed the internals to the 2.81 ratio gears?

For a 3.08 swap from a manual E82 it seems I would only need to "merge" the DCT's and the MT's drive shaft, everything else would be plug and play... got to get my hands on a manual's drive shaft

 

[RuskiRacer]

Thanks @derekgates for your notes!

If I understand @LMB335is post correctly, you already did the m3 diff swap and then changed the internals to the 2.81 ratio gears?

For a 3.08 swap from a manual E82 it seems I would only need to "merge" the DCT's and the MT's drive shaft, everything else would be plug and play... got to get my hands on a manual's drive shaft

Dct and manual driveshafts are the same length

 

[naschine]

Maybe I used the wrong terminology. Here's what I mean:

Upper is manual, lower is DCT. Manual = 1385mm and DCT = 1353mm

 

[derekgates]

Ah... my notes are specific to shortening of the M3 shaft when using the M3 diff...

If you are using a 3 series diff, it should bolt up without an issue as long as it is the same size; large or small. That might be the difference in length that you found?


The M3 'pain' is due to the different coupler used between the diff and shaft (and size)... but a 3 series diff should bolt up due to same flange/coupler?