I would not mind sacrificing top speed for more wheel torque with a bigger FD ratio change. I think it would be a big improvement in acceleration and with the DSG fast shifts and all. No more “slow motion “ when watching tachometer move through the RPM band lol. Based on a search, I do not think one is made but I imagine it may not be possible with the Quattro system?
Do you know i was only thinking about this the other day as i did this on a M3 E46 i had and it made a massive difference to the way the car pulled through each gear ! I had a 3.91 ratio put in and could still hit 170 + mph at 8500rpm crazy !!!
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Amazingly, I don’t remember the rear end ratio off the top of my head but it’s the steepest offered across the 5 series range. Uses a ring and pinion gear just like any other diff but I suspect there aren’t aftermarket options available mostly due to the complexity of the rest of the differential. Each axle has a sub-gearset and changing the main ring gear may alter the way those gearsets perform. Just guessing. Here’s a nice video showing the sport differential and how it functions.
One more video, this one has commentary.
Totally forgot to post this up but the rear final drive ratio is 4.375:1 for the RS5.
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Blimey i didn’t think it would be as high as that like i say i put a 3.91 in my E46 M3 i thought that was high !
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I don’t think that its an interchangeable number between cars because of all the variables. engine output, rpm and overall gearing of transmission come into play. And as high as the RS5’s Final drive ratio (FDR) number is, could you imagine how much longer the gears would be if it was… say 3.91? With a 3.91, First gear would get you to a 10%+ higher MPH. You’d probably hit 85 MPH on 2nd lol.
Considering conservation of energy, I think you sacrifice top end for acceleration up to a point. Also, more i think about it, the more complicated it gets in my head lol. So a higher number FDR would mean overall getting more torque from an engine but then you would be going through the low gears faster and at a lower MPH… I dunno anymore.
Where are all the engineering peeps. like James?
I understand what your saying and i do know there is a fine line between going to higher FDR . When i done this on my M3 years ago now, i had to work out what each gear would give in mph at any given rpm but i can’t remember the formula on how i worked it out back then ? I also did this for my Dad’s E39 528 he had and we took the FDR out of a 520 it worked well but his top end was only 130mph but it got there so much quicker ! I also had a friend who had an American muscle car with a supercharger on and he did the same sort of thing with his FDR but it was to high and it ruined the car’s acceleration completely as he had to shift to early in the lower gears and it actually made the car not as quick 0 to 60mph. If we did change the FDR it would have to be worked out correctly so not to do the same sort of thing !
http://www.kabamus.com/garage/gears.html
That is the calculation website that I use for comparing different tire and wheel sizes, transmission gear ratios, and final drive gear ratios for gear speed. It spits out the what the vehicle mph would be based on the engine rpm, very close to what it actually is in real application.
Also I have some experience actually changing final drive gear ratios on Audis and it isn’t a cheap or easy process because of the AWD system. A RWD BMW is stupidly easy and much cheaper in comparison because only the rear diff needs to be changed vs changing out the front and rear diff for the AWD Audis. I have also changed rear diffs on BMWs as well. Like on my 1991 850i where I went from the stock open 2.65 FD to a custom LSD 3.15 FD.
With my B6 S4 I changed out the original 0A3 manual transmission and rear diff completely for a RS4 0A3 manual transmission and a rear diff from a B6 A4 1.8T automatic transmission quattro car. That changed the final drive gear ratio from a 3.889 (35 ring/9 pinion gear teeth) to a 4.111 (37 ring/9 pinion gear teeth) . Along with changing the 2nd and 3rd transmission gear ratios as well but that isn’t as important to discuss. The ring and pinion gears are designed to mesh a certain way so you can’t usually just change one or the other to a different number of teeth.
This S4 modification lowered the total mph per transmission gear causing some unforeseen issues. Like the top of fourth gear at 7500 engine rpms is 121 mph and I started trapping 119 mph in the 1/4 mile so I had to decide if I try to shift or just ride out fourth gear to the rev limit. Totally expected the 300-400 more rpms at cruising speeds down the highway but I was running higher engine coolant and oil temps as well.
Just food for thought.
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Wow. Very good info! Thanks!