FAQ

FAQ

How do differentials work?
These guys can explain it better then I can: http://auto.howstuffworks.com/differential.htm

What final drive ratio does my differential currently have?
Check here for all US spec cars..

What are the transmission gear ratios for my E46 323/325/328/330?
Check here for all US spec cars..

Are differentials from all E46 323/325/328/330 interchangeable?
Yes, even the Z4 differential should work.

Does changing my differential give me more power?
It doesn't give your engine more horse power, but through torque multiplication, it effectively increases the torque you put to the ground.

How does changing my final drive ratio effect my top speed?
Changing to a higher ratio will drop your top speed. Check here for a more detailed answer
You can also use my Gear Ratio Calculator to see how your top speed changes with different ratios.

Will my gas mileage go down?
Usually going to a higher ratio decreases your fuel economy. This results from higher engine rpm's at cruising speeds. From my personal experience, I've lost about 1-2 mpg but I'm able to get about 27mpg on freeway. (according to the OBC) A lot depends on how you drive your car.

How many liters of diff fluid does the differential need?
0.95 US liters. If you are filling your differential with a fluid pump, you will probably need more gear oil.

What type of oil does the differential use?
Synthetic 75/90 Gear Oil

How much do differentials cost?
A brand new differential cost about $1300 from BMW. Salvage yards offer much better deals on used differentials. Prices from salvage yards can vary widely, but expect to pay anywhere from $200-600. Harder to find ratios are always more expensive. Check here for some tips on finding a used differential..

Is there anything I should look out for when buying a used differential?
You may want to avoid super high mileage differentials, but gears will probably last longer then your car. Some people have reported stripped threads. You can have shop retap them or if the threads are stripped from the flanges, use the flanges from your car.

How much does it cost to install a differential?
Cost varies widely between shops. Typically anywhere from $150-300. Call around.

How long does it take to install a differential?
A shop can install a differential in about 1-3 hours. Most people that do a differential swap for their first time can do it in 3-6 hours.

Is my 323/325/328/330 diff Limited Slip Differential (LSD)?
No, LSD is only offered on E46 M3s.

Can I get an LSD for my non-M3 E46?
Yes, if you go with a custom built differential. BMW doesn't offer LSD differentials for standard E46s. One other route is to go with E46 M3 LSD. This has been reported to work when you also use M3 axles and the M3 drive shaft. (E46 M3s use 3.62 final drive ratio)

Will each gear feel too short if I change to a higher ratio?
This is all relative to the driver and what you can deal with. A change in 10-20% will be a noticeable change in your gear lengths. This can be offset with a higher redline from performance engine software. BMW uses 3.46 differentials in manual transmission 323/325 convertibles and touring editions. So.. if these guys aren't complaining, it can't be all that bad.

Do I have to swap my input/output flanges on my differential?
This depends on what car your new differential came from. Most swaps from 323/325/328 don't need the flanges swapped. Swapping between 330s and to other models usually requires a flange swap. 330s have more bolts connecting differential input flange to the drive shaft.

Why not just change the gears instead of the entire differential?
It's just easier to swap out the entire differential instead of opening it up and rebuilding it with a new gear. I also don't know where you can buy just the gears.

What are the recommended final drive ratios?
This really depends on the type of driving you do. Typically 323/325s want to go with a 3.38 to a 3.46 to see a noticeable difference. 328/330s can go with 3.15/3.23/3.38/3.46 and see a noticeable difference over stock.

Do I need engine software if I upgrade my differential?
No, but software that raises your redline helps if you go with a very aggressive ratio.

Does changing your gears affect DSC (Dynamic Stability Control) system in anyway?
No.

Can I do a differential swap on an automatic transmission E46?
Yes, higher ratios are available for automatic transmission cars. If you can't find the right ratio, you can have a custom ratio built for you.

Why are 3.46 ratio differentials so much cheaper then all the other ratios?
There are a lot more 3.46 differentials out there since it's in most automatic E46s.

Will a differential from a 325xi fit a non 4wd car?
Yes, someone of E46fanatics has done this swap, so it does work.

My differential didn't come with axles (drive flanges)! What do I do?
Don't worry.. just take the output flanges from your old diff. They'll fit right in.

When I buy a used differential, how do I know what ratio it is?
There's usually a label on the top of the differential that lists the ratio. You can always check how many times you have to turn the input flange to get the output flanges to turn 360 degrees.

E-mail me questions / comments / or corrections to my info.

 

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