3.0 Si rear diff/ LSD swap ?

[GuidoK]

1. Simple mathematics dictates that reducing the final drive multiplies the torque to the road by the same factor, so yes it is literally a bolt on power adder - available torque at the wheels is higher. HOWEVER, as has already been said, a shorter final drive will result in very short gears, and ultimately higher rpms for the same speed in 6th gear. So there is room to modify it a bit, and you'll get a noticeable benefit in that the gears will be more closely stacked so you'll be in the powerband all the time, but if you go too mad you'll end up buzzing about at 5k rpm in 6th doing 60mpg (I exaggerate but you get the point).

You only get more wheeltorque as long as you're in the same gear. Reducing the final drive ratio will make the gears shorter. The part of the acceleration where you need to upshift earlier you'll be slower until the point where you would be upshifting with the longer final drive gear, at which point the shorter gear will be faster again. So there only is a partial benefit along the acceleration line. A 0-62mph time could even get slower :wink:
Its not a bolt on power adder, as there is literally no power added. The power output stays the same from the engine.

As for using the z4m diff, you need the complete rear axle including trailing arms brakes etc as the driveshafts wont fit the wheelbearings or hub flange.
And you may need the propshaft too (I dont know if you can get away with swapping the propshaft flange, if its compatible)

 

[MACK]

As thread resurrections go this one at 10 and a bit years really must be a record!

 

[Ed Doe]

You only get more wheeltorque as long as you're in the same gear. Reducing the final drive ratio will make the gears shorter. The part of the acceleration where you need to upshift earlier you'll be slower until the point where you would be upshifting with the longer final drive gear, at which point the shorter gear will be faster again. So there only is a partial benefit along the acceleration line. A 0-62mph time could even get slower :wink:
Its not a bolt on power adder, as there is literally no power added. The power output stays the same from the engine.

As for using the z4m diff, you need the complete rear axle including trailing arms brakes etc as the driveshafts wont fit the wheelbearings or hub flange.
And you may need the propshaft too (I dont know if you can get away with swapping the propshaft flange, if its compatible)

I'm not sure I completely agree, but please correct me if I'm wrong - my assertion (perhaps not explained properly in my original post) was that for any given gear, you are multiplying the available torque to the road by the same ratio factor as the final drive is shortened by. To my mind this statement is accurate - the more revolutions of the engine per revolution of the tyres, the more torque you are putting to the road. I take your point that you could potentially be slower due to having to change gears earlier, but that's no the point I was making - at the top of the same gear you will be putting more torque to the road with a shorter final drive. Please correct me if my understanding is wrong though!

EDIT: Facepalm, didn't realise how old this thread was :lol:

 

[GuidoK]

More torque isnt automatically a power adder.
Say you're accelerating from 0 to 62mph.
Say with the stock ratio you shift from 1-2 at 30 and with 2nd gear you can reach 62.
with a short ratio, you need to shift at 25, 2nd goes to 50 and shift to 3rd you get to 60.
with the short ratio, you'll be quicker to 25. from 25-30 you'll be quicker with the stock ratio, from 30-50 you'll be quicker with the short ratio and from 50-60 you'll be quicker with the stock ratio.
Overall through the revrange the short ratio is quicker, but it depends on where your shift points will be needed that has influence on the quickest gear combination. Thats why in racing they use different gearsets for different tracks.

I think for a z4 you'd ideally want to change every individual gear. I think 1st gear gets pretty useless when you shorten the ratio but especially 3rd, 4th and 5th could benefit because for a roadcar hooning around seldomly happens at 100mph+
With 1st gear getting useless you're pretty much transforming to a 5 speed. This is mainly the problem I run into.