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3.0 Si rear diff/ LSD swap ?

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Curtis
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Re: 3.0 Si rear diff/ LSD swap ?

Post by Curtis » Tue Nov 06, 2007 11:30 am

My original diff ratio was 3.23 with no LSD. I installed a complete E46 M3 rear axle with 3.62 ratio and obviously an LSD. The extra grip is superb and the general accleration is slightly improved however my 0-60 time has been reduced because I now have to change up to 3rd before reaching 60 when before I could get there in 2nd. I've no doubt that my 0-70 time is reduced because I would have had to change to 3rd to reach 70 anyway.

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3.0 Si rear diff/ LSD swap ?

Post by Jacobnbarber » Sat Feb 17, 2018 8:02 pm

Will the z4m diff be a direct replacement for the standard one or does it require sub frame and drives shafts etc.

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3.0 Si rear diff/ LSD swap ?

Post by pokeybritches » Sat Feb 17, 2018 9:09 pm

It requires subframe, driveshaft, etc.
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3.0 Si rear diff/ LSD swap ?

Post by Mr Whippy » Sat Feb 17, 2018 9:27 pm

power = mass x speed x acceleration

So

power / ( mass x speed ) = acceleration


You can improve power figure by being near peak power longer which can mean closer gear ratios.

But then time during shifts is 0 power too, so add in another shift into an acceleration interval and you can lose your gains.

Also too low a first and second (and maybe higher) gear can mean loss of traction which is wasted power too.


For a track car you get ratios (not just FD) set to your corners, top speed etc.

For a road car it's better to choose a cruising gear and a 1st gear that just loses traction in good conditions.
Then stack the rest evenly for optimal acceleration.

I'd only ever lengthen a FD while adding more power on a road car.
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3.0 Si rear diff/ LSD swap ?

Post by Ed Doe » Sat Feb 17, 2018 9:54 pm

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 the small additional torque at the wheels, and a further benefit 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 60mph (I exaggerate but you get the point).

2. For the avoidance of doubt the MC/MR Final drive ratio is 3.62, the 3.0si Manual final drive is 3.46, the 3.0si Auto final drive is 3.64, the 3.0si Roadster is 3.23, and the 3.0i Roadster Manual is 3.07 and 3.0i Auto is 3.46. So hypothetically (without knowing the inner workings of the 3.0si auto boxes) there's plenty of scope to tear down an auto box as a donor unit and make up an oem shorter box with the auto final drive, if that's what you're shooting for?

3. Remember the M has another 80hp, but also has another ~1,200rpm to play with at the top end (redline 8,000rpm-ish vs Si soft-limiter at 6,700rpm-ish), so yes it has a shorter final drive but it's revving harder, so wheelspeeds are higher at the top of each gear too. Given the 3.0si with a standard rev limiter wont hit 60 in second until you're into the realms of the soft limiter, you're already using 3 gears to reach 60, to the M only using 2, so if you're fussed about that sort of thing the 0-60 comparison really is a non-starter....
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3.0 Si rear diff/ LSD swap ?

Post by GuidoK » Sat Feb 17, 2018 10:04 pm

Ed Doe wrote: Sat Feb 17, 2018 9:54 pm1. 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)
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3.0 Si rear diff/ LSD swap ?

Post by MACK » Sat Feb 17, 2018 10:12 pm

As thread resurrections go this one at 10 and a bit years really must be a record!😁
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3.0 Si rear diff/ LSD swap ?

Post by Ed Doe » Sat Feb 17, 2018 11:19 pm

GuidoK wrote: Sat Feb 17, 2018 10:04 pm 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:
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3.0 Si rear diff/ LSD swap ?

Post by GuidoK » Sat Feb 17, 2018 11:41 pm

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.
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