Starting M3 rear end/diff swap doityourself

[Hornung418]

Nice!! I've got my eye locked on one as well. Are you going to keep it 3.62? I'm converting mine to M5 3.15 R&P.

[terraphantm]

Nice!! I've got my eye locked on one as well. Are you going to keep it 3.62? I'm converting mine to M5 3.15 R&P.

Since he's got a 6-speed, the 3.62 wouldn't be that bad. His top gear revs would actually be lower than yours with a 3.15

[az3579]

Since he's got a 6-speed, the 3.62 wouldn't be that bad. His top gear revs would actually be lower than yours with a 3.15

About 2800 or so at 70. Seems a little high, but not too much higher than stock. Definitely doable!


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[Hornung418]

The whole thing that I think is pointless is the fact that you're missing that extra grand of revs. The gearing is stupid short. 3.15 would allow for better driveablility.

[terraphantm]

The whole thing that I think is pointless is the fact that you're missing that extra grand of revs. The gearing is stupid short. 3.15 would allow for better driveablility.

Eh not really - unless you redline each gear every time when cruising normally, slightly shorter gears will not pose a driveability problem. My car came w/ a 3.62 from the factory - I find myself shifting around 3000-3500 in most gears. I don't find myself having to shift excessively or anything like that. Also keep in mind that the 323/325 5MT verts also came with 3.46s and those cars were plenty driveable. Cruising revs were a bit high for my tastes... but that's with a 5-speed. Now as far as maximum acceleration goes - yes your maximum speeds in each respective gear will be reduced. But you'll accelerate at a quick enough rate that it's not really an issue. 0-60 may be slower, but 30-50, 50-70, 70-90, etc... should be quicker. 1/4 mile will also be slightly quicker. So that's really a nonissue there.

The only real concern IMO is the cruising RPM and fuel efficiency. 3.62 in 6th is almost the exact same ratio as a 3.07 in 5th. No biggy.

Now if you have a supercharger, then gears that short would be useless. You'll burn through 1st and 2nd anyway. But a 330 is deficient enough in torque that it could make good use out of a shorter diff.

[LeontiyZHP]

I havent read much about this, but what is the benefit of doing this? What would be improved? Any reads on this?

Are you done with te swap?

[Hornung418]

I'm lost here. Is the goal to get a Limited Slip Differential or get a short ratio Final Drive?

Each one has their benefits but for a car that comes with a 3.07 diff from the factory wouldn't you want to retain stock performance at the cost of MPG, and still have an LSD? Even if I had a 6 speed transmission, I'd opt for the M5 r&p simply because there are not enough revs in the car to enjoy the 3.62.

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[derbo]

I'm lost here. Is the goal to get a Limited Slip Differential or get a short ratio Final Drive?

Each one has their benefits but for a car that comes with a 3.07 diff from the factory wouldn't you want to retain stock performance at the cost of MPG, and still have an LSD? Even if I had a 6 speed transmission, I'd opt for the M5 r&p simply because there are not enough revs in the car to enjoy the 3.62.

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The benefit would be the LSD in my opinion. As far as enjoying a 3.62, you can definitely enjoy it even with a lower redline! I know for myself, turn 2 at infineon is in 3rd gear but however I am around 3000RPM going uphill and the ZHP feels like it got hit by an invisible wall. . Adding a 3.62 would set up the 3rd gear a little higher and allow me to push right up that hill in the prime torque powerband. By changing the gear ratio, the car would feel more alive and ready to pounce whenever you hit the gas. You will have to shift more due to lower mph redlines at each gear and it would suffer at MPG due to higher RPMs are normal freeway speeds. Top speed suffers as well but hey, how many of us reach 155mph on a normal day anyways? The car will have about a 18% increase in acceleration.

I know a 3.46 gear ratio with a 6spd increases 12% acceleration from a 3.07.



But awesome to the OP. I love to have myself an M3 LSD. I'll probably do one down the line. If not, its 3.46LSD here i come.

[terraphantm]

I'm lost here. Is the goal to get a Limited Slip Differential or get a short ratio Final Drive?

Each one has their benefits but for a car that comes with a 3.07 diff from the factory wouldn't you want to retain stock performance at the cost of MPG, and still have an LSD? Even if I had a 6 speed transmission, I'd opt for the M5 r&p simply because there are not enough revs in the car to enjoy the 3.62.

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LSD and shorter ratio aren't mutually exclusive.

A shorter ratio does not *reduce* performance. It will reduce the top speed in each gear, but overall acceleration should improve (0-60 may suffer slightly due to an extra shift, but there are plenty of cars out there that don't hit 60 'til 3rd anyway). Absolute top-speed will not be effected since the 330 does not have near enough power to max out 6th gear with a 3.07.

I don't really see why you think it'd prevent you from enjoying the car either. For cruising around - just increase your shift points by a couple hundred RPM (so if you used to shift at 3000, shift at 3500 after the swap). For maximum performance, I think the shorter gears will make it more fun. Part of what makes a fast car feel fun is that kick you get in the back of your seat. And I do remember when I had my 330 that the gears felt unreasonably long; it felt like it took eons to max out 3rd gear. So if I can get a little better use out of my gears, I would.

Now if i had a 5-speed, I wouldn't do it. The cruising revs become too high. 4000+ RPM at 80mph would get old. Shortest I'd go with a 5-speed 328/330 is probably 3.23 (like the E36 M3)

[Sockethead]

Back in the day when I was drag racing, I ran a 4.57 in the rear with a 4 speed and 1:1 4th gear... Going through the gears with 400+ HP was awesome but 3500 rpm at 55 mph was annoying. I switched to 4.11 and that was acceptable even with a 1:1 fourth gear. After I blew up my good motor, I stuck a stock V8 in the car and I was still blowing the doors off of most other cars, even big block Chevy's and Mopars that were using a stock gear in the back...those gears made that much of a difference in acceleration
A 3.62 & a 6 speed should be fine... and believe me, the first time you go through the gears, you're going to have a huge grin on your face. The increased acceleration will make you think that you just added a shit load of HP to the car.