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Old 01-20-2006, 01:29 AM   #1
Zim Zimma
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Join Date: Dec 2003
Location: Mississauga
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Differential Swaps

Personally I am interested in a differential swap and I Pm'ed Thinair just to get some insight his experience with differential' swaps.

I know there are a few others that are interested so Thinair thought it would be a good idea just to ask my questions publicly so we can all gain something from it.

My questions for Thinair were:

Basically I just wanted to know a bit about your differential swap, and how you liked it. My understanding that you initially swapped your 2:91 for a 3:15 and currently your running a 3:38. I guess some of the question i have are:

How beneficial is the 3:15 over the 2:91 when it comes to acceleration? Are we talking barely noticeable or would this shorten the gap between a 328 and M3?

How does the 3:38 compare to the 2:91 and the 3:15? How close would a relatively stock 328 keep up with an M3 with a 3:38?

What are your overall driving impressions of each? Is the drive etc just as smooth?

Is fuel economy greatly affected?

And finally, are these diff's things that can be pretty banged up? Ex. When im looking for one should i be worried about one being thrashed or are they relatively strong and hold up pretty well?

Thanks Thinair and hopefully any others interested in diff swap will gain some from this too!
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Old 01-20-2006, 01:52 AM   #2
M3ti Compact
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I tested diff swaps in LFS (known to be the best driving sim out there)...I used identical cars, with identical AI drivers. The only difference is the gear ratios

The yellow car has a 4.444, the turquoise car has a 3.333 diff.

Turquoise car reaches 102km/h (60mph) in 4.19s in second

At the same time, (look at stopwatchs), the 4.444 car is only at 95km/h because it needed to shift to thrid. In doing so, it loses 1/2 a car.

Pretty much every shift, the 4.444 car loses a 1/2 car lead.

At the end, the 3.333 car finishes a car ahead, in 4th gear @ 207km/h.
The 4.444 car in 6th gear, having had to shift 2 times more, was ony at 204km/h
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Last edited by M3ti Compact; 01-20-2006 at 01:56 AM.
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Old 01-20-2006, 02:10 AM   #3
M3ti Compact
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I just tried some more tests with the diffs you mentioned 3.38, 2.91, and 3.15. The race was ultra tight, with the cars finishing within 0.1s of each other. Finished in this order 2.91, 3.15 then the 3.38. If you have the game I can send you the replay- the cars with the higher diffs you can see they clearly accellerate faster, but they lose a lot of ground when shifting. The faster they are going, the more ground they lose per shift to the 2.91 car.

I put a car with a 2.222 in and it lugged at the start and it came in almost half a second behind the second slowest 3.38 car- so there is an optimal diff ratio out there...

I also put a car with a 4.444 and again, it was quickest off the line, but every shift, it would lose ground.

Last edited by M3ti Compact; 01-20-2006 at 02:18 AM.
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Old 01-20-2006, 04:42 AM   #4
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I can only answer your last question. Yes diffs can be beat up. If someone was doing to much donuts, or stuck in a snow for a while trying to get out, his clutch in the diff would be worn out, making LSD into an open diff.
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Old 01-20-2006, 07:15 AM   #5
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Well, there is a 7% difference between the 2.93 and the 3.15. It is noticable, for performance a shorter LSD is always my first recommendation. BMW decided to put a 2.93 in the 328's to space it further away from the M3.

The 3.15 is a great gear, it's good on the highway and it does improve acceleration. 3.15LSD's are too difficult to find (some stock 325's, 94-95 M3's), in my opinion I'd suggest a 3.23LSD which is stock on the 96-99 5spd M3's. You will definately notice an acceleration difference in every gear, but your cruising RPM on the highway will be a bit higher.

As you know I do have the 3.38LSD right now, I love it, but it can seem short for some on the highway. At 140km/h I'm cruising at 4000rpm. My mileage is still decent, while there are more revs there is less load on the engine. Some people will argue that a shorter diff will kill your top end speed, which is true in a sense, but our cars generally won't pull to redline in 5th for it to matter. I can (barely) hit 241km/h at 7000rpm with my 3.38, but how often do I do that for it to matter.

A stock M3 will still pull on a stock 328 with a 3.38, but it's definately an improvement from stock. With my current setup I've hit 14.1 at the drag strip, which is stock M3 times. I'm confident with a proper launch I could get 13.999 . I've lined up from 2nd at idle with a stock M3. The M3's torque made it pull a little quicker off the line, but past 4000rpm my car pulled on it (thanks to the cams and manifold).

There is no noticable difference in lack of smoothness or drivability.

In regards to buying a used diff. Do an external inspection, make sure it's not coated in obvious leaking oil. If there is oil leaking out of the vent hole on top that may be an indication that the previous owner may have driven it hard (high g's)

Without draining the oil and taking off the rear cover it's pretty hard to tell what shape the diff is internally. You can check for play at the input and output shafts. If it's still installed on the car you're buying it from, go for a drive and listen for whirring noises coming from the rear of the car. Jack up one rear wheel and try to spin it with the car in neutral to see if the LSD is still functional. It should be pretty hard to rotate one wheel without a leverage bar.
"Driving is like a narcotic, an addiction that can take over lives. It ruins people, breaks families apart, and even kills. It’s habit that needs to be fueled. Time, money, rationality, sweat, knuckle skin, and anyone who disapproves are of little importance to a driver, and are often overlooked. Hi, I’m Nelson, and I’m a drivaholic."
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Old 01-20-2006, 07:50 PM   #6
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I have a 3.23 LSD in my 328is and i love it. I was very pleased when i put it in and noticed a pretty big difference in every gear. Its a lot tourqueier than it was, which is also nice. I would definitly find one for ur self, u wont be disapointed. Besides that, i dont have any big mods, just an M exhaust and a CAI, thats it. This summer i decided to drag my car just to see what she would run and im not to proud of the times but i ran a 15.2, best. Would have liked to see high 14's but like i said the car is pretty much stock. And i guess for runing 1.somthing seconds slower than Nelson, thats not to bad either. haha, just kiding Nelson.
Good luck with the decision
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