3.62 Limited Slip Differential

[klammer]

Originally Posted by Wall$treet

As some have mentioned shorter gearing just changes the distribution of torque through the gears but the overall "average" throughout useable gears really does not change and possible can worsen.

So it depends on what you want to do with your car. Shorter gears is IMO only good for street driving in 1st and 2nd gear. Other than that the ability to stay in 2nd and 3rd gear for much longer far exceeds the benefit in 1st and half of 2nd.

For example (without real numers for simplicity)

Short gearing- 1st gear torque is increased but you can use that torque for a shorter duration of time.

REgular gearing-you get less torque but you get to use it for long period of time in each gear.

Someone could do the calculations-I am too lazy but it would not be very hard.

There is a HUGE drop in torque from gear to gear. For example you pay a big penality for shifting to 3rd from 2nd or 2nd from 1st as the gear ratio multiplicaiton is a big difference. So by losing a chunk of time being able to use those much lower gear ratios, you lose a big chunk of time where you are at a higher level of torque.

Another way to think about it is lower final gearing can give you a bigger punch that doesnt last as long v. normal gearing is a lesser punch that lasts longer.

So if you are the "pinned to the seat is fun" type than shorter gearing will increase your "fun" factor signifigantly. If you are purely into performance numbers than it will probably worsen almost every measure of performance but at the cost of not as much "oomph into the seat".

So do you like a BIG oomph for a second or a pretty good size oomph for a few seconds? That is the easiest most simple way to think of the difference if you want to make it easy

You are correct, but for urban assaults love the gearing change as there's not a lot of shifting in the city.

[Serious Sam]

I have to disagree with much of what I hear about shorter diffs being no advantage. In most cases (there are always exceptions to the rule), a shorter diff is a faster diff whether in a straight line or in the twisties. Its been one of the primary things BMW has used in the M cars to differentiate (make faster) the M car from the lesser series. The only drawback I see is the fact that it would be advantageous to actually lengthen 1st and 2nd gear when you severely shorten the final drive. There is always a point of diminishing returns, and you have to watch out for additional shifts which might lengthen the time to 60 or in the 1/4 mile. I know that Dinan put a 3.64 in an e34 540i Auto (the same ratio thats in an e34 M5, and it went a full second faster to 60.

Before this seems like its all about shorter diffs, as I mentioned, there are exceptions. When a car makes its power lower in the rev range (NOT an M car!), you can install to low a diff that will make it all buzzy and go nowhere. The 2002 was a classic example. Guys always wanted to put 3.90's or 4.10's (the holy grail of 2002 diff's), but I always wanted the guys to stay with a 3.64. The 2002 makes is power lower and its torque lower. The shorter diffs just meant we were spending more time outside the optimal HP range. At Charlotte, I wanted the team to put in a 3.64, but they wouldnt listen. Then the diff broke and our only spare was a 3.64. They went 3 sec a lap faster in the race that in qualifying! Needless to say we stuck with the 3.64 at any track that had any higher speeds.

[GreasyGinzo]

I love the internet debate about gearing that goes on on every forum for all makes of cars. For a street driven DD car that likes to weekend race, drag or RR or autox gears are great and you will enjoy them. You will get from light to light faster and your ET's will go down as well. Yes there is a point where your final drive can be too short and hurt you but we are talking about minor shortening of the gears. I find most people that bash gears have no expirence with shorter final drives.

[eeeboy]

Stock dct vs my stock dct 3.67..... I jumped half a car at launch, 1 car end of second and kept pulling almost 2 car lengths by 100+. Hmmm...is my math right??

[BarryC]

Originally Posted by RichMattB

To respond, I don't have any information about how a shorter final drive ratio would cause you to overrev, per se. Your redline and fuel shut off (absent the Dinan remap) remains the same as it was prior to changing your rear end.

One quick comment - the overrev is not a fuel shutoff issue. With the stock DCT software, the trans will downshift at points where you are instantly in a mechanical overrev situation. I know because it happened to me a few times with my '08 car.

Thx,
BC

[dcstep]

I've got the 4.10 FD in my 6MT. That's a 6% change. To me 14% sounds like too much. In the DCT you DO have another gear, but I can't imagine 1st gear being of much use after that much change in the FD ratio.

The 4.10 is perfect for my 6MT because my main "sport" with the car is autocross, where I spend all but the first 100-ft in 2d gear. I've also got the Dinan chip that gives me another 200-rpm before the revlimiter kicks in, which it does for a second or two when I hit 70-mph on a course. It also feels better around town for third and fourth-gear acceleration on the interstate.

Remember, the gears were selected by BMW with the FD in consideration. When you make a 14% change in the FD you're going to start limiting the usefulness of one or two of your gears. The stock FD was selected with fleet mileage goals in mind, so I'm all for getting a little more agreesive with FD, but within limits.

Remember also that our E9x M3 have relatively low torque and any rolling acceleration "competition" (street race) will demand more than 5,000 rpm for any chance at getting "a jump". Whether you've got the stock FD or Dinan 3.62, you'll need to be in the right gear for any success. You can get the jump on some high-torque cars, BUT you must be high up in the rev band or you'll be toast, no matter what FD you have.

I vote for a FD change, but only a moderate change.

Dave

[swamp2]

Originally Posted by GreasyGinzo

I love the internet debate about gearing that goes on on every forum for all makes of cars. For a street driven DD car that likes to weekend race, drag or RR or autox gears are great and you will enjoy them. You will get from light to light faster and your ET's will go down as well. Yes there is a point where your final drive can be too short and hurt you but we are talking about minor shortening of the gears. I find most people that bash gears have no expirence with shorter final drives.

I find that most supporters of FD modifications have absolutely ZERO evidence of any any performance gains. Substitution of an imagined improvement with the ever so inaccurate butt-dyno is typically accepted in place of any real numbers, measured crudely or accurately.

On top of that you are simply plain wrong. You won't get a better 1/4 ET from a FD mod in an M3. If there is any benefit it will likely be in the noise level, i.e. a tenth or so.