Dinan 3.62 Limited Slip Differential

[swamp2]

Originally Posted by M3Korea

not fast but It can make car quick

What does that mean, I mean, what does it really mean precisely? Does it mean the cars top speed won't be improved but acceleration will be? That isn't the case.

[swamp2]

Those with the 3.45 really like it based on reviews I've read and have had no issues. They say the car should have came like that from the factory.

Originally Posted by SneakyDeeky

I'll second that , as I changed the gearing in my 335i when I installed my Wavetrac diff and it definitely should have come from the factory like this.
For the non-Ms, 6MT = 3.08 and Auto = 3.46. I had my diff built with an Auto ratio in it and it's probably my favorite mod.

I'm curious, is this adoration based 100% on the good old butt dyno? You are aware that this is a very inaccurate way to actually measure improvements in performance? I challenge anyone who believes a FD mod provides significant OVERALL performance benefits to instrument a run before and after the mod, with an honest attempt to achieve best results. Probably a few runs should be made to average out natural variations. Perhaps also do a 5 mph rolling start to try to eliminate important quality of launch variation. We've seen both simulation data and stories of folks who either got no additional performance or got worse performance.

Where is the data on how much better this mod is?

[regular guy]

Originally Posted by swamp2

I'm curious, is this adoration based 100% on the good old butt dyno? You are aware that this is a very inaccurate way to actually measure improvements in performance? I challenge anyone who believes a FD mod provides significant OVERALL performance benefits to instrument a run before and after the mod, with an honest attempt to achieve best results. Probably a few runs should be made to average out natural variations. Perhaps also do a 5 mph rolling start to try to eliminate important quality of launch variation. We've seen both simulation data and stories of folks who either got no additional performance or got worse performance.

Where is the data on how much better this mod is?

Repeatedly running 0-130 MPH with vBox on the same street: 6 passes up, 6 passes back -- should just about cover all the bases of performance measurements.

[bruce.augenstein@comcast.]

Originally Posted by Hujan

Oh, but it is. At least so long as you continue to insist that "[w]ithout adding power (to weight ratio...) one can not add EITHER CONSISTENT nor SIGNIFICANT performance."

We should be having a friendly debate about matters of degree, but first we've got to get you to stop taking indefensible absolute positions like the above.

Honestly, I've lost count of how many times you've contradicted yourself in this thread alone.

Unless you think a tenth or so is significant, Swamp's statement is essentially correct. FD changes have little effect on overall acceleration over a quarter mile, for the reasons already mentioned (and mentioned, and mentioned).

Such minor improvements that may be available through a final-drive change tend to favor ETs, as the early advantage (off the line and gear by gear) fall to the geared car. If traction is an issue, however, all bets are off.

I first became aware of this (without knowing the why of it) back in the '80s when a couple of friendly rivals were attempting to beat up on my '85 4+3 Vette, which would run pretty consistent 13.9s at the time. One had an '87 automatic Vette with a 2.56 rear, and he could repeatedly run 14.2s with it. Changing to a 3.42 gear (same as mine) netted the same 14.2s.

The other car was an '87 Mustang 5.0 coupe, which would run fairly consistent 14.0s and .1s with a 2.73 final drive. Changing to a 3.55 cog netted him consistent 14.2s.

Having said all that, the smile factor with a set of gears is often improved, because out on the open road, just prodding the loud pedal without a downshift results in a much more satisfying belt in the back.

Bruce

[catpat8000]

Originally Posted by regular guy

Repeatedly running 0-130 MPH with vBox on the same street: 6 passes up, 6 passes back -- should just about cover all the bases of performance measurements.

RG and Swamp are completely correct - changing FD will make no significant difference in measured performance, over mph ranges longer than provided by a single gear. That is simply a fact. However, as I mentioned to Hujan last time we had this exact discussion three months ago, I used to own an LS3 powered Corvette which had some light mods, including a diff change. In my case, I changed from 3.42:1 to 3.91:1, which is about a 15% change. When I made that change, I completely understood that the car wouldn't actually become faster in the 1/4 mile. It would pull 15% harder than it did before, in each gear, and it would require shifting out of each gear 15% sooner, in mph. Overall, no much difference in 1/4 mile times or traps.

Even so, I was extremely happy with the change because even though the car *wasn't* actually faster, it *felt* a lot stronger, which I personally enjoyed. BTW, if that seems like a nonsensical statement, consider the reverse example of the 1996 Mustang. The standard Mustang V8 with the modular 4.6 liter V8 had 260 hp. The Cobra had 305 hp but they had identical transmission and diff ratios. The difference was that the Cobra revved higher. So when you drove the two cars, you were presented with the scenario that the 260 HP Mustang felt exactly as fast as the 305 HP cobra yet the Cobra had significantly faster 1/4 mile times. How could that be? Because it revved higher, it stayed in lower gears for longer. So the base V8 pulled as hard as the Cobra, just not for as long.

Anyway, my perspective is that such a change is a positive one and a reasonable mod to make but you'll need to remember, your car isn't actually faster.

I discount the argument that since it doesn't actually subtract seconds from 0-whatever, it isn't a good mod. People mod cars to suit themselves. Anyone who changed suspension, exhaust, etc is basically doing a mod to suit their personal tastes more than they are truly affecting performance. Same with an FD mod - the car can feel significantly stronger lower down than it used to, since you never do a side by side comparison with the old version.

[swamp2]

Originally Posted by catpat8000

In my case, I changed from 3.42:1 to 3.91:1, which is about a 15% change.
...
I was extremely happy with the change because even though the car *wasn't* actually faster, it *felt* a lot stronger, which I personally enjoyed.
...
Anyway, my perspective is that such a change is a positive one and a reasonable mod to make but you'll need to remember, your car isn't actually faster.
...
People mod cars to suit themselves. Anyone who changed suspension, exhaust, etc is basically doing a mod to suit their personal tastes more than they are truly affecting performance. Same with an FD mod - the car can feel significantly stronger lower down than it used to, since you never do a side by side comparison with the old version.

These comments in particular are excellent and hit the nail on the head. If folks want to shell out the bucks for a FD they simply should know what they are getting and what they clearly won't be getting. I do genuinely hope to help people understand this. Going into it with this knowledge and understanding makes the modification quite justified (of course not in any way implying that anyone needs to justify any choices to someone else, perhaps "rational" or "consistent"... whatever). Now that being said, one should also very carefully consider the price of the mod. It can be cheap and easy in some cars and totally overpriced in others...

[coloradoe92m3]

Can someone comment on why you hear about higher revving cars using shorter gears to keep up with cars making HP at lower rpm with taller gears?

[SneakyDeeky]

Originally Posted by swamp2

I'm curious, is this adoration based 100% on the good old butt dyno? You are aware that this is a very inaccurate way to actually measure improvements in performance?

Butt dyno.
I understand and knew this before purchasing, but I would absolutely do it again.

Originally Posted by bruce.augenstein@comcast.

...the smile factor with a set of gears is often improved, because out on the open road, just prodding the loud pedal without a downshift results in a much more satisfying belt in the back.

Bruce

Bingo!

[swamp2]

Originally Posted by coloradoe92m3

Can someone comment on why you hear about higher revving cars using shorter gears to keep up with cars making HP at lower rpm with taller gears?

It's fairly simple. One needs a certain amount of power (or torque) at the rear wheels (independent of the engine crank torque) for a given amount of performance. Two very broad classes of engines are lower crank torque combined with higher redline vs. high crank torque and lower redline. Think M3 vs. Corvette. One can arrive at the same level of rear wheel power and torque with a lot of gear multiplication (higher numerical gear ratios and/or final drive) accompanied by less crank torque or more crank torque and less gear multiplication. There is also the need for reasonable speeds in each gear along with typically being able to reach a drag limited top speed (in the last or perhaps next to last gear). The speed in gear thing is the same kind of give and take. High gear multiplication is great for getting more power to the rear wheels with less crank torque but the tradeoff (without a high redline) would be screaming to redline at way too low of a speed (perhaps 25 mph in first gear or something similar). That would also be accompanied by way too many shifts to get to a given speed. High gear multiplication really needs to be accompanied by a high redline and that is what we find in such cars.