Big Thread on Diff Gearing vs. RPM's

[Hollagraphik]

If you want the actual spreadsheet, let me know, I'll send it to you

Shot in the dark here, but Do you still have this info available?

[TostitoBandito]

You can get it right here at Diffsonline's site:

http://diffsonline.com/bmw-different...formation.html

It's the "Gearing Calculator" link in red at the top of that page.

[Moron95M3]

Shot in the dark here, but Do you still have this info available?

Let me check...it was on an old work computer but should have transferred over...

[pbonsalb]

I love that gearing calculator. I have run 3.15, 3.23, 3.38 and 3.64 diffs and 245/40, 255/40 and 275/40 tires. A 3.64 with 255/40 is like a 3.58 with 245/40.

[RRSperry]

What? Only a 7 year old bump? We're getting lazy...lol

[mitch500]

man photobucket killed the forums. What a bunch of asshats


Sent from my iPhone using Tapatalk

[mistreku]

What? Only a 7 year old bump? We're getting lazy...lol

And another 2 year bump!

But on topic, you boys have tried lots of setups, it seems. I was wondering if anyone did any actual timed quarter miles, or maybe compared different setups against the same car in a drag race, and found out which is the quickest? I know a shorter diff will *feel* faster, but any exact numbers? Times, car lengths?

Only asking because I have a friend who's driving an E46 330ci and he's winning against my E36 328i (dual vanos) by a car's length on a 1/4m. We both have stock 2.93 diffs, main difference would be mine revs up to 7000 and his to the stock 6500. Do you think I could gain more than a car's length with a shorter diff?

[RRSperry]

Depends. What is your RPM at the end of the trap? What gear are you in? What does your power curve look like? If your power is falling off at 6500 to 7000, then it would probably behoove you to shift earlier than at redline.

But in general, with stockesh E36 328 power, yeah a 3.23, or a 3.38 (LSD) will be quicker without lowering top speed too much. My 3.38 lowered top speed from 158 to 149 at 7000 rpm, with 245-40 17 tires. (at 80 mph it was turning 3750 rpm, hence the 6 speed conversion)

Oh and if drag racing is your metric, get stickier tires, and work on reaction time.

[mistreku]

Well, there is no strip close to where I live, so we do it on the streets, at night. Old school style. Now, because of that, I can't tell you exactly what speed and RPM I get at the finish line, but it's about 100mph and 6500rpm's. Not sure about the peak power vs rpm, but I imagine these engines will run out of puff around 6000, maybe a tad higher. However, I have no idea why, but if I change at 7000, I stick with him better than when I shift at 6000-ish. Maybe when shifting later, the next gear falls into a better torque curve? Also, I'm thinking better tyres as well, just that I can't get the subframe out of my fearful mind, lol.

I also have a dirty itch for a very long diff, just to see what happens. I know, it's dumb, but I always wondered what the 328 will do with a 2.56. Everything points to being slower, of course, but still I'd be curious to see how it will drive. It should have better traction in 1st and 2nd, maybe enough to put a car's length between us? If anything, would be a cool experiment, especially since 2.56 diffs are going for peanuts where I live, because no one wants them.

[pbonsalb]

An M52 would be slower with a 2.56, not that I have ever read of a 2.56 in an E36. Too long for the available torque. Many years ago, a guy with a turbo USA M3 tested 2.79, 2.93 and 3.15 diffs. He had about 500 rwhp and traction was a problem. They were all comparable. You might be able to find the article online. It was written by Pablo Mazlumian and in European Car.

With a shorter diff, you might be faster in certain speed ranges and the car might feel faster overall, but it might not be any faster overall. There are online calculators that can help you determine this but you will have to do much of the work. One guy did it for the E92 M3 with 4.10 instead of 3.85 and posted the simulation and calculations on M3post

[mistreku]

Well, a turbo 500whp car will be of no help comparison wise, considering mine is probably pushing something like 180 and with a totally different torque curve. If I had a turbocharged E36, I'd definitely go for a tall diff. Sadly, I don't.

Now, with a shorter diff, I'll only be looking to be faster on 1/4m, I don't do 1/2m. Basically, looking to be faster up to maybe 120mph. And the diff mod is the easier and probably cheaper way to do it. Another option would be to save some cash and spend it on a proper exhaust, as atm, it's the stock one with just a decat and resonators deleted.
BTW, for general knowledge, deleting cats and resonators made my car considerably slower, for some reason. Also very loud. I know the general consensus is that the more free the exhaust, the better, but somehow, this is the second E36 I fuck up this way. Like all the torque shifts all the way up in the rev range and the car feels like crap. And even in the upper range, it was still slower than when it had a stock exhaust. Heads up for ppl considering "freeing up" the stock exhaust. I'm sure decat-ing is a good thing performance wise, but do it properly, with an X-pipe and some long headers.

[RRSperry]

Lol... You HAVE a long diff... You want a shorter diff. Or more power, or less weight... Face it, What you really what is faster, better, car.

[mistreku]

I wouldn't mind a GT2 RS, indeed! Alas, I'm stuck with my old banger due to the funds missing. And I believe wanting a faster car was already implied by my inquiry, wasn't it? Albeit, I'd be satisfied with just gaining two lengths on the 1/4m, just so I don't have to face my friend's smug face every time he wins.

[pbonsalb]

A tune helps if you go catless and reduce backpressure. Without one, you might not gain anything.

You need to do the calculations. Shorter diffs multiply torque more but also require more gear changes. Depending on the speeds in which you want to go faster, you may or may not be faster. For example, if you can hit 60mph at the top of 2nd now and change to a shorter diff and have to shift to 3rd to hit 60, you might be slower after due to the shifting time. Also maybe due to being out of the power band in 3rd right after the shift.

It is a lot of work to figure it out. You need the power curve, torque multiplication, speeds in gear, rpm after shifting — for the speed range you are studying.

In general, I’d encourage you to run USA S52 cams, a catless midpipe, a tune with a 7200 limiter, an M50 manifold, and a 3.38 diff. The cams and catless midpipe and M50 manifold help top end power dramatically. The tune and 3.38 help offset the bottom end loss from the M50 manifold. The higher rpm limit of the tune allows you to extend the power band, which is especially important because the 3.38 would otherwise more significantly reduce the speed before you shift to the next gear.

Turbo the M52 and you can have 400-500 rwhp, run long gearing that makes highway driving more relaxed, and still be faster than nearly all cars you will encounter.

[TostitoBandito]

I like the nice round numbers I get with the 3.46 in my M3. The 7000rpm redline on gears 2-5 is 60/90/120/150.

[RRSperry]

You want 2/10s? Tires. Simple.

[NeilM]

I like the nice round numbers I get with the 3.46 in my M3. The 7000rpm redline on gears 2-5 is 60/90/120/150.

I like the 3.46 in my M3 — plus its 6-speed, which adds very relaxed highway cruising.

75 mph in 6th is a calculated 3025 rpm with my street tires (Michelin PS4S 245/40-17 at 841 rev/mile).
Calculated speeds in the gears at 7000 rpm are 34/58/86/117/144/174 mph (the last being purely theoretical, of course).

Neil

[mistreku]

I'm only interested in diff ratio gains, the M50 and the lot don't apply here. I run a M52TUB28 from the E46, dual vanos engine. It already has the M54B30 intake manifold, injectors, MAF, ECU, throttle body, accelerator pedal, DISA, camshafts, CAI and remap. It is pretty much identical to the M54B30, except for lacking 200cc in displacement. The thing with the 330i I am having trouble with, it is not stock either. He already has the M54 bits I added to my E36 but extra, he has long headers and a better CAI.

- - - Updated - - -

I know I should address the exhaust side of things, but ATM, all I could do was to decat it. Planning on getting some long headers and a proper exhaust, but diff mod seems easier and cheaper, right now.

[pbonsalb]

You want 2/10s? Tires. Simple.

You can get stickier tires for a better launch if traction is an issue or smaller tires to effectively shorten gearing. I run 295/30/18 rears in my E90M3 that shorten my rear drive from the 3.85 with 285/35/18 to nearly 4.10.

[Moron95M3]

can't believe i did this thread 9 years ago....I still remember doing it while at work lol. Glad photobucket finally made hosting okay again...

...anyway, carry on.