# final drive ratio vs. performance



## hector (Jul 14, 2003)

question; does a shorter rear axle(higher numerically) give better acceleration in all gears or only in 1st gear, what i'm getting at is since with the shorter rear one will have to upshift sooner do you then lose the torque multiplication advantage of the shoreter rear?


----------



## Kaz (Dec 21, 2001)

Given 2 cars with identical transmission ratios and just a difference in rear end ratio, the car with the shorter rear end, at a given speed and gear, will be at a higher RPM, and thus typically, in a meatier part of the powerband.

It's really noticeable when driving. Even with the sizable power deficit, my 325 with a 3.46 doesn't feel as differently as it should compared to a 330 with a 2.93 in, for example, a 50-65MPH passing move in 3rd gear.


----------



## hector (Jul 14, 2003)

hmmm, interesting perspectives which seem to take opposing views, i understand what was said about being in the meatier part of the powerband but what i'm suggesting is that if would'nt downshifting one gear accomplish the same thing (as was stated in the first response)? of course in 1st gear you couldn't do that but i'm noot concerned with drag-strip times so much as 2nd gear and up acceleration.


----------



## Nick325xiT 5spd (Dec 24, 2001)

One thing I noticed was that the 330 generally had to be downshifted a lot earlier than my wagon. drove me nuts when I drove one.

Dropping a gear only works if your speed will allow--up to a point, the shorter ratio will also dramatically improve top end acceleration.


----------



## Kaz (Dec 21, 2001)

In my experience, it doesn't improve top-end acceleration one bit if you're short on power. At 110mph, a 330 will just walk away from me. But in general, if you compare what can be compared in real life (a 330 with a 2.93 vs a 325 with a 3.46) the diff ratio does go a long way in closing the seat-of-the-pants gap between the two cars.


----------



## Moderato (Nov 24, 2003)

Kaz said:


> In my experience, it doesn't improve top-end acceleration one bit if you're short on power. At 110mph, a 330 will just walk away from me. But in general, if you compare what can be compared in real life (a 330 with a 2.93 vs a 325 with a 3.46) the diff ratio does go a long way in closing the seat-of-the-pants gap between the two cars.


So your saying because of the gearing, during around town driving the 325 and the 330 will feel similar in terms of acceleration, but the 330 will have a noticable advantage over 100mph? If that's true then can you remind me why did I buy a 330 instead of a 325?


----------



## Pinecone (Apr 3, 2002)

Well, because you can drop in the 3.46 and have great fun. 

here are trade offs with everything. At some point, a slightly high er gear ratio will hurt acceleration because you are in a higher gear. But that is only a small part of the range.

Froma standing start to a given speed or distance, typically a higher numerical ratio will give better acceleration. The nly problem comes when the point at which you measure has an extra shift JUST before measuring. That is why cars are typically geared to do just over 60 MPH (or 100 KPH) to enhance the specs for sales. But if you go to a gearing that tops out in 3rd just past 60, then your acceleration will be enough better to overcome th etime of the shift.

Also, there are no real blanket statements about whether a downshift inthe lower numerical ratio will be better. That could put you too high in the power band (depends on the engine and tranny gear ratios), while the higher numerical ratio would be int eh meat of the power band. The torque differences could offset the gear ratio differences. OF course you could also change the rear ratio enough that the overall ratio is the same one gear up versus the lower numerical ratio. Say 10% gear ratio steps in the, raise rear ratio 10% so now 3rd is the same as the stock cars 2nd. 

So like anything, you have to look at YOUR application and change things to best suit the conditions. There are people in autocross that change diffs to different ratios for different courses. 

It is VERY common in road racing where the rules allow. And if allowed teams even change tranny ratios to get the engine in the power band at the places where it is most important. The idea being you want to top out in the highest gear at the end of the longest straight, and you want to be in teh power band exiting the corner onto the longeest straight. hen you adjust to top out in a gear at the end of the second longest straight, with exiting onto that staright in the power band. And so forth, and then tweak to get the lowest lap time.

In drag racing, typically the higher numerical the ratio the better. Some of the street stock guys are running 5.60 ratios and in some cases even higher.

So what do you want out of your car?

If you want it to FEEL peppier, change the rear ratio to a numerical higher ratio.


----------



## SAZMan (Mar 6, 2004)

Wow, this is like deja vu all over again.

http://www.bimmerfest.com/forums/showthread.php?t=52816&highlight=diff


----------



## Moderato (Nov 24, 2003)

SAZMan said:


> Wow, this is like deja vu all over again.
> 
> http://www.bimmerfest.com/forums/showthread.php?t=52816&highlight=diff


Yeah but in that thread they never got into the issue of raising the redline to compensate for the fact that a shorter rear diff will accelerate faster to redline, but also redline at a slower speed. By increasing the redline you gain back what you lost from the shorter rear diff. Of course some might not be comfortable with raising the redline because of engine & warranty issues. I was considering putting in a diff, chip, suspension etc. in my 330i, but by the time I spend all that money I'm now thinking of keeping my 330i, keeping it stock and seeing if I can put that money towards buying a used "sports car", something built from the ground up for speed as a third car for track and weekend fun. If I can't swing a third car I will look into selling the 330i and getting an E46 or E90 M3 at some point in the future. Either way I have to have at least one BMW in my garage, they are great cars.


----------



## Jspeed (Dec 23, 2001)

I did some math on this subject before and found that a 10% increase in final drive ratio (numerically) results in a 5% increase in aggregate-torque at the wheels. In other words, having to shift earlier with the shorter final drive ratio negates about half of the torque multiplication gained.

http://www.bimmerfest.com/forums/showthread.php?t=44033


----------



## Nick325xiT 5spd (Dec 24, 2001)

rumratt said:


> There's no possible way to justify this statement, unless you assume the car with the taller diff isn't allowed to downshift (which is a retarded assumption).
> 
> And the idea of needing to downshift more in the 330 doesn't make any sense either, unless you were trying to keep the cars in the same gear all the time (rather than shifting at a fixed RPM.)


 let's see. in a corvette Z06, for example, switching to a shorter rear end can reduce the quarter mile time by 1 second. but obviously that can't be the rear end because they could have downshifted, right? if a car is geared so that it's top speed occurs before its gearing limit, then you can gear it generally gear it shorter. particularly if it's a top end biased car.

gearing is often at least partially related to fuel efficiency. ever drive a WRX? there's a classic example of a car that's outriht crippled by excessively tall gearing. It's so tall that not only does top end acceleration suck, but it drops out of the power band in between shifts.

in the city at slow speeds, the 330 will not dawdle around in fourth like my 325 or my M3 will. it's significantly closer to idle. as a result, you have to drive it around in a lower gear at these speeds. I also had to shift to first a number of times, rather than leaving it in second when I would have in my wagon. so yes, i was trying to keep it in a similar gear, but what is your point?

And what's your obsession with downshifting? Any idiot knows that the less shifting you have to do, the better.


----------



## Pinecone (Apr 3, 2002)

Dropping out of the power band doesn't have anything to do with rear ratio, but it is based on trany ratios. And excessively spaced tranny ratios appear when the company tries to give the car good off the line performance AND very tall upper gears for crusing, and does so without adding more gears.

For any given engine, there is an optimal tranny ratio spacing for general street use. To increase the spread between first and top, you need to add more gears to do it right.

As for the extra shifts, remember there are points where you get more acceleration, but no extra shift. Going from a 3.62 to a 3.91 in an E46 M3 still allows you to hit 60 in 2nd. BUt it will get there quicker. Now 0 - 65 will suffer because of the extra shift.


----------

