engine and drivetrain Any one running a gear set lower than 4:11/1 pros and cons

[peteinjp]

First off I'll say that I have a stock tii and the engine will stay stock for a while yet. My 02 is no longer a daily driver and I drive very hilly roads with tight corners. Accelerating out of a tight corner going uphill is- well- less than breath taking. On the other hand going down hill with the 4:11 (40% unit from ardvarc) is more than adequate. With no slope I feel that 3rd and 4th gears could use a little more umph. So I'm toying with the idea of gearing down the rear even more because I will not be doing any highway trips and I really never need to go 190 kph (5 speed overdrive). Basically I'm into building a rally car. With all that said I'd be psyched to hear from people with experience with gearing lower than 4:11.

[albawhore]

I know that Korman sells a 4.22 kit and he recommends it with the 5 speed OD trans mod I have heard of racers running 4.56's and even 4.75's

Cheers

Justin

[MichaelP]

If you're looking for more torque at the wheels, how 'bout smaller diameter tires? It's cheaper than a special-order diff, and easy to swap back if you don't like it.

I'm not smart enough to calculate the difference for torque, but the math for rpms with different tire diameters with the same diff is

rpm = (speed x diff ratio x 336) / tire dia

assume 50 mph and 23.2" as stock tire diameter (185/70 x 13):

(50 x 4.11 x 336) / 23.2 = 2976 rpm

If you go to, say, 185/60 x 13s, tire dia is 21.8":

(50 x 4.11 x 336) / 21.8 = 3167 rpm

If you grok that back through the first equation, you end up with an equivalent of a 4.37:1 ratio by going with a 1.4" smaller dia tire.

[bmw1602.com]

i'm a bit confused,

as far as I know...

3.91 > 4.11 = less top speed, shorter time in between gears....zipper

4.11 > 3.91 = higher theoretical top speed...longer time in between gears...

[ColinK]

Good to know but the gear set isn't cheap..

so much for poor man's CR...

[steve k.]

i've ran 4.22 in the past. It's fun. Very low top speed so freeway driving is not much fun. it will all depend on the track you will be running. Shift time vs. torque.

steve k.

[tomorrowcalling]

I'm running a 4.45 - good for a rally stage but the engine does get a bit busy on the touring stages. I'm considering a swap to a Sierra 5 speed with quaffe close ratio kit but I haven't completed the homework on how it will all fit yet.

[winstontj]

Good to know but the gear set isn't cheap..

so much for poor man's CR...

IMO there isn't really any substitute for the CR. Lower rear end ratio will get your rpms up but it's not going to change that 2-3 gap that can be brutal on the OD unit.

i'm a bit confused,

as far as I know...

3.91 > 4.11 = less top speed, shorter time in between gears....zipper

4.11 > 3.91 = higher theoretical top speed...longer time in between gears...

This is correct. I believe that MichaelP was trying to explain is that it's all ratios. 2+3+4=9 just like 2+3.5+3.5=9 so you can change numbers around to get different end results. Changing tire size is the same mathmaticlly as changing diff size, it's # of teeth / # of teeth in the diff while on a tire it's # of inches (circumfrence) / 360 degrees

Smaller tires = same as lower ratio diff (higher number) just like higher ratio diff (lower number) = larger tires.

I **thought** that when you build a car you start with your red line RPM and final drive to calculate top speed (for the given venue you will be running) and then factor in gearsets (CR race vs. rally) and tire size depending on smaller variables. If you need more pull in lower gears AND top speed it's best to raise the rev limit (may take a differently designed engine) and keep the lower ratio (higher number) diff so you have the pull out of corners for example, and the top speed will be increased with the increase of the rev limit.

Just my .02

[steve k.]

what are the 4 speed, e21 5 speed and CR 5 speed ratios?

steve k.

[winstontj]

what are the 4 speed, e21 5 speed and CR 5 speed ratios?

steve k.

Umm....

http://www.bmw2002faq.com/content/view/56/32/

posted all on the FAQ.

I was certain that somewhere I had the difference between the race & rally CR gearsets... I know it's in the Restoration Guide but I don't have that at work.

Is that what you mean steve?

[steve k.]

great. brain dead but what about CR ratios?

wake me up when it's all over

[ingraham]

Torque is inversely proportional to tire diameter, so a 10% smaller tire give you 10% more torque (torque at the tires, not torque at the engine).

[peteinjp]

IMO there isn't really any substitute for the CR. Lower rear end ratio will get your rpms up but it's not going to change that 2-3 gap that can be brutal on the OD unit.

This is true but with the stock tii the torque is there from 3000 so a longer spread on the gears actually suits the car well. I'm just looking for more pull in 3rd and 4th. Thinking maybe 4:45 would be fun to try and is available but at a certian point I imagine I'll end up losing time to the shift that I gained with acceleration. One of the reasons for doing this is that here in Japan i cannot find a wide low profile tire to keep the gearing down.

I have a set of 13's I could run with 185/60's on the 4.11 and I have a set of bbs rs 15x7's that I could run with 205'50's. In doing so if I went to a 4.45:1 diff I would end up with the same overall ratio with a lot more rubber on the ground. Or I could order hoosiers in the 225/45/13 size and have the best of both worlds and a fender pulling party.

[steve k.]

depends on how low your car is you might be able to run 225/45R13 without fender works. they are that much smaller then in diameter thent he stock tire.

steve k.

[rallysmurf]

I'm running a 4.45 - good for a rally stage but the engine does get a bit busy on the touring stages. I'm considering a swap to a Sierra 5 speed with quaffe close ratio kit but I haven't completed the homework on how it will all fit yet.

i know this is an old thread .

but i am building a 02 rally car and need all the input i

can fine. i have a 4.27? not on the car yet. do you think

i need a 4.45? what trans are you running with your 4.45?

have a web site or pics of the car?

alex in texas