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4.10 or 3.91 ?

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Hello, i have a 323i - 2.7L and i plan to install a dogleg transmission for my car, and knowing that it will have very close gear ratio, is it better with a 4.10 or 3.91 differencial ?
thank you.
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Those two are pretty close - if I had to choose I would go with the 3.91.
With a dogleg's 1:1 ratio for the final drive I would be looking more at a 3.2X-3.4X range. That is way to high of a final drive with that transmission.
Thank you both, so I will go with the 3.91 and see how it drives.
this

for reference my 316i is sitting at 3200rpm in 5th gear with a 4.1 final drive, and thats with an overdriven 5th gear
With a dogleg's 1:1 ratio for the final drive I would be looking more at a 3.2X-3.4X range. That is way to high of a final drive with that transmission.
3200 in 5'th doing ???

Andy, at what speed :dunno:
What is the final drive ratio ?
I suppose Beemers, like all others, have different final drive ratio's on their 5'th...
Dogleg's for E30's had a final ratio of 1:1 with some minor exceptions.
oh thats at 100km/h

final drive = differential. no matter what ratio the gearbox is in, its reduced by x.xx amount before it goes through the rear wheels. the top gear in your gearbox/transmission is NOT your final drive ratio, thats the ratio that your diff reduces by. if the ratio was exactly 4, then the driveshaft turns four times for every one turn of each rear wheel, or one turn of the driveshaft turns the wheels only one 1/4 round.

example of different final drive in action: drive a jap import 318i with a final drive of 3.4:1 while a new zealand new 316i has a 4.1:1 diff, and you will find the 316i is quicker than the 318i even though at the fly the 318i has more power and they have the exact same gearbox. swapping the diff is the quickest and cheapest way to increase percieved power in a car
Andy, at what speed :dunno:
What is the final drive ratio ?
I suppose Beemers, like all others, have different final drive ratio's on their 5'th...
3200 rpm at 100 km/h is not so nice, but in exchange you will have a good acceleration :thumbup: and thanks for the technical info
yep no problem :thumbup:
3200 rpm at 100 km/h is not so nice, but in exchange you will have a good acceleration :thumbup: and thanks for the technical info
Gear ratio's

Again my feeble lack of communication rears its head :eek:
And then to cosider that I wrote some fancy Dyno prigrams to enable you to choose your gear ratio's- I should retire :tsk:

Just to summarise my thoughts, originating in the days of running Hewland boxes.
You have a gear ratio in each gear of your gearbox with the final drive usually being overdiven anything from 15 to 27 %.
Then you have your diff ratio, that is numerically lower as your engine torque increases.

Most people only have access to different differential ratio's, whilst we should strive to be able to change the internals of the gearbox to give us the most suitable powerband :bigpimp:
The easiest way is to chuck the motor on a Dyno and see where it makes power in the Rpm band & then choose the gears accordingly.
I normally choose 1'st so that I need not slip my clutch (in the old days of open wheel racing a severe offence) and then go up in the range so that I never fall out of the power band.
The final gearbox ratio being determined by what top-speed I need to attain.

I see a lot of BMW's ran Borg Warner & Getrag boxes, so getting different internals should not be too difficult :thumbup:
I have seen some old school bmw mechanics taking the gearbox apart and changing the internals like you are saying, with amazing results :thumbup:
Most people only have access to different differential ratio's, whilst we should strive to be able to change the internals of the gearbox to give us the most suitable powerband :bigpimp:
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