Final drive ratio change

[winstontj]

How much do I need to go up (smaller #) in rear end ratio when switching from 5spd with 4.10 to dogleg box and maintain the same top end RPMs and top speed? This is a DD street car not a hillclimb car.

TIA,
TJW

[FredK]

Well, it's going to be a simple calculation.

The regular 5 speed has a .81 5th gear.
The dogleg / close ratio box has a 1.00 5th gear.

To get the same as a 4.10, just multiply

(4.10)(.81) = 3.32

So, if you have a 1:1 5th, you'll need a 3.32 to get the same top speed. Gear speeds will vary because of the different gear ratios (IIRC they're really close though).

There are 3.25 and 3.46 LSDs for the E30. If you want to run it as a daily driver, you might want to get the lower of the two gears, and get the 3.25.

[winstontj]

Going from 4.10 to 3.25 is going to be a HUGE difference in the lower gears' acceleration. I thought it would be closer to a 3.9 or 3.7.

Thanks for the info though. Do you know if I can just swap out the ring & pinion in the diff?

[Garageaholic]

Winston, email me at frank.macaluso@lmco.com and I will send you a very detailed spreadsheet consisting of transmission ratios, speeds, and final drives. It will prove to be very helpful, i promise.

[winstontj]

Problem with going down to something in the 3.25 final drive range means it'll totally kill my first gears' acceleration. I don't want to lose the 4.10's acceleration but I do want the dogleg box's gearing so it's a bit of a catch 22. Are there other options? perhaps I should further explore the options for a 6-speed install? Is 6th gear geared lower than 0.81:1?

[Garageaholic]

Isn't a 6 speed rare and more expensive?

[winstontj]

Dunno. I read about an e36 6-speed from an M car on this site somewhere. the M20 dogleg box wasn't cheap but I can't be rolling around at 5grand on the highway or my motor will develop a half life. I'd rather keep it as is and sit on the cr box than lose the acceleration of the 4.10 in first & second gear.

I asked about the /// e36 6-speed just because I know nothing about them. I assume that they are expensive as all hell, hard to find and not an easy swap - but I could be totally wrong.

[FredK]

A Getrag 420G is UBER expensive. Prices are around $2K PLUS. You can use an E46 M3 transmission as well, and these go for around $2500.

You can switch the ring and pinion in the diff, but it requires opening it up and setting gear lash. It's probably easier to just buy a whole diff. 3.25 diffs are expensive ($200+), but definitely not out of reach.