Help needed! Choosing the correct differential

[kam09]

As per title. Have had my e30 laid up for a good two years after converting it from an auto to a manual, using a dogleg gear box that originated from a 2.3 alpina (if my information is correct). It currently is mated to the original differential and is ridiculous to drive. Maxing out below 100mph and spinning in every gear and snakes like maf in 2nd.

I know its not an e21 but thought id give it the forum a go. Am hopefull in finding a resolution as the car is now required for daily driving. Thanks in advance!



Kam.

[Jeroen]

Engine? HP? Current diff ratio?

[kam09]

Sorry forgot to mention its a 325 on a h reg. pretty stock bar a 6 branch and scorpion system.

[Jeroen]

I believe early (chrome bumper) 325i's had a 3.64 diff, facelift models a 3.73. With the stock overdrive gearbox you might want to go one step shorter ie 3.73 and 3.91, but for normal road use that would be too much combined with the dogleg with 1:1 5th gear and close ratio design. I guess in that case shorter would be too much, unless it's for trackday use. Don't think I'd go more than 1 step shorter, your gears will only become shorter and you'll keep on shifting. Had my track car on a 3.91 with a stock 5 speed.

[nas80]

You need something between a 2.9 and a 3.2 with that box

[kam09]

Thank you all for the quick responses. Ive had the car laid up for a long time and i need it driveable again. With regards to its use it will be used as a daily/weekend car and Also it is a convertible so not track day use.

[Jeroen]

Then I can only agree with Naeem, with the dogleg box I'd personally go 1 step longer than stock ie 3.46 or 3.64.

[nas80]

you can use an online calculator to work out where your top speed / gearchanges at relative speeds are going to be. you just need to know your diff ratio (you plan on using) the gearbox gear ratios (somewhere on this site?) and the wheel tyre size.

you really need to know when the engine starts producing most of its torque and its redline limit.

http://johnmaherracing.com/calculators/ ... alculator/

[horstl]

Just my experience:
I'm running a 3.64 in my 200+hp 327i with 5-speed OD.
In my opinion this is the best combination for daily use including highway drive.
I tested same config with 3.91 - it is funny for mountain roads, but annoying at higher speeds.

For the same reason I did not install my 5-speed dog leg yet, as in combination with the 3.45 rear end it makes driveability worse on highway.
I'm trying to build a 3.38 rear end and test this combo then.

rgds

Horst
http://www.bmw-e21.hostingsiteforfree.com/index.htm