BimmerFest BMW Forum banner

differentials

4K views 49 replies 9 participants last post by  bmw-mania 
#1 ·
hey so i just finishing up a 302 5.0 swap into my 92 325i and i was wondering what differential i should use. its a daily driver and weekend drift car. i was lookin into the 3.91 but figured id ask since you guys probobly know more about the facts while im only good at installing and making stuff work. lol so let the suggestions begin...also axels? let me know what i need!!
 
#7 ·
nope thats right
No it is incorrect. What you are marking as 6th gear is actually reverse. Assuming the other info you gave is correct, here are your transmission's gear ratios:

1st 3.36:1
2nd 2.07:1
3rd 1.35:1
4th 1.00:1
5th 0.80:1
6th 0.62:1
reverse 3.28:1
 
#12 ·
Sure thing. And if you want help determining the outside diameter of any given tire size (which is needed when using the gear ratio calculator), you can use this tire size calculator:

http://www.miata.net/garage/tirecalc.html

This one is nice to have around when shopping for tires, since it lets you see the differences between two different sizes.
 
#14 · (Edited)
Well, in order to use the gear ratio/RPM calculator, you need to enter the outside diameter of your tires, so you'll have to source that information from somewhere. The tire size calculator I posted provides you with the ability to enter your tire specs (205/60, 215/50, etc...) and obtain the outside tire diameter in inches. Take that figure and plug it into the gear ratio/RPM calculator.
 
#15 ·
ok does this sound right?
speed i did 120mph (normal top speed in my opinion)
tire diameter is 23.1 inch
axel gear ratio 3.91
and i used 6th gears ratio which is .62
and i wasent sure what to use for transfer case ratio so i did 1.00

my final rpm was at 4233.03

is that correct? lol like i said i dont relly understand how to do this but if its correct than i should be able to use the 3.91 without any problem but i have a feeling im wrong lol
 
#16 · (Edited)
What size tires are you using that have a 23.1" outside diameter? A stock tire for the 325 non-sport has an outside diameter of 24.7". A tire with an outside diameter of 23.1" would be very small, and your speedometer would be wildly inaccurate.

Also, I would base your differential gear ratio decision on the highway speed limit in your area, not a theoretical top speed (which 120mph is far too low anyway). This way, you will know what your highway cruising RPM will be, which is far more applicable to real-world conditions than top speed (unless you're building a car specifically for Bonneville Speed Week or the Texas Mile, which I doubt...).

So... Say you are running stock tires, which have an outside diameter of 24.7", your diff is a 3.91:1, and your 6th gear is a 0.62:1 overdrive. At a speed of 65mph, which I believe is your state's typical maximum highway speed limit, your engine will be spinning at 2144.37rpm.
 
#18 ·
The 3.91 is really your best choice. If you go to the 3.38 you're going to be almost bogging the engine at highway cruising speeds. The only diff with a higher ratio is the 4.04 from the 318 and I believe that is a physically smaller diff which likely isn't a good choice to be mated up with a 5.0 liter powered vehicle.

http://www.bokchoys.com/differential/GearRatios.htm
 
#19 · (Edited)
hey so i just finishing up a 302 5.0 swap into my 92 325i..let me know what i need!!
It seems like you need your head adjusted for putting a 5.sl0w motor into a BMW. :cry:
Why?

i was wondering what differential i should use..?
its a daily driver and weekend drift car.
i was lookin into the 3.91 but figured id ask since you guys probobly know more about the facts
while im only good at installing and making stuff work.
lol so let the suggestions begin...also axels?
You're a drifter...
but, you cant be bothered to learn about what you're building.. (?)
Were you trying to EDUCATE yourself?
or just wanting a answer to your query "what differential should I use?"

Ze German gave you some great places to start.. but it seems you dont know where to begin...

Your Mustang with V8 came with 2.73 - 3.73 in the diff..
BMW's with V8 use 2.93 - 3.64 in the diff..
If you have the money.. you can pick whatever ratio you want.
(dont drifters just weld the rearend anyway?? :dunno:)

If you want to make this easy..
FIND A COMPLETE BMW REAR SUBFRAME ASSY with 3.23 LSD.. (I got one if you need it)
You get the M brakes, M axles, M Diff (with LSD)
re-inforce your mounting points on your "drift" car since it IS NOT A M from the factory.
When you got the whole ass end apart for the subframe swap,
RE-inforce the mounting points for the sub-frame, and your rear Shock towers!
Unless you want the shock to blow through the wheel well in the trunk and cause you more work.
YES, it DOES HAPPEN!
You are aware of the weak spots on the standard E36 chassis, right?

Brace the hell out of your e36 if your seriously planning on Drifting it..
especially after your swap.. get the suspension dialed.
Anytime you make a adjustment... even a small one.. you have to re-check EVERYTHING.

Cheers!

EDIT:
Here is some light reading.. Enjoy..
http://forums.bimmerforums.com/forum/archive/index.php?t-853341.html
 
#23 ·
It seems like you need your head adjusted for putting a 5.sl0w motor into a BMW. :cry:
Why?
I believe it is you need your head adjusted. American car manufacturers have always been good at one thing, and that's POWER. Why wouldn't you want to take a firebreathing V8 and put it in a car that can actually take a corner well? It is the perfect combination, taking the best aspects from both cars to create a monster in the straight and the corners.

OP, just like in your last swap thread, I approve this swap:thumbup:.
 
#20 ·
Aw man, now you've gone and given the recipe for the secret sauce away! I was looking forward to the future posts titled "I Ripped The Entire Rear Subframe Out of My E36" (photos included)
 
#22 ·
Since he did it right and swapped in the tranny too, it should do OK as a DD. If he'd used an adapter kit and mated it up to the OEM tranny which has shorter and closer gear ratios, not so much fun, particularly with that 3.91.........

I had a friend that had an older Chevelle that was set up for the 1/4 mile and was also his daily driver. The 5.13 rear diff made it awesome on the track but it just wore you out driving it on the street with the continuous shifting required.
 
#48 ·
alex, you mean the "XX" isn't dos equis?? :mad:
Ya, sorry DF, Don is not the most interesting man in the world:(.

Im not your friend, guy. :p
I am not you guy, buddy!

if if if..
if we were talking about a $15,000 all aluminum race engine.
BUT, we're NOT.. Were talking about a 5.0 lump
You are the one who brought up 302 with lightweight components. I was just showing you how little a 302 with lightweight parts installed would weigh.

Why dont you go ahead and do that?
Maybe in a few years when I swap my engine, if I go with a 302 I will do just that.

Dont ever reference a F@#$ing MG forum again.. ever. : puke:
I was originally planning on buying a Mazda Miata as a second car, but considering your reaction to MGs maybe I should get a MG Midget instead :p.

waiting for alex to reply "i'm not your guy".....:p
You saw where I was going with that. Unfortunately my timing got a little thrown off by GW2, but oh well here it is anyway.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top