diffs and changing the gearing

[g57yle]

i have a few diffs but only 1 nice 1. its a 3.91:1 LSD. is it possible to modify it using other parts from diffs to get a lower ratio. say 2.8:1 or 3.5:1 and what would i need to change? my engine has nice power at 12psi but first gear is waaayy too short and my top speed is limited to about 140. i would like a 160-180mph limit i i cant modify it what ratio do you think id need to teg to that top speed? i also have a 325i sport gearbox. im not sure if that makes a difference. and just 1 more thing. the clock only shows a max of 150mph. is there a way to change it to 1 that has a higher top speed? thanksThanks in advance

G 57YLE MOTORSPORT inc

[Mark W.]

first you have to find out the gearing in the tranning, know the shiftpoints and revlimits of the motor, and tire diamiter. then you can calculate the your most desirable final drive. Here is a link to a calculator(for the transfer case ratio just enter 1)...




Gear/MPH calc
calc for tire size

[mr-winny]

.....

Mark W.

[bilodeaulynn]

My first question is...ARE YOU INSANE? You want to exceed 150 mph in a 3 series? Hope yo live in Montana, or have access to a pretty long track. I don't think I could bring myself to go over 135 in my 745 although I know it will exceed that. (Not that I've ever done it officer). But, then again, that's your business.

Secondly, yes, you can change gears from one diff to another, but unless you are experienced with setting up a ring and pinion, you are asking for trouble. Shims are used to set the pinion depth and also used to get the ring gear "centered" so to speak (not the proper terminoligy, but gets the point across) so that you will have the proper backlash. Very close tolerances. Unless you have a large assortment of the shims, the chances of one coming out just right are slim. Miss just a little, and it will likely howl like a lonely wolf at a full moon.

What year is your 3 series? Your best bet is to find a complete differential in the gear ratio you want and swap it out. You might even find someone with a naturally aspirated car that could benefit from your short gear for quicker accelleration (wihtout trying to top 150!!!!). BTW, I am really not trying to be a spart [Oops!] here, but the higher the number, the "lower" the gearing in the diff, so you really want a higher geared diff (lower number). Look in your owner's manual for a description of the gear ratios in the transmission. Notice that first gear has a much higher numeric number, but it is obviously the lowest gear. Good luck with your project.

Lynn

[g57yle]

your right i get pritty scared at 130mph. but when im crusing, lets say at 80mph its on about 4500rpm! in 5th and it sounds like your coming to its limit already!. The main thing is my first gear seems way too short and i cant cruse without getting a headacke. the reason ive said that about the top speed is that then i could be doing 100mph on about 2000rpm. then that would be nice. OK 1 more question. if i buy a diff of the correct ratio. could i make it lsd by swapping a few my bits from my diff or is that complicated again?Thanks in advance

G 57YLE MOTORSPORT inc

[g57yle]

and my car is a 1990Thanks in advance

G 57YLE MOTORSPORT inc

[mr-winny]

You might want to check on the Private BMW parts group in the Yahoo groups. I have a link to it through my site there are alot of e30 guys on there or just cut and past. (http://autos.groups.yahoo.com/group/PrivateBMWParts/)

Mark W.

[bilodeaulynn]

The LSD units use a different carrier and you will have the same problem with shims etc I dexcribed before.

Sorry about the delayed answer; have been really busy and not on the board.

Lynn