I have a 92 318is
I need to know what kind of diff would be the best for my car, i want acceleration more then i want top speed. I need Some one to school me on the different gear ratios and what would be best for my car.
Whats the difference between an open diff and a lsd
Ive tryed to look at the threads as well as google but couldn't find what i was looking for.
An open diff delivers torque to both rear drive wheels, and also allows the wheels to turn at different rates for going around corners smoothly.
http://en.wikipedia.org/wiki/Differential_%28mechanical_device%29An open differential delivers equal torque to both wheels, no matter what. This means that when one wheel loses traction, and only needs a small amount of torqure to spin, then the other wheel simply gets that same small amount of torque. The sum total torque delivered to the road therefore shrinks the less traction either of the tires have. On ice, the total torque delivered might approach zero. If you have one tire up in the air on hard cornering, the other tire gets zero torque.
The limited slip differential is designed a bit differently. It will always send a useful amount of torque to the wheel with the better traction. So you get a benefit from the limited slip feature in situations where one wheel loses grip, during racing or on loose surfaces for example.
There are different kinds of limited slip diffs, but the most common in the E30/E34 world is the clutch (plate) type. A few owners have installed helical gear differentials (torsen, Quaife, etc.) as well.
If you want more acceleration in a straight line in a 318, then an LSD probably won't help. However, if you have additional power, or if your tires are not great, or if you want that acceleration on loose surfaces (all situations where you might spin your rear wheels) then an LSD would be a significant help. If you want more acceleration during hard cornering, then absolutely an LSD will help.
As for gear ratios, there are a few to choose from in the BMW world. I'm familiar with E30 ratios, but not so much E34 ratios. In my car, which is a '91 318is with M42 motor and Getrag 240 transmission (which is stock for my year), I run a 4.27 rear end. In fact the 4.10 that was stock in my car was also pretty good. The effect of the gear/diff ratio is a torque multiplication. The higher your total gear ratio, the more torque you feel at the wheels. And since torque is what makes your car get up and go from a stop, improving your torque multiplier is a good thing.
Here's some reading:
http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=707In my case, I only improved my torque multiplication factor by about 4% by going from a 4.10 to a 4.27 ratio in the rear end.
One other thing that really helped my car accelerate better was to change to a lightweight flywheel+clutch assembly. I also mounted the lightest wheels and tires that I could. These two things reduce the rotational mass that the engine has to accelerate. Accelerating rotational mass takes more power from the engine than accelerating static mass, pound for pound that is, mainly because the rotating mass has to move faster than the static mass. So lighter wheels and tires plus lighter flywheel should theoretically allow your engine to spin up faster. In practice, the flywheel is rotating a lot faster than your wheels are from a start, so lightening that has a large effect in first and second gears and less effect once you get beyond second. Lightening the wheels has a more subtle effect, and assists handling dynamics in other ways too, so it is good thing to do, but does not give you much of a boost in acceleration.
