91 and later 750iL's have 3.64 LS final drives (archive)

The 3.15's are used in the 88-90 cars. As mine both have the 3.64 I couldn't comment on improved accelleration from a taller gear set. Interestingly, Emir of the Vancouver 8's, emailed me 2 weeks ago about how he was putting in a 4.10 gear set in his white 850. This wouldn't give you any service life indication on the transmission but with the M70/4HP24 trans combo in the post 91 7ers, going to a 4.10 shouldn't be a problem for the trans, but I agree with Greg that increased wear to the rear suspension and CV joints are possible if your like me and turn the ASC off and stage the engine to stall rpm for max holeshot.

Guy Simpson

Al,

You pose an interesting question. Changing the differential ratio by itself shouldn't cause any problems for the transmission. The trans would continue to feed the same engine torque and horsepower levels to the rear regardless of the change in the final drive ratio. The only parts that would see higher torque values would be the rear suspension bushings, rear axles, cv joints, wheel bearings, rims and tires. The increased wear to these parts is more dependent on your driving style than the upgrade.

The only way I can think of increased loads to the transmission resulting from a modified differential would (1) from motor braking, or possibly (2) from the addition of a limited slip unit and large rear tires.

The latter consideration would cause a certain amount of torque or "load" to be transferred back up the the driveline and in theory through the transmission and finally to the engine. This would in theory result from the more efficient hook-up of two larger rear tires simultaneously vs. one which tends to spin much more readily and thus "de-load" the driveline to some degree.

I don't know what the long-term effects would be on the transmission, but if its any indication, many of the shared V12 750i's came with limited slip 3.15LS units and 4spd autobox. The increased load doesn't appear to have been a concern for BMW or from the look of it, from the owner's perspective.

Hope this helps anwer your question,

Greg K

Greg --
What are your thoughts on auto transmission durability for a '93 850 if the final drive ratio is changed as you suggest?
Thanks
Al

First a disclaimer. Anyone who can measure and *repeat* real world tuning improvements in tenths of a second is likely a professional driver with a support staff of technicians on a closed and carefully manicured tarmac. That said, some changes like final drive ratios and the Conforti chips do make a dramatic difference in everyday driving, but you won't likely be able to re-enact Dinan's or magazine's improvement times. The times I've used are pretty conservative compared to magazines, Dinan, etc., and based on G-tech before and after results for my 6spd.

Quite frankly, I'd be surprised to see a chip for either the transmisison or engine really affect acceleration dramatically. Maybe a tenth of a second or so, but not much more. The real gain to be had isn't from chips but from changing the rear end's final drive gearing to something more reasonable. After that, the benefit of chips becomes more noticeable as the car enters the upper rpm range where the chips and things like free flow exhaust have a pronounced effect. Stock, the cars are geared severely tall, totally sacraficing low end acceleration in favor of comfortable 150mph cruising and 172-190mph top speeds. New 4k+ pound performance BMW's including the M5 and 740 Sport that continue to share 8-series transmission ratios don't have their 2.65 ratios, but instead come with 3.15LS final drive units. Not a coincidence. BMW has rectified this tall gearing issue since the 8's in favor of faster acceleration. Changing my 6spd from a 2.65 rear to a 3.15LS was the most worthwhile modification to date. I think the upgraded rear alone, particularly with a limited slip unit having the higher locking percentage to hook the large 285 tires to the pavement, was worth a 0.3 second or better improvment to 60mph. With the addition of chips and a free flow exhaust, the 6spd's improvement is a solid 0.3-0.4 seconds for about 5.8 seconds to 60mph. You should expect the same 0.3-0.4 second improvement for the 850ia 4spd autobox, or maybe even a little better since a new final drive would compensate even more for greater disadvantage of the 4spd autobox, so somewhere around 6.4-6.5 would be a reasonable expectation for the 4spd.

Greg K

The 850i 6spd is 6.1 seconds to 60mph per BMW's official sales literature for the car. The car has the fastest of the 850i's or Ci's.

I was doing some research on which 850 was faster, and I found conflicting numbers. clube31.net has the 0-60 on a 850i at 6.1 s, and e31.net has it at 6.8 s. The reason why I am asking is because I dont want to buy the 6 speed and have it slower than an auto.

Hi Greg,
Do you know what the 0-60 woud be..after changing to a Jim Conforti performance chip..and also maybe after adding the transmission chip (if automatic)?
Randy H.
91 850iA

Follow Ups:

[ Follow Ups ] [ 8-series (E31) Message Board ] [ Msg. Board FAQ ]

Questions, comments, or problems, please visit the Roadfly help desk.