[lbjgh]

Is there any way to have the 3.73 rear gears from the 28i installed on a 35i factory order?

[paul386]

I think you know the answer. haha

Yeah I would hedge a good bit of money that it would be impossible to do "factory".

Maybe you could order the hardware as individual parts and do it yourself.

[Lotus7]

Originally Posted by lbjgh

Is there any way to have the 3.73 rear gears from the 28i installed on a 35i factory order?

The 28i's 3.73 gears CAN be substituted for the 3.39 ratios in the 35i, but ONLY if your name is Ludwig Willisch.

Seriously, with the very low 4.71 first gear ratio of the ZF 8 speed, the 35i is already over geared with a real "stump puller" as it stands. the N55 engine produces 80% of full torque by 1400rpm (6 to 7 mph) which happens virtually instantly if you "floor it" while using the auto-hold function.

In "normal driving" first gear is so low that the conventional "Drive" programming shifts to second around 10 mph. With the 35i's low rpm torque availability, a lower set of axle gear ratios might only make sense if you were planning on constantly pulling a 2 ton trailer.

[lbjgh]

I probably wouldn't be anything more dificult than buying a 28i ring gear and pinion gear and having the dealer install it.

[paul386]

Originally Posted by Lotus7

The 28i's 3.73 gears CAN be substituted for the 2.81 ratios in the 35i, but ONLY if your name is Ludwig Willisch.

Seriously, with the very low 4.71 first gear ratio of the ZF 8 speed, the 35i is already over geared with a real "stump puller" as it stands. the N55 engine produces 80% of full torque by 1400rpm (6 to 7 mph) which happens virtually instantly if you "floor it" while using the auto-hold function.

In "normal driving" first gear is so low that the conventional "Drive" programming shifts to second around 10 mph. With the 35i's low rpm torque availability, a lower set of axle gear ratios might only make sense if you were planning on constantly pulling a 2 ton trailer.

Its a worthy thought if the 10% higher torque would give a benefit! Sometimes I question the gear ratios in the X3 (35i).

[Lotus7]

Originally Posted by lbjgh

I probably wouldn't be anything more dificult than buying a 28i ring gear and pinion gear and having the dealer install it.

Actually (2) ring gears and (2) pinion gears. Changing one set will destroy the transfer case clutch and won't get you very far.

Also, of course, speedometer calibration, and stability control calibration should be changed which is not something that be could accomplished by a "home hacker" (BMW source code is encrypted and not available to the dealers, let alone the public).

[tuco44]

Warranty issues?

I guess my question is - why? Seems like a lot of hassle to me, unless one is smarter than the BMW engineers.....

[lbjgh]

... get the 35i to acclerate like the 28i in the first couple of gears.

Originally Posted by tuco44

Warranty issues?

I guess my question is - why? Seems like a lot of hassle to me, unless one is smarter than the BMW engineers.....

[Lotus7]

Originally Posted by lbjgh

... get the 35i to acclerate like the 28i in the first couple of gears.

X3 (F25) 35i BMW manufacturer's rated acceleration 0-60mph = 5.6 seconds
BMW Car (UK magazine) test results = 5.5 seconds. (and that includes all turbo lag)
Shift points: (1-2) 35 mph, (2-3) 52.5mph

X3 (F25) 28i BMW manufacturer's rated acceleration 0-60 mph = 6.7 seconds
Best published magazine acceleration published = 6.7 seconds.
Shift points: (1-2) 31.8, (2-3) 47.7mph

That means the 35i is 105.6 feet farther down the road at 60 mph than a 28i.

I sure wouldn't want to make my 35i accelerate like a 28i.

I've personally driven (3) F25 35i's and (2) F25 28i's and every one of the 35i's was dramatically faster in gears 1, 2, and 3 (as well as the other 5) than any 28i. That's at full throttle, pedal to the metal, flat out acceleration. The "slow" 35i throttle mapping is an entirely different issue, and is solved by just pressing the pedal a little harder.
If you found a 35i that was measurably* slower accelerating then any 28i, you found a real dog.

It's also possible that you were fooled into believing that 28i seemed faster since it shifted sooner because of the lower gearing, but the 28i was also going slower at each shift point.

Due to the 28i's lower gearing, torque at both axles is increased by 10.0% (3.73/3.39)
BUT and it's a big but: The torque of the 35 i is 400Nm and the 28i is 310 Nm (410/310) that's 29% more yielding a net forward thrust INCREASE of 19% at the wheels. In addition that thrust is maintained longer (to a higher speed) in each gear because of the beneficial gear ratios of the 35i. This doesn't even consider the 35i's much flatter, higher integrated area torque curve compared to the 28i's narrower peak. More thrust for a longer time at higher speeds integrates to faster acceleration.

*As measured by precise timing equipment, not someone's "seat of the pants" perception.

[jk]

Originally Posted by Lotus7

X3 (F25) 35i BMW manufacturer's rated acceleration 0-60mph = 5.6 seconds
BMW Car (UK magazine test results = 5.5 seconds.
Shift points: (1-2) 35 mph, (2-3) 52.5mph

X3 (F25) 28i BMW manufacturer's rated acceleration 0-60 mph = 6.7 seconds
Best published magazine acceleration published = 6.7 seconds.
Shift points: (1-2) 31.8, (2-3) 47.7mph

That means the 35i is 105.6 feet farther down the road at 60 mph than a 28i.

I sure wouldn't want to make my 35i accelerate like a 28i.

I've personally driven (3) F25 35i's and (2) F25 28i's and every one of the 35i's was dramatically faster in gears 1, 2, and 3 (as well as the other 5) than any 28i.
If you found a 35i that was measurably* slower accelerating then any 28i, you found a real dog.

It's also possible that you were fooled into believing that 28i seemed faster since it shifted sooner because of the lower gearing, but the 28i was also going slower at each shift point.

Due to the 28i's lower gearing, torque at both axles is increased by 10.0% (3.73/3.39)
BUT and it's a big but: The torque of the 35 i is 400Nm and the 28i is 310 NM (410/310) that's 29% more yielding a net forward thrust INCREASE of 19% at the wheels. In addition that thrust is maintained longer (to a higher speed) in each gear because of the beneficial gear ratios of the 35i. More thrust for a longer time at higher speeds integrates to faster acceleration.

*As measured by precise timing equipment, not someone's "seat of the pants" perception.

lotus7, i just want to thank you for all your posts. i'm a newcomer here, have been reading a bunch of threads, old and new, and always find your comments precise, to the point, and highly educational.

and, btw, i should be picking up my 35i in spartanburg in january.

[Lotus7]

Originally Posted by jk

lotus7, i just want to thank you for all your posts. i'm a newcomer here, have been reading a bunch of threads, old and new, and always find your comments precise, to the point, and highly educational.

and, btw, i should be picking up my 35i in spartanburg in january.

Thanks. Enjoy your factory pick up. And, I'd be willing to bet that once your 35i is fully broken-in (please wait until it has at least 1200 miles before you try any full throttle launches), it will be faster than a 28i.

[lbjgh]

Thanks for the clarification Lotus... science and facts trump seat of pants every time.

[Lotus7]

Originally Posted by lbjgh

Thanks for the clarification Lotus... science and facts trump seat of pants every time.

The 28i is clearly not slow by any means and even though the N52 has been around a few years it's a great reliable engine, but unfortunately offers no fuel efficiency over the N55, so is being retired by BMW. It will be interesting to see how the 28i "character" changes with the installation of the "hot" N20 when its released.

The "magic" combination these days seems to be direct injection, continuously variable valve timing on both intakes and exhausts, low-boost turbocharging (allowing high compression), and "on-demand" accessory drives. For BMW that means the N55 and N20.

Back to the current N52 vs. N55 acceleration, a LOT of the perception is caused by the obviously economy-minded throttle mapping of the 35i. The first 20 or 30% of throttle travel doesn't actually increase power very much (in the normal basic "Drive" program, but then things start to happen. It does take some adjustment, and I don't particularly like the non-liner throttle/torque mapping. The Sport or Sport+ settings are quicker responding, but with the extended shift points do take a big toll on fuel economy in normal driving. To drive a 35i "aggressively" you have to really mean it.

To really feel what a X3 35i can do, just hold your left foot on the brake pedal, then bring the engine revs up to 1200 rpm or so and simultaneously release the brake as you bury the throttle pedal. There is about 1/5th of a second of turbo lag as the turbo spins up and revs build to about 1500, then the full N55 torque is released and continues without any break up to about 6000 rpm with only a minor reduction all the way to the 7000rpm redline.

The ZF 8-speed than shifts and it happens all over again.