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      05-01-2008, 12:01 PM   #9
Nixon
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I chewed through some numbers that I was able to find earlier, and it looked like the difference between the final drive ratios between the 135i and the 123d gives the 135i about a 9% advantage for "in gear" acceleration. The final drive ratio is 2.81:1 for the 123d, compared to the 3.08:1 for the 135i.

That's a .27:1 difference, or about a 9% difference.

Which is interesting when you look at some of the speed comparisons between the 300 ft-lb torque 135i and the 295 ft-lb torque 123d other than just the 0-60 times:

http://www.bmw.com/com/en/newvehicle...datasheet.html

135i:
Acceleration 0 - 100 km/h (in s)5.3
Acceleration 0 - 1,000 m (in s)24.4
Acceleration 80 - 120 km/h in 4th/5th gear (in s)5.0/6.0

123d:
Acceleration 0 - 100 km/h (in s)7.0
Acceleration 0 - 1,000 m (in s)27.3
Acceleration 80 - 120 km/h in 4th/5th gear (in s)5.5/6.6

The difference in 0-100 km/h is 1.7 sec, or 24% of 7 seconds.
The difference in 0-1,000 m is 2.9 sec, or 11% of 27.3 seconds.
The difference in 80-120 km/h in 4th is .5 sec, or 9% of 5.5 seconds.
The difference in 80-120 km/h in 5th is .6 sec, or 9% of 6.6 seconds.

Note that the 123d requires TWO shifts to get to 100km/h, while the 135i only needs 1 shift. So as the differences in the number of gear shifts in the tests are minimized or eliminated, and speeds become higher, the closer the differences in acceleration approach the final drive ratio difference between the two cars.


Very interesting, eh? It makes you wonder how a 123d with a 3.08 final drive ratio would run against a 135i that is short-shifted at 5,000 rpm. With it's 144lb. weight advantage, the 123d might not be that far behind the 135i. But of course the minute the 135i runs all the way up to 7,000 rpm, it would leave even a 123d with a 3.08 final drive ratio in the dust.

The biggest performance problem for the 123d will always be the relatively low 5,000 rpm redline. BMW plays with the final drive ratio to make it drive more like it has a 5,500 rpm redline, but the higher final drive costs the 123d acceleration. And even 5,500 still counts as short-shifting in my book.

But I'm willing to live with the short-shifting and 9%+ less umph in exchange for the huge gas mileage gains and a lower price (more around the price of a 128i).

Well, by live with 9%+ less umph, I really mean I will chip the 123d, and I'll be happy to live with it if it cuts that 9%+ gap in umph down to something much less. And hopefully a chip might give me a few hundred more rpm's before redine if I'm really lucky.
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