BMW M3 Forum (E90 E92) - View Single Post

pbonsalb · 07-16-2012, 06:48 AM

Interesting that an E9X M3 will be equally fast with a 3.15 or a 4.10, or any other contrast in gear ratios for that matter. We have now debunked the "performance axle ratios" offered by the factory for many cars as a fraud. And Dinan must not have been using the right simulation software when testing its aftermarket differentials. Maybe it was actually testing them in the real world instead.

With the DCT, the shift speed factor is nearly eliminated. However, an extra shift adding time is only an issue when the race ends at a speed that requires the lower geared car to do an extra shift and the higher geared car not to do one.

You can choose a target speed that will favor the higher geared car, but you can also choose a target speed that will favor the lower geared car. Or you can choose a speed range in one gear or in a couple of gears that favors one car or the other.

Bottom line is that BMW choose the gearing as a compromise. It could have easily gone up or down one size. It needed to hit certain fuel economy targets and it needed to hit certain performance targets.

I do agree that diff ratio changes come with compromises. Highway rpm may be higher, MPG may be less, traction may be worse off the line, acceleration will be slower or faster to certain arbitrary target speeds or through certain arbitrary speed ranges. The factory's compromise is usually good for the average driver.

But having done diff ratio changes in the real world (3.15, 3.23, 3.38 and 3.64 on my 99M3), I like them and believe that reasonable ones make a performance improvement that the performance driver appreciates. When my 99M3 was centrifugal supercharged and relatively torqueless, making only 368 lbs rwtq, I liked the 3.64. Now that it is turbocharged and making 517 lbs rwtq, I like the 3.15 and for traction reasons, I may go to a 2.93. However, I am not basing my like or dislike on arbitrary target speeds or arbitrary speed ranges, because I have found based on experience that there is much more to the real world than computer simulations with limited inputs.

07-16-2012, 06:48 AM	#40
pbonsalb Lieutenant General 5234 Rep 10,616 Posts Drives: 18 F90 M5, 99 E36 M3 Turbo Join Date: Feb 2011 Location: New England iTrader: (4)	Interesting that an E9X M3 will be equally fast with a 3.15 or a 4.10, or any other contrast in gear ratios for that matter. We have now debunked the "performance axle ratios" offered by the factory for many cars as a fraud. And Dinan must not have been using the right simulation software when testing its aftermarket differentials. Maybe it was actually testing them in the real world instead. With the DCT, the shift speed factor is nearly eliminated. However, an extra shift adding time is only an issue when the race ends at a speed that requires the lower geared car to do an extra shift and the higher geared car not to do one. You can choose a target speed that will favor the higher geared car, but you can also choose a target speed that will favor the lower geared car. Or you can choose a speed range in one gear or in a couple of gears that favors one car or the other. Bottom line is that BMW choose the gearing as a compromise. It could have easily gone up or down one size. It needed to hit certain fuel economy targets and it needed to hit certain performance targets. I do agree that diff ratio changes come with compromises. Highway rpm may be higher, MPG may be less, traction may be worse off the line, acceleration will be slower or faster to certain arbitrary target speeds or through certain arbitrary speed ranges. The factory's compromise is usually good for the average driver. But having done diff ratio changes in the real world (3.15, 3.23, 3.38 and 3.64 on my 99M3), I like them and believe that reasonable ones make a performance improvement that the performance driver appreciates. When my 99M3 was centrifugal supercharged and relatively torqueless, making only 368 lbs rwtq, I liked the 3.64. Now that it is turbocharged and making 517 lbs rwtq, I like the 3.15 and for traction reasons, I may go to a 2.93. However, I am not basing my like or dislike on arbitrary target speeds or arbitrary speed ranges, because I have found based on experience that there is much more to the real world than computer simulations with limited inputs.
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