Quote:
Originally Posted by dcstep
Here's my real world experience with the 4.10 FD installed in my 2009 E92 M3 6MT. I've got around 14,000 miles on the 4.10 and well over 100 autocross laps. Other mods include the Active Autowerks exhaust, oiled air filter, Dinan springs, Dinan chip with 8600-rpm redline, UUC SSK and Dinan throttle bodies.
I don't do 1/4-mile races at tracks, I do occasional rolling start street races (but usually only up to around 100-mph) and I run BMWCCA sanctioned AX. AX is my main performance interest. It should be noted that I live at 5400-ft above sea level, so non-forced induction power adders are not near as effective as at sea level.
Around town the negatives of the 4.10 are poorer fuel economy and slight noise from the gears on the overrun. I expected these things going in and was only worried about the noise level. I can report that it's not bad at all and doesn't bother me, but I DO know it's there. My riders don't say, "What's that noise?", but it is more than stock. The advantages for street driving are in-gear responsiveness and acceleration in-gear. When I street race I hope to start at around 4000 rpm on the rolling start and use the full 8600 rpm redline and only shift once up to 100-mph. Squirting through traffic in 3d, 4th and 5th is clearly more responsive than stock.
If you look at those scientific charts, they're right. The gear ratios were not selected with this FD in mind, such that 1/4-mile time is actually worse, DUE TO THE TIME REQUIRED TO SHIFT. However, the in-gear acceleration can be better with the higher numerical ratio. Also, I'm able to space out my shifts a little more with the extra 200-rpm of redline that my car has.
AX is where the FD really pays off because, other than the 1-2 shift, the entire course is typically run in 2d gear. The 6MT has better acceleration than the DCT in 2d. The DCT will be faster on a road course, but the 6MT will be faster on an AX course.
Top priorities for AX speed are tires, then lightweight and wide wheels that reduce inertia and increase the contact patch and tracking width. Gearing and power come in distant thirds. I lean toward gearing, partly because my high altitude makes power VERY expensive to gain. Gearing is a known that won't be sapped by low oxygen density (my loss is around 16% vs. sea level). With my current setup (awaiting new, lighter 18" wheels and a switch to Dunlop Direzza tires) my car will spin its wheels anywhere on the course.
Anyway, I think that my case demonstrates there's no one absolute fastest answer. The answer is more like, "It depends on what you're doing." If you do standing start 1/4-mile races and that's important to you, then listen to the techno data. That is really, really true. Shifting time DOES cost speed. However, if you run events where shifitng is limited, you'll still want to minimize shifting time by either strategy or higher redline, or both, but it will not be as important to you as in a 1/4-mile event. A higher numeric gear ratio can be an advantage in events where there is little or no shifting.
I like the feel of the gearing with a 4.10 FD and I think it's quicker for AX.
Dave
|
Would you say it helps during rolling start street races? Imagine both cars have the same power, but one w 4.10 and one stock FD. Say from a rollin start at 40, the 4.10 would be at a higher RPM which would give it a nice leap ahead of the stock FD car? And then it would just be a chase from there on assuming both drivers were on par with shifting.