Subtraction by Addition: CHECK! Sometimes I hate my car - Page 9 - The Unofficial BMW M5 Messageboard (m5board.com)

jrhaile · 8th July 2009, 23:34

As an avid Auto Crosser, I'm wondering if there is any advantage at all with a 3.62 gearing or 3.73. As it is, I can't lay the hammer down coming out some turns anyways..

2k1blkm5 · 8th July 2009, 23:36

Quote:

Originally Posted by CSBM5

The goal is balance not extremes. Why not put a 6:1 rear gear in the car?

You'll get a ton of torque multiplication at the wheels...car will be massively faster, right?

Read Lscman's post.

I understand what you are saying. With 6:1, you are spending too much time making shifts and also spinning the tires.

Another thing of concern is if you have super long gears, when you shift into the next gear, you are down in the 3000rmp rev range, which is not where you want to be.

2k1blkm5 · 8th July 2009, 23:40

Quote:

Originally Posted by jrhaile

As an avid Auto Crosser, I'm wondering if there is any advantage at all with a 3.62 gearing or 3.73. As it is, I can't lay the hammer down coming out some turns anyways..

I guess another thing that comes to help lay the power down with a custom diff is the 30/90 ramp. You get 30% lock to start with, not until a wheels start spinning like the oem diff.

CSBM5 · 8th July 2009, 23:40

Quote:

Originally Posted by jrhaile

As an avid Auto Crosser, I'm wondering if there is any advantage at all with a 3.62 gearing or 3.73. As it is, I can't lay the hammer down coming out some turns anyways..

Depending on the course, it might well be a huge handicap. We do most of our events on airport courses where peak speeds can be 60-65mph in the M5 at spots. You don't want to have to shift into 3rd if you can help it (2nd peaks at 68mph with stock diff), and only in a couple of courses do I hit the limiter (and a few I have to shift into 3rd). Steeper gearing might well make you slower due to rev limit/shifting issues along with more trouble modulating power application out of elements.

jrhaile · 8th July 2009, 23:45

Quote:

Originally Posted by CSBM5

Depending on the course, it might well be a huge handicap. We do most of our events on airport courses where peak speeds can be 60-65mph in the M5 at spots. You don't want to have to shift into 3rd if you can help it (2nd peaks at 68mph with stock diff), and only in a couple of courses do I hit the limiter (and a few I have to shift into 3rd). Steeper gearing might well make you slower due to rev limit/shifting issues along with more trouble modulating power application out of elements.

Same with us, we're always on an runway. I think I've hit rev limited twice this year in 2nd gear at 7400RPMs which puts me right around 72ish I believe. Hard to say for sure as I'm not looking at the speedo

E55AMG2 · 9th July 2009, 00:00

Quote:

Originally Posted by 2k1blkm5

I am still not convinced that 2.65 vs. 3.64 gearing and there's no change. At some point, the time it takes to make the extra shift is more than you gained from accelerating faster, thus it does not make a big difference. However, when the time you gained by acclerating faster is greater than the time wasted on the extra shift, the lower gearing clearly helps.

To see such effect amplified and more clearly, we need to look at the extreme ends of each case. Imagine that one car has super high gearing that it takes only 1 shift to get to 100mph. The second car has lower gearing and needs 2 shifts to get to 100mph. (Or one car has a 1.5:1 diff and the second car has 3.5:1 diff if you will) Now, the second car runs through the gears faster, by 40mph it has shifted into 2nd gear. The first car is still in first gear, it does not have to shift till 60mph. By theory, once the second car shifted into 2nd gear, even when it's ahead, the first car should catch up, until it shifts. But the fact is that the second car would have out accelerated the first car by so much that the first car would never catch up (until it runs out of gears or top end speed). Unless somebody thinks both cars will get to 100mph equally quick??

Somebody put in 1.5:1 and 3.5:1 into the the excel sheet and see what results we get.

I dont think it quite works that way. If you want to, download cartest and put the parameters in and run a comparo. Youd be surprised at what you see. If you dont increase the rev limit and the trans has closely spaced ratios, the effect a shorter rear end will have will be minimal.

gsfent · 9th July 2009, 00:05

Quote:

Originally Posted by 2k1blkm5

I am still not convinced that 2.65 vs. 3.64 gearing and there's no change. At some point, the time it takes to make the extra shift is more than you gained from accelerating faster, thus it does not make a big difference. However, when the time you gained by acclerating faster is greater than the time wasted on the extra shift, the lower gearing clearly helps.

I believe you are partly correct. You are accelerating quicker through each gear and you might also be accelerating to speed more quickly at some point (you have about a 25% or so difference in gearing depending on how you measure) But you left one thing out: with all other things being equal.

If you increase the hp (and necessarily the torque) to the taller geared car, you offset the gearing advantage of the deeper geared car. The advantage may depend on the actual numbers.

As an example, my car went from 323 rwhp to 354 rwhp on the same dyno different day (about 10%). So in theory, I should be at least as fast a stock geared car.

Maybe Chuck can run some sims with these numbers? Or run the old sims and adjust 2.65 for the hp change

Regards,
Jerry

xr4tic · 9th July 2009, 01:02

Quote:

Originally Posted by gsfent

I see your logic and seems to make a lot of sense. But I am not sure how you worked out the math; to me it looks like you are double counting the 3.45's 10% torque increase.

I assume you have equalized road speed in gear, so at any given speed, the 3.45 is at roughly 10% higher rpm. Isn't that the 10% increase due to gearing?

You then multiply again by 10%, presumably for the extra torque? I could see that at the same rpm, but not at the same road speed. What did I miss?

Regards,
Jerry

Nope, the ft/lb numbers are straight off the graph for the given RPM, so that is the torque the motor is actually producing, I then multiply by the 10% wheel torque increase from the shorter gears.

Lets say you are driving in 3rd gear at 40 MPH (made up numbers), with the stock gears, you're turning 3000 RPM, with the 3.45 gears, you are turning 3300 RPM. Same road speed, different RPMs, and different torque output from the motor. Then on top of that you multiply by the 10% extra torque the wheels will see.

2k1blkm5 · 9th July 2009, 01:24

Quote:

Originally Posted by xr4tic

Nope, the ft/lb numbers are straight off the graph for the given RPM, so that is the torque the motor is actually producing, I then multiply by the 10% wheel torque increase from the shorter gears.

Lets say you are driving in 3rd gear at 40 MPH (made up numbers), with the stock gears, you're turning 3000 RPM, with the 3.45 gears, you are turning 3300 RPM. Same road speed, different RPMs, and different torque output from the motor. Then on top of that you multiply by the 10% extra torque the wheels will see.

That is a good point to clarify actually. It is easy to be confused by thinking that the added torque is result of the shorter gears causing the engine in a different (higher) rpm range. In that case, the torque curve would just shift to the left. But it actually shifts up (toque added) and to the left (engine in higher rev).

E55AMG2 · 9th July 2009, 22:26

Quote:

Originally Posted by 2k1blkm5

That is a good point to clarify actually. It is easy to be confused by thinking that the added torque is result of the shorter gears causing the engine in a different (higher) rpm range. In that case, the torque curve would just shift to the left. But it actually shifts up (toque added) and to the left (engine in higher rev).

In theory, yes. In practice, no.

I would love for someone to prove me wrong with a back to back dyno...but IDK of anyone who has both differentials to swap back and forth between