Why did BMW do this?

[evoDRichie]

wow

[MAXELHOFF]

Is it true???

Is not it true that BMW never put a 4.1 differential into a 325is manual transmission car, because it would be quicker to 0-60 MPH than the hot shit "racer" M3? At least in the states?

[110stx]

That might be true. But the 325i is very nearly as quick as an ///M3 anyway, up to about 50-60. Then the S14 takes over.

In my experience, right as the M20 is falling off the top of the power band, the S14 is just kicking in.

[reelizmpro]

That might be true. But the 325i is very nearly as quick as an ///M3 anyway, up to about 50-60. Then the S14 takes over.

In my experience, right as the M20 is falling off the top of the power band, the S14 is just kicking in.

Perhaps...or it could have just been that they thought the gearing would be too short with the Getrag in the 325iS and as mentioned, the powerbands are different. A 325e with 4.10 is pretty quick too but it runs out of RPM's too fast and has a top speed of like 80MPH. I think BMW got it right though.

[Earendil]

Is not it true that BMW never put a 4.1 differential into a 325is manual transmission car, because it would be quicker to 0-60 MPH than the hot shit "racer" M3? At least in the states?

I'm pretty sure if BMW wanted to get the 325i to 60mph faster they would put something like a 3.5 (which doesn't exist) in it, or made 2nd gear a little longer.

I believe the 325i is rated at 0-60 at 7.6 seconds? I once did my own very unscientific test and got a time of 7.8 seconds, but ask me what my 0-54mph time was? Go ahead, I'm waiting...
...
I'm done waiting, it was like 5.5 seconds. Why? Because we have to shift at 54mph and pull those last 6 mph at the bottom of 3rd gear. BMW was hardly thinking about 0-60 USA marketing fluff.

Now, my 328 E46 has a longer second gear, and tops out at exactly 60mph. Accident? I think not.

Putting a 4.10 into the 325i would make you shift earlier, not later. You want a gear to end at your target speed for best results, not at the bottom end of one.

[nando]

the solution is to increase the redline to 7500+ and use a 4.10. now you can hold the gear until 60 and still accelerate faster. assuming you've built it to make power over a broader range than stock. :p

FWIW: an E46 M3's gearing is significantly shorter than a M20B25 with a 4.10 would be. but it revs to 8000rpm.

[MAXELHOFF]

lighter is quicker

I'm pretty sure if BMW wanted to get the 325i to 60mph faster they would put something like a 3.5 (which doesn't exist) in it, or made 2nd gear a little longer.

I believe the 325i is rated at 0-60 at 7.6 seconds? I once did my own very unscientific test and got a time of 7.8 seconds, but ask me what my 0-54mph time was? Go ahead, I'm waiting...
...
I'm done waiting, it was like 5.5 seconds. Why? Because we have to shift at 54mph and pull those last 6 mph at the bottom of 3rd gear. BMW was hardly thinking about 0-60 USA marketing fluff.

Now, my 328 E46 has a longer second gear, and tops out at exactly 60mph. Accident? I think not.For marketing fluff?


Putting a 4.10 into the 325i would make you shift earlier, not later. You want a gear to end at your target speed for best results, not at the bottom end of one.True!

You realize a 325i with a 4.1 accelerates quicker through each gear, than a numerically lower rear-end. Right? Also, it would not be, "last 6 mph at the bottom of 3rd gear", with a 4.1. The 6 mph would be a larger number, (chime in- people who enjoy math) making 60 mph in the middle-ish of 3rd gear...not the bottom. (I'm assuming you did not have a 4.1 for your "unscientific test")


As an aside. A 4.1 effectively makes a manual transmission a closer ratio gearbox.

[Earendil]

You realize a 325i with a 4.1 accelerates quicker through each gear, than a numerically lower rear-end. Right? Also, it would not be, "last 6 mph at the bottom of 3rd gear", with a 4.1. The 6 mph would be a larger number, (chime in- people who enjoy math) making 60 mph in the middle-ish of 3rd gear...not the bottom. (I'm assuming you did not have a 4.1 for your "unscientific test")


As an aside. A 4.1 effectively makes a manual transmission a closer ratio gearbox.

Chiming in, as I love math and wrote an excel calc for this exact thing, but adding in wheel torque for each gear and diff combo.

Anyway, a numerically higher diff will allow you to excelerate faster in each gear, but for less time. So if a 4.1 has to shift at say 50, and the 3.73 has to shift at 55, the 4.1 would win a drag race to 50, and lose terrible to 55. Every time you shift you lose massive amounts of torque to the ground. Whether its 2nd gear or 3rd gear, you want to use all of the gear, meaning you want to end you drag race at the top of a gear, not in the middle.

I'll see if I can't find my excel calc and dig out some numbers. It's been a year or so since I was messing with it.

[Mark 42]

Why did BMW put a 4 beater in the e30 M3?

Because Chevy put one in the Camaro.

[Earendil]

Here are some numbers, for those that like math:

Assumptions
1. All examples use the stock tire/wheel 195/55/14
2. Max RPM is 6500
3. Transmission is a Getrag 260
4. Torque values at maximum RPM are unpublished. Will use gear ratio since the difference in applied torque comes from the gear ratio in this confined example.
5. My excel calculator is actually correct :)

First some numbers. Where does a 3.73 and a 4.1 diff ratio car shift at in mph if the redline is hit?

Gear - 3.73 - 4.10
1st - 30mph - 28mph
2nd - 53mph - 48mph
3rd - 83mph - 76mph
4th - 116mph - 106mph
5th - 144mph - 131mph

That is probably about what we all expect, right? okay, so now let's look at what total gearing multiplier is for the 3.73 and the 4.10.
Well look at the first 3 gears to keep it simpler.

Gear - 3.73 - 4.10
1st - 14.3 - 15.7
2nd - 8.2 - 9.0
3rd - 5.2 - 5.74

As you might expect, the 4.10 diff is a 10% higher total gearing allowing it to apply 10% more torque to the ground for a given gear. This is also why it feels like 1st gear knocks you back in your seat. You are applying almost 3 times as much torque to the ground as you are in 3rd gear. Yowzer!

Now look back at when you have to shift gear. A 4.10 is going to have to shift at 48mph, at which point its torque multiplier drops to 5.74. But the 3.73 gets to keep it's 8.2 multiplier until 53mph, and hasn't had to spend time shifting yet. If the goal were to reach 53mph, the 3.73 would win. Sadly, it's not, and both cars suck at getting to 60mph.

If the goal was to hit exactly 60mph, you might be willing to give up a little bit of applied torque in one gear just so you could stretch that gear a tiny bit further thus avoiding
A) Shifting
B) Lower torque in the next gear.

If shifting took no time, and mechanical things never broke, the best option would be a gearbox with 20 (or 1000) gears, so that the engine was allowed to stay in a tiny window of it's best RPMS.

Alternatively, you have an engine wind up ridiculously high with a single gear (See Tesla Roadster), or a third option would be a Formula 1 car that has the budget, engineers, and ridiculousness to archive basically both approaches :)

I realize almost nothing of the above proves any point I might have been trying to make. I still maintain that, in general and with others things considered as well, you want a gear to end at the speed you are trying to archive. The only way to make third gear truly useful in a 0-60mph test is if 3rd gear is running out as you approach 60mph. Otherwise, I believe you are better off extending 2nd gear just a tiny bit.

Side notes:
It would take a 5.2 rear diff to get the Getrag 260 to shift out of 3rd at 60mph.
The 328i E46 shifts out of second at exactly 60mph, and has the following total gearing ratios:

Gear - 2.93 diff
1st - 12.3
2nd - 7.3
3rd - 4.9

BMW could get away with this because it makes the car more driveable, and it has more low end torque even though the horsepower isn't all that much higher. It may have a lower total gear ratio, but it doesn't have to use that slow-ass 3rd gear to reach 60mph :)

Thanks for reading/skimming/skipping to the end!

This rant brought to you by: a nerd...that has no intention of proof reading what he wrote.

[Kershaw]

excellent post, sir!

[Mark 42]

If shifting took no time, and mechanical things never broke, the best option would be a gearbox with 20 (or 1000) gears, so that the engine was allowed to stay in a tiny window of it's best RPMS.

That's the idea behind a CVT. They used them in Formula 440/Formula 500
racing (snowmobile drive train). I'm told that it feels strange and takes getting used to.
Saturn built a CVT with a chain instead of a belt. Not a bad concept,
but they put it into production too soon.

[Kershaw]

my friend's parents have a dodge caliber with cvt. it's kind of weird. you can really push down the accelerator and you'll accelerate but rpms will remain constant at 2500. if you floor it, it'll jump to 4k and hold steady there as well.

[MAXELHOFF]

m3 vs. 325 4.1 to 60 mph?

Here are some numbers, for those that like math:

Assumptions
1. All examples use the stock tire/wheel 195/55/14
2. Max RPM is 6500
3. Transmission is a Getrag 260
4. Torque values at maximum RPM are unpublished. Will use gear ratio since the difference in applied torque comes from the gear ratio in this confined example.
5. My excel calculator is actually correct :)

First some numbers. Where does a 3.73 and a 4.1 diff ratio car shift at in mph if the redline is hit?

Gear - 3.73 - 4.10
1st - 30mph - 28mph
2nd - 53mph - 48mph
3rd - 83mph - 76mph
4th - 116mph - 106mph
5th - 144mph - 131mph

That is probably about what we all expect, right? okay, so now let's look at what total gearing multiplier is for the 3.73 and the 4.10.
Well look at the first 3 gears to keep it simpler.

Gear - 3.73 - 4.10
1st - 14.3 - 15.7
2nd - 8.2 - 9.0
3rd - 5.2 - 5.74

As you might expect, the 4.10 diff is a 10% higher total gearing allowing it to apply 10% more torque to the ground for a given gear. This is also why it feels like 1st gear knocks you back in your seat. You are applying almost 3 times as much torque to the ground as you are in 3rd gear. Yowzer!

Now look back at when you have to shift gear. A 4.10 is going to have to shift at 48mph, at which point its torque multiplier drops to 5.74. But the 3.73 gets to keep it's 8.2 multiplier until 53mph, and hasn't had to spend time shifting yet. If the goal were to reach 53mph, the 3.73 would win. Sadly, it's not, and both cars suck at getting to 60mph.

If the goal was to hit exactly 60mph, you might be willing to give up a little bit of applied torque in one gear just so you could stretch that gear a tiny bit further thus avoiding
A) Shifting
B) Lower torque in the next gear.

If shifting took no time, and mechanical things never broke, the best option would be a gearbox with 20 (or 1000) gears, so that the engine was allowed to stay in a tiny window of it's best RPMS.

Alternatively, you have an engine wind up ridiculously high with a single gear (See Tesla Roadster), or a third option would be a Formula 1 car that has the budget, engineers, and ridiculousness to archive basically both approaches :)

I realize almost nothing of the above proves any point I might have been trying to make. I still maintain that, in general and with others things considered as well, you want a gear to end at the speed you are trying to archive. The only way to make third gear truly useful in a 0-60mph test is if 3rd gear is running out as you approach 60mph. Otherwise, I believe you are better off extending 2nd gear just a tiny bit.

Side notes:
It would take a 5.2 rear diff to get the Getrag 260 to shift out of 3rd at 60mph.
The 328i E46 shifts out of second at exactly 60mph, and has the following total gearing ratios:

Gear - 2.93 diff
1st - 12.3
2nd - 7.3
3rd - 4.9

BMW could get away with this because it makes the car more driveable, and it has more low end torque even though the horsepower isn't all that much higher. It may have a lower total gear ratio, but it doesn't have to use that slow-ass 3rd gear to reach 60mph :)

Thanks for reading/skimming/skipping to the end!

This rant brought to you by: a nerd...that has no intention of proof reading what he wrote.

Very interesting!/nice computations. So a 4.1 is quicker to 60 mph than a 3.73 rear-end. But, is it quicker to 60 mph than an e30 M3?


MAXELHOFF

Is it true???
Is not it true that BMW never put a 4.1 differential into a 325is manual transmission car, because it would be quicker to 0-60 MPH than the hot shit "racer" M3? At least in the states?

[Mark 42]

You can short shift the 4.10 and get almost the same effect as the 3.73
will give you, so any advantage the 3.73 had, is really not going to help
with acceleration.

In a car with not a lot of torque (e.g. 4 cylinder or even a small 6 cyl)
you need to spend time in the area where the motor makes horsepower.
Driveability is where the the higher gear ratio (3.73) is an advantage.
A daily driver that is constantly needing to be shifted is more work
to drive. The 4.10 will require you to shift more often, and will sound
and feel more "buzzy" at highway speeds.

Drag racers run low gear ratios (larger rear end numbers) for high acceleration.
If you want faster 0-60 times, 4.56 ratio, (if it's available - I'm new to BMWs)
4.88 or even 5.39 ratio would make 0-60 faster, depending on how fast you can shift.

There's a lot more to it than the basic numbers can show though.

http://www.wallaceracing.com/reargear.htm

http://www.wallaceracing.com/calcrgr.php

http://forums.corvetteforum.com/c4-zr-1-discussion/2762503-what-is-the-ideal-rear-end-gear-ratio.html