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Unread 11-16-2009, 05:41 PM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

Good resource on transmission and rear gear ratios on the E46:

http://www.bokchoys.com/differential/GearRatios.htm (the chart below took a lot of format tweaking)

E46 Model...1st. 2nd. 3rd 4th. 5th. 6th R... Final/Auto Final Drive
------------------------------------------------------------------
323i / ci.. 4.23 2.52 1.66 1.22 1.0 n/a 4.04 3.07 3.46
325i / ci.. 4.23 2.52 1.66 1.22 1.0 n/a 4.04 3.15 3.46
328i / ci.. 4.21 2.49 1.66 1.24 1.0 n/a 3.85 2.93 3.46
330i / ci.. 4.21 2.49 1.66 1.24 1.0 n/a 3.85 2.93 3.38
330i 6sp... 4.35 2.50 1.66 1.23 1.0 .85 3.93 2.93 3.38
330i zhp... 4.35 2.50 1.66 1.23 1.0 .85 3.93 3.07 3.64
323cic..... 4.23 2.52 1.66 1.22 1.0 n/a 4.04 3.46 3.46
325cic..... 4.23 2.52 1.66 1.22 1.0 n/a 4.04 3.46 3.46
330cic..... 4.21 2.49 1.66 1.24 1.0 n/a 3.85 3.07 3.38
323i t..... 4.23 2.52 1.66 1.22 1.0 n/a 4.04 3.46 3.46
325i t..... 4.23 2.52 1.66 1.22 1.0 n/a 4.04 3.46 3.46
325xi /t... 4.21 2.49 1.66 1.24 1.0 n/a 3.85 3.23 3.46
330xi...... 4.21 2.49 1.66 1.24 1.0 n/a 3.85 3.07 3.46
330xi 6sp.. 4.35 2.50 1.66 1.23 1.0 .85 3.93 3.07 3.46

*M3/M3 conv 4.23 2.53 1.67 1.23 1.0 .88 3.75 3.62 n/a


http://www.apexgarage.com/tech/gear_ratios.shtml

I plugged in the numbers for a 285/30/18 (24.9" dia) and 7000 max RPMs and here's the speeds in 1st-3rd gears for the following rear ratios:

2.93 (what's in the car now)
1st = 42 mph
2nd= 71 mph
3rd = 107 mph

3.15 rear
1st = 39 mph
2nd= 66 mph
3rd = 99 mph

3.38 rear
1st = 36 mph
2nd= 62 mph
3rd = 92 mph

3.46 rear
1st = 36 mph
2nd= 60 mph
3rd = 90 mph

3.64 rear
1st = 34 mph
2nd= 57 mph
3rd = 86 mph

So contrary to popular belief it might be ideal to keep the 2.93 rear ratio for autocrossing (a max speed of 70 mph in 2nd gear is usually the goal). Can someone check my numbers because this looks wrong....

Last edited by Fair!; 11-17-2009 at 04:26 PM.
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  #12  
Unread 11-17-2009, 05:57 AM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

This was bugging me and I couldn't sleep... at around 4 am it came to me why the gearing in E36 LS1 was so different. Teucci had chosen 3.91 gear for the same tire height (285/30/18 tire), RPM and the same target 70mpg speed in 2nd gear. Its because that car had an "American" transmission...

Here are the gear ratios for 1st thru 6th for the BW/Tremec T56 from the 1994-2002 Camaro:

2.66 1.78 1.30 1.00 0.74 0.50

Look at the massive difference in 2nd gear ratio between the BMW 5-spd and the T56... 2.49 vs 1.78. At 7000 rpm the T56 equipped car with the same tire would do 75mph in 2nd with a 3.91 rear gear.

OK, so it took my sleeping brain to figure out the weird rear gear ratio calcs for the DSP car, oh well.
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Unread 11-17-2009, 08:47 AM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

What is the redline you are figuring for this? I know, I can do the math, but it looks like you already have .
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Unread 11-17-2009, 01:53 PM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

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Originally Posted by Fair! View Post

So contrary to popular belief it might be ideal to keep the 2.93 rear ratio for autocrossing (a max speed of 70 mph in 2nd gear is usually the goal). Can someone check my numbers because this looks wrong....

I'd say it's not quite that simple.. The Bimmer have a low torque engine in a somewhat heavy car.

The tradeoff is: taller gearing so you can get a couple of extra MPH down a long straight without shifting to 3rd versus shorting gearing that pulls harder out of slower corners.

If it were me, I'd be looking at the 3.38 or the 3.23 for a little better grunt off the corners with the tradeoff of running out of gear once in awhile.

Tough call to make.
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Unread 11-17-2009, 02:15 PM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

Teucci makes a good point; how often have you bumped the limiter in 2nd?

I only hit the limiter in the Formula a couple times all the years I owned it (if my memory is correct at about 72 mph). Now the Solstice GXP is the opposite extreme (it runs out at about 52-53 mph). Those are both Stock class cars though
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Unread 11-17-2009, 02:40 PM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

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Originally Posted by GoDog View Post
Teucci makes a good point; how often have you bumped the limiter in 2nd?

I only hit the limiter in the Formula a couple times all the years I owned it (if my memory is correct at about 72 mph). Now the Solstice GXP is the opposite extreme (it runs out at about 52-53 mph). Those are both Stock class cars though
I agree with Teucci as well. The 3.23-3.38 might just be the best compromise between low end grunt out of the slow speed corners vs. top end 2nd gear mph. If you stay closer to the 2.93, then you might even be tempted to try 1st gear again on tight stuff and IMHO that's worse than riding the rev limiter for a few seconds at a time. Of course you could always bump the limiter up a bit
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Unread 11-17-2009, 02:41 PM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

Quote:
Originally Posted by Teucci View Post
I'd say it's not quite that simple.. The Bimmer have a low torque engine in a somewhat heavy car.

The tradeoff is: taller gearing so you can get a couple of extra MPH down a long straight without shifting to 3rd versus shorting gearing that pulls harder out of slower corners.

If it were me, I'd be looking at the 3.38 or the 3.23 for a little better grunt off the corners with the tradeoff of running out of gear once in awhile.

Tough call to make.
I'd think mid 60s for a DSP car is enough. I'd doubt a SCCA Nationals course would ever have a place for a low torque somewhat heavy car to exceed that (Teucci's words). It's the high torque, high hp STI that runs out at 55 mph that's the problem child.

On another note, the shocks for this project leave the factory Friday and will be here by the next Friday.
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Unread 11-17-2009, 03:23 PM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

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Originally Posted by GoDog View Post
Teucci makes a good point; how often have you bumped the limiter in 2nd?
It happens a lot around here... with bigger courses in Texas (A&M Annex, Mineral Wells, even TMS Bus Lot) we do have to keep an eye on 2nd gear terminal speeds.

There have been courses where I have raced in the STU prepped M3 vs the Subaru boost buggies where they were hitting their 60-65 mph 2nd gear limit 3 to 5 times on a course, and losing time with constant up/down shifting or sitting on the limiter in 2nd. And now with Nationals moving to a bigger venue in Nebraska I think we will see more courses where 70mph speeds are common. That plus this car should be a good bit faster (grip and power) than the STU M3 I had run for several years...

But yea, losing acceleration out of a corner is tough. Now when I am getting a LSD built is the time when I need to choose the right gear. A newly setup diff is going to run around $3000, so I want to get the gears right the first time!
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Unread 11-17-2009, 03:36 PM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

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Originally Posted by Fair! View Post
There have been courses where I have raced in the STU prepped M3 vs the Subaru boost buggies where they were hitting their 60-65 mph 2nd gear limit 3 to 5 times on a course, and losing time with constant up/down shifting or sitting on the limiter in 2nd. And now with Nationals moving to a bigger venue in Nebraska I think we will see more courses where 70mph speeds are common. That plus this car should be a good bit faster (grip and power) than the STU M3 I had run for several years...
Bimmerhaus site says that car has a 3.38 if that's any any help. Having driven that car, it never ran out of gear when I drove it and you could floor it out of corners without loss of traction. Seemed to work.
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Unread 11-17-2009, 03:44 PM
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Default Re: Vorshlag BMW E46: DSP Auto-x Project

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Originally Posted by hancheyb View Post
Bimmerhaus site says that car has a 3.38 if that's any any help. Having driven that car, it never ran out of gear when I drove it and you could floor it out of corners without loss of traction. Seemed to work.
Thats only 62 mph max speed in 2nd @ 7000 rpm. Hmmm.... but you drove it and it worked, so I might just give that a shot. I don't want to buy a $3000 part based simply on a bench racing calculation.

The other LSD option is: Quaife, OS Gilken, Kaaz, or a custom setup 2-clutch or 3-clutch pack BMW diff (or the further optional "30/90 ramped 3-clutch pack diff). The cost for each is about $3095 retail, but the 30/90 ramped custom diff is $300 extra.

Dan at Diffsonline says "For the 330 I would probably go with the 3 disc 30/90 ramped setup that has been proven to be fast both on the autoX course and road course as well as very streetable" For that 30/90 3-clutch pack LSD in 2.93, 3.07, 3.15, 3.23, or 3.38 ratio the cost is $3395 retail, which is one of the biggest ticket items on the car's prep list, for sure.
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