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View Full Version : A theoretical differential question...


Boone.Msi
04-30-2007, 11:43 PM
Say i have a 3.91 lsd from an e32 laying around, how high would my rpm's be at 80mph if i put it in?



What would be the top speed at 6,000rpm?



Any input is apreciated
Boone.Msi
05-01-2007, 12:02 AM
Moroza:
Assuming that what the Bentley manual tells me about your gear ratios is correct, and that your tire circumfrence is identical to that of a 225/60R15...at 80, you'd be turning over 3035 RPM. Your speed at 6000 RPM would be 158 MPH.

hmm, interesting...
632 Regal
05-01-2007, 10:57 AM
my 3:64 runs 3krpm @ 83mph
hmm, interesting...
GoldenOne
05-01-2007, 11:28 AM
my 3.91lsd runs at 70mph @ 3.6krpm!!!!!!!! yeah the highway sucks but city driving is spirited!
steve0suprem0
05-01-2007, 11:46 AM
my 3:64 runs 3krpm @ 83mph

NICE. could one expect the same from the m50 with that diff?
DanDombrowski
05-01-2007, 12:17 PM
I have a 3.73 on an M50, I know what you mean!
632 Regal
05-01-2007, 12:59 PM
this on the 525 0r 540?
my 3.91lsd runs at 70mph @ 3.6krpm!!!!!!!! yeah the highway sucks but city driving is spirited!
Boone.Msi
05-01-2007, 05:17 PM
Assuming that what the Bentley manual tells me about your gear ratios is correct, and that your tire circumfrence is identical to that of a 225/60R15...at 80, you'd be turning over 3035 RPM. Your speed at 6000 RPM would be 158 MPH. so is this not correct then?

I figured it would be more like this:

my 3.91lsd runs at 70mph @ 3.6krpm
Boone.Msi
05-01-2007, 05:20 PM
as long as 80mph stays under 3500rpm im goin for it...
632 Regal
05-01-2007, 05:41 PM
im figuring it would be more like 3810rpm...but my mathamagician skills arent the best.


as long as 80mph stays under 3500rpm im goin for it...
yaofeng
05-01-2007, 07:45 PM
I thought you meant differential equations. Never mind.
GoldenOne
05-01-2007, 08:13 PM
this on the 525 0r 540?

525iM....I got a 3.46lsd going in on thursday or friday because I will be doing alot of highway miles during the summer
GoldenOne
05-01-2007, 08:19 PM
Say i have a 3.91 lsd from an e32 laying around, how high would my rpm's be at 80mph if i put it in?



What would be the top speed at 6,000rpm?



Any input is apreciated

Jumping from a 3.15 to a 3.91 is approx. a 24% increase....So multiply and add your RPM at 80mph now to see where it would be with the 3.91...
Ex: If you were at 3035RPM @ 80mph, you can expect to be churning around 3763RPM @ 80mph.
ryan roopnarine
05-01-2007, 08:20 PM
my 4.10 m50 automatic car does about parallel speeds between the speedometer needle and tachometer (as in, tach needle will be parallel to speed needle over about 50 mph) at 70, i'm doing about 3k, 75, 3500 or so.
Boone.Msi
05-01-2007, 11:43 PM
Jumping from a 3.15 to a 3.91 is approx. a 24% increase....So multiply and add your RPM at 80mph now to see where it would be with the 3.91...

ok, that sounds better. At 80mph i am exactly at 2500rpm. So that would make it 3100rpm at 80mph with the 3.91, which is alot closer to the results Moroza got from the bentley.

what do you guys say? Il add a pole to get your vote on whether i do this swap or not...
Boone.Msi
05-01-2007, 11:51 PM
Wait a minute, this dosent make since then...


my 3:64 runs 3krpm @ 83mph

Regal, is yours a manual? Why are your rpm's significantly different than the results were getting?
GoldenOne
05-02-2007, 04:34 AM
according to his title its also a 530ia....and he is sorta right I guess, by my calculation he should be around the 2888rpm mark with a 3.64, its close enough to the 3k mark...
Ferret
05-02-2007, 05:37 AM
Dont forget that a 25% increase in comparable rpm will lose you 20% of your fuel economy...
Boone.Msi
05-02-2007, 01:51 PM
I thought having lower gears would benifit in city driving and mpg?


according to his title its also a 530ia....and he is sorta right I guess, by my calculation he should be around the 2888rpm mark with a 3.64, its close enough to the 3k mark...

Ah, didnt see that, I was hoping that he estimated his rpm, rather than knowing exactly what it was when he wrote that.
632 Regal
05-02-2007, 02:31 PM
My actual rpm's at 83 are exactly 3000rpm

City mileage went up a very small amount, NO difference on the freeway. Dont think I would want to be churning 3800 though.


I thought having lower gears would benifit in city driving and mpg?



Ah, didnt see that, I was hoping that he estimated his rpm, rather than knowing exactly what it was when he wrote that.