Diff ruminations

[moroza]

In spite of my decision to stop seriously upgrading this car, I feel I could make an exception for a diff swap, in no small part because it would cost less than $100 and probably save fuel. I drove over 20k last year, mostly long highway trips; even 1mpg average improvement would more than pay for the cost of a junkyard diff.

I feel the 3.15 is too short for the gobs of torque this engine produces (296 at the wheels). I find myself shifting a lot in town, taking off from second and still leaping ahead of most everybody else, and high-speed gearing doesn't feel quite right. The E39 540i/6 non-Sport that donated this driveline felt much more appropriately geared, even still a bit too steep.

Seems I have two options: 2.81 and 2.93. Not quite sure what came in what, but I can do that homework. I'd like to hear from folks with these ratios and a manual box, about what it feels like around town and on the highway.

The diff currently in the car is a 3.15 LSD from a 750iL. It has the 750's smaller input flange, custom driveshaft flange to match, and replaced output (but not input) seals. What should I do?

1. Use the existing case, including input flange, and just swap ring and pinion gears (hoping the pinion will fit the smaller input flange)?

2. Use the 2.81/2.93 case, swap in the 3.15 LSD carrier, and modify the driveshaft to standard 540i/6 dimensions?

[Meeve]

I swapped my diff for a shorter ratio. Higher ratio is no fun imo. Well, give some good mpg. Whats your actual top speed?

[moroza]

I stopped looking at the speedo at 235, then an electrical gremlin set in, probably related to the ABS, and prevented me from revving past 5400 (RPMs in 6th at that speed) until I restarted the car. I've yet to test it again.

[Meeve]

then why do you want higher gear ratio? Imo its fast enough. Since you were not even top speed yet at 235. but your car your choice :P

[bfp9]

then why do you want higher gear ratio? Imo its fast enough. Since you were not even top speed yet at 235. but your car your choice :P
He isn't looking for just top speed improvement. He wants better gas mileage when just cruising, and doesn't need his car to be so fast off the line. I am in the same boat.

OP: I have been doing some homework about what cars a 2.93 diff came in. I'd rather have a nice highway cruiser than a 0-60 beast.

[Steve547i]

id be interested in your current 3.15 if you were interested in selling it

[dme88]

Going from a high final drive ratio to a low one is not a upgrade, it's a downgrade :-)

[eddycooper]

I really like the stock 2.93 that came stock in my car - I definitely recommend it or something similar. In 6th on the highway, it revs nice and low. Makes high speeds seem effortless. My fuel economy is 9.5-10 L/100km on the highway in 6th. At the same time, I really like how it makes you feel the torque when you accelerate. I'm sure it won't win any acceleration contests, but on the other hand you can do over 100 km/h in 2nd gear.

[moroza]

Going from a high final drive ratio to a low one is not a upgrade, it's a downgrade :-)

Not when you spend more time shifting, than WOT in the first two gears, it isn't.


I really like the stock 2.93 that came stock in my car - I definitely recommend it or something similar. In 6th on the highway, it revs nice and low. Makes high speeds seem effortless. My fuel economy is 9.5-10 L/100km on the highway in 6th. At the same time, I really like how it makes you feel the torque when you accelerate. I'm sure it won't win any acceleration contests, but on the other hand you can do over 100 km/h in 2nd gear.

Just the sort of thing I wanted to hear, thanks.

I'd also like to hear from anyone with a 2.81.

[moroza]

I really like the stock 2.93 that came stock in my car - I definitely recommend it or something similar. In 6th on the highway, it revs nice and low. Makes high speeds seem effortless. My fuel economy is 9.5-10 L/100km on the highway in 6th. At the same time, I really like how it makes you feel the torque when you accelerate. I'm sure it won't win any acceleration contests, but on the other hand you can do over 100 km/h in 2nd gear.

Just the sort of thing I wanted to hear, thanks.

I'd also like to hear from anyone with a 2.81.

[moroza]

Bump?

[Baupfhor]

I have 2.93 in my 540/6. Freeway I run just shy of 2k rpm at 65mph. Coming from my 3.67 vr6 VW, these feel really low and I want to go to 3.15...mostly for LSD though. Id say get a 2.93 diff and swap the LSD and input flange over from the 3.15.

[jehu]

I think the original late model 540 stock is alright. If it gives the engine the best chance to show its fullest output at max that'd for me be preferavble to the ability to take off the line faster.. If I cared about that I'd go FI..

[moroza]

I have 2.93 in my 540/6. Freeway I run just shy of 2k rpm at 65mph. Coming from my 3.67 vr6 VW, these feel really low and I want to go to 3.15...mostly for LSD though. Id say get a 2.93 diff and swap the LSD and input flange over from the 3.15.

Would you trade your driveshaft and diff for mine?

[Baupfhor]

Would you trade your driveshaft and diff for mine?

For sure, just depends on where never-never land is. That would leave you sans LSD though.

[moroza]

http://img856.imageshack.us/img856/4036/picture902.jpg