Differential Question

[kirkpart2]

Hey,

Any point swapping to a 3.91 LSD. I believe my 06 auto-manual has a 3.73 stock. Is it worth it to swapping for such a small difference?

Thanks.

[3002 tii]

Originally Posted by kirkpart2

Hey,

Any point swapping to a 3.91 LSD. I believe my 06 auto-manual has a 3.73 stock. Is it worth it to swapping for such a small difference?

Thanks.

I guess it depends on how much your installer is charging you incrementally. I had the same issue when I put in my Quaife. My stock gearing was a 3.46 and was considering a 3.64 or 3.73. But I'll throw this out there, when speaking with different rebuilders the general consensus was a 4~5% change is barely noticeable... but for 8~10% increase, you'll definitely get your money's worth.

I would've done a 3.73 which was about an 8% change but the price was even HIGHER for 3.73 over a 3.64 so I didn't bother. The Quaife + rebuild was already putting me at the end of my budget. The upgrade was worthwhile but not for the cost. So if your installer is not going to charge you anything more for taller gearing, I'd go with the 4.10s...

Other things to consider, at what speeds do you typically drive your car? Does it see alot of freeway miles, is this a dedicated track car, etc? But given the lack of mods for the n52, I think torque multiplier effect from taller gearing is really your best mod (plus the limited slip).

[kirkpart2]

Its not a track dedicated car. Daily driver really .... spirited driving now and then. Some autocross ....

the reason i was thinking 3.91 was because i thought 4.10 would make it impossible to drive daily (too jerky and i would be at higher RPM's all the time) .... what do u think???

oh and 4.10 is the same price ....

[-c-]

hey their! id stick with your 3.73 gears, your already higer than the rest of the z4's but a 4.10 rear will kill your gas milage and youll be reving way higer on the higeway, could be a 1000 rpm increas in some cases was with my 1971 charger anyway.

your rear diff is already a real good number for fuel and performance, at least you can say you have a higher diff out of the box than a stock M and SI's for that matter. For the cost youll get more of a difference and way cheaper going with that 3 stage intake mod anyway.

oh yea teh 3.91 you wont notice anything and youll be pi$$ed you spent all that money!

either way you slice it changing the rear diff will be over a grand to possably 2K depending on labor and parts.

[3002 tii]

Originally Posted by -c-

hey their! id stick with your 3.73 gears, your already higer than the rest of the z4's but a 4.10 rear will kill your gas milage and youll be reving way higer on the higeway, could be a 1000 rpm increas in some cases was with my 1971 charger anyway.

your rear diff is already a real good number for fuel and performance, at least you can say you have a higher diff out of the box than a stock M and SI's for that matter. For the cost youll get more of a difference and way cheaper going with that 3 stage intake mod anyway.

oh yea teh 3.91 you wont notice anything and youll be pi$$ed you spent all that money!

what are you talking about? why would it be way higher? the difference in 3.73 to 4.10 is only 10%, thus your revs would only go up by 10%, ie 3000rpm to 3300rpm.... the only way it'd be 1000rpm's higher was if you're crusing at 10,000 rpms!

easiest way to solve your question is look at how you drive now. look at your current shift points and your typical cruising speed/gear, and then compare the effect of taller gearing. the change in rpms is linear, so 5% change in gearing is a 5% change in your rpms.

few members here went with taller gears, and the only regret is those who didn't go high enough. search around and you'll see threads on this. if you're only going 5% change, i say don't even bother then and save your money. the limited slip alone would put a big smile on your face.

[-c-]

Originally Posted by Dubbedown

what are you talking about? why would it be way higher? the difference in 3.73 to 4.10 is only 10%, thus your revs would only go up by 10%, ie 3000rpm to 3300rpm.... the only way it'd be 1000rpm's higher was if you're crusing at 10,000 rpms!

easiest way to solve your question is look at how you drive now. look at your current shift points and your typical cruising speed/gear, and then compare the effect of taller gearing. the change in rpms is linear, so 5% change in gearing is a 5% change in your rpms.

few members here went with taller gears, and the only regret is those who didn't go high enough. search around and you'll see threads on this. if you're only going 5% change, i say don't even bother then and save your money. the limited slip alone would put a big smile on your face.

Yea I guess I was not really being apples for apples as my experience with my 4:10 rear on my charger was always making my motor sing on the high way so extended drives got a little worried about my engine grate for stop light take offs though. I was just saying his rear end is a good rear end for a daily driver that's all.

So yea it be like knocking it over to sport mode.

[kirkpart2]

-C- .... I actually just got a huge scholarship from Boeing ..... So, I can, if I want, get the manifold swap done as well as put in a diff .....

I'm trying to see if the diff is a good idea. If not ill get KW coilovers.

[vachss]

Originally Posted by kirkpart2

-C- .... I actually just got a huge scholarship from Boeing ..... So, I can, if I want, get the manifold swap done as well as put in a diff .....

I'm trying to see if the diff is a good idea. If not ill get KW coilovers.

Ah, the familiar refrain... I remember when I got my fellowship in grad school the first thing I did was run out and blow the entire first months pay on comp tires for my Autox car. A year later I was doing my first engine swap in my department secretary's garage. The bug never quit's biting.

[kirkpart2]

Oh and what about recalibrating the speedo?

[pokeybritches]

You don't need to recalibrate about the speedo. Dubbedown is correct.

[kirkpart2]

Originally Posted by pokeybritches

You don't need to recalibrate about the speedo. Dubbedown is correct.

Thanks for the input .... Just a question: my friend just swapped a diff on his 99 camaro and he is having major issues with speedo calibration. I don't know why I don't need to recalibrate my speedo. I guess I don't know enough about how the speedo works ... any help appreciated

Also it seems 4.10's are the way to go .... no jerkiness/insane increase in gas mileage then?????

[kirkpart2]

Originally Posted by kirkpart2

Thanks for the input .... Just a question: my friend just swapped a diff on his 99 camaro and he is having major issues with speedo calibration. I don't know why I don't need to recalibrate my speedo. I guess I don't know enough about how the speedo works ... any help appreciated

Also it seems 4.10's are the way to go .... no jerkiness/insane increase in gas mileage then?????

Bump

any further inputs ..... about the speedo and the jerkiness??? i tried lookin up posts but didnt find anything to answer the questions ....

[HP Autosport]

Originally Posted by kirkpart2

Bump

any further inputs ..... about the speedo and the jerkiness??? i tried lookin up posts but didnt find anything to answer the questions ....

Speedo is not affected as late model BMW's reads speed from the wheel speed sensors and not a sensor in the transmission.

The only jerkiness I can see happening is if you were to have a clutch type LSD that is engaging and disengaging all the time. If you ever driven a car with a detroit locker, you will know what I mean. You will never have such a thing with the Quaife, it's super quiet, you wont' even know it's there.