3.0si final gear ratio + Forced induction

[dre99gsx]

Trying to wrap my head around this: Those of you with the Z4M and 3.62 rear, do you find yourself feeling like your stressing out the motor at 80mph highway cruising speed? I believe 3233rpm is what you would see. I tend to maintain those speeds on NJ highways based on traffic flow (which usually is moves even faster unfortunately).

With the 2J in this thing and quite a bit of power, I also find myself rowing through the gears way too often. I'm in 6th gear just cruising around town for no reason. I don't know how some of you go 3.91 or 4.11!

In comparison, the Supra has a 3.13 final drive, putting it at 2608rpm / 80mph 6th gear. I was looking at some options:

80mph cruising, 25.58 tire height, .85 6th gear rpm diff
3.0si Auto 3.64 3251 <- My current setup...
3.2 z4M manual 3.62 3233
3.0si 6spd 3.46 3090
3.0si E85 6spd? 3.23 2885
Supra 3.266 (.793 final) 2608

I noticed on realOEM, E85 comes with 3.23 rear, although I may choose a happy medium at 3.46. Any thoughts on compatibility of these diffs?

[GuidoK]

If you want to have similar end gearing as the supra, you can get a 3.07 ratio diff or a 2.93 diff. That last one 'll do about 2600rpm at 80mph.
I think the difference between 3.23 and 3.46 would be quite small.

I dont know how much power and torque you're going to put down, but there's no use in having a 1st gear so short that its totally unusable. If you're going to make big torque figures, you even risk breaking traction all the time in 2nd.
Usually the more torque down low you make (say in the range of 2k-4krpm), the lower the final drive (with the same gearbox)

Too bad there is no realistic way to change the gearbox ratio's, to get for instance a ration patternt like a s2000 has (relative long 1st and ratio's get shorter when gears get higher). That would be ideal but very costly.

Be sure to choose your final drive ratio before for example buying a LSD because the different ratio's require different lsd's.
I'm not sure but 3.23 and 3.46 might use the same setup, but 3.07/2.93 use a different setup.
A 3.62 z4m diff is incompatible (210 diff instead of 188 diff) and requires a different rear axle (subframe, driveshafts, trailing arms the lot...) and propshaft.

I also think it depends on your use; if you're planning to do a roadtrip/vacation in this car you'll get crazy if you have to listen to 3500rpm for hours on end on the highway

[Westersund]

What about a final drive from a E36/39 diesel?

[GuidoK]

e39/e36 diff doesnt fit (at least the backcasing, and the output bearing style is also different which might cause problems).
But if something lower than 2,93 is required you can probably use a diesel e46 diff. The e46 has the most similarities with the e86
For using some of the e46 diffs you need to re-use the z4 drive shaft flanges. Boltpatterns vary among cars/engines.

[pokeybritches]

I went 3.07 --> 3.64 --> 3.46. The difference between 3.64 and 3.46 isn't that great.

I like having the shorter gearing with my centrifugal supercharger, but with a turbo you might be better with 3.23 or lower... especially if traction is already an issue. The shorter the final drive, the weaker it is.

[Westersund]

Originally Posted by GuidoK

e39/e36 diff doesnt fit (at least the backcasing, and the output bearing style is also different which might cause problems).
But if something lower than 2,93 is required you can probably use a diesel e46 diff. The e46 has the most similarities with the e86
For using some of the e46 diffs you need to re-use the z4 drive shaft flanges. Boltpatterns vary among cars/engines.

You're right, I was thinking Z3 as I'm into building my track car right now. Brain fart. The E46 330D final gear should be a good option.

[dre99gsx]

I'm eyeing the 3.23, but not easy to find a E85 variant. There are plenty of E46 3.23 diffs. How probable is it that the drive flanges need swapping to Z4 variants?

[GuidoK]

I think only a 330i has a different bolt pattern on the drive shafts. (94mm instead of 86mm).
But swapping out those flanges is probably very easy. They just pop out, and I think a 86mm flange fits right into a diff that previously had the 94mm flange. (I've never had a 330i diff in my hands but I think thats all the same).
So you can just re-use the flanges of your z4 diff. You probably have to reuse the flange on the propshaft side too because the z4 has a universal joint on there, but some of the e46's have a cv joint on there (not all of them)

I usually pop those flanges out by using the 2 sets of the original bolts and that little bracket that goes on top and putting that in the flange opposed, and then put a steel bar or tyre iron or so through it, and use a large socket or block of wood to sit between the diff housing and tyre iron (using the tyre iron as lever) and just pop it off by hand. (I know a picture would be easier to understand...)

And its best to use the z4 back cover as it has a build in heat sink (and the undertray has air ducts to channel air into that heatsink) and the e46 back diff cover hasnt.

[dre99gsx]

Perfect, will look into it, thanks for the tips!

[Steeler]

Originally Posted by dre99gsx

I'm eyeing the 3.23, but not easy to find a E85 variant. There are plenty of E46 3.23 diffs. How probable is it that the drive flanges need swapping to Z4 variants?

Good info here.
I have bought 3 different diffs that came out of E46 with no fitment issues.

http://www.bokchoys.com/differential/GearRatios_E46.htm

[dre99gsx]

Originally Posted by dbarton62

Good info here.
I have bought 3 different diffs that came out of E46 with no fitment issues.

http://www.bokchoys.com/differential/GearRatios_E46.htm

I think it depends, for example:

325xi /touring 4.21 2.49 1.66 1.24 1 3.85 3.23

RealOEM shows the flange on that diff to be LK=94MM/M10 bolt patter, while the Z4 uses LK=86MM/M10, thus that would require swapping some components...

[Steeler]

Originally Posted by dre99gsx

I think it depends, for example:

325xi /touring 4.21 2.49 1.66 1.24 1 3.85 3.23

RealOEM shows the flange on that diff to be LK=94MM/M10 bolt patter, while the Z4 uses LK=86MM/M10, thus that would require swapping some components...

Not sure.
I always went for the E46 330 derivative.

[dre99gsx]

The other aspect to this is: how do those of you with the standard 3.62 Z4M rear (or higher as some seem to go) live with driving at normal highway speeds? Seems like these cars were not naturally designed for GT / touring based on the gearing.

[Steeler]

I had a 3.91 LSD in my 3.0i and it was taching 3k @ 70mph.
Got irritating after a while.
I sold it off this winter and gone to a 3.64 LSD. Can't tell you what RPM's are at 70mph until spring.

[WickedNy]

I have a 3.46 that'd I'd be willing to part with taken out of car with approximately 75k miles, swapped input flanges from it so you'd prob need to use your own.

[dre99gsx]

I'm looking for a 3.23, not a 3.46!

[GuidoK]

Yesterday I measured the rpm on the 3.07 diff, and at 80mph you get 2900rpm.
That is on a 265/35/18 tyre (25.3" hight) at about 38psi. (
This was done with gps speed, so 100% accurate.
But note this: if you calculate it, it should have been 2773. So there is a pretty big difference there. Ive read about a lot of speculations, how much this real world difference is (and it's obviously result of tyrepressure (and mine is even quite high), camber and state of wear (mine are at about 50%). Some calculators even calculate above 10% difference for this effect.
But it shows that if you want to be at or below 3000rpm at 80mph a 3.23 diff ratio might still be too short. If I apply the same correction on your tyresize, with the 3.23 ratio you'd be at ~3040rpm at 80mph.

Point is that you'll be making more revs irl than when calculated.

[dre99gsx]

Regardless of accuracy, I do know I run 3200rpm right now at 80mph using values from the AEM, thus I should be close to theoretical as it is. 275/35 tire is a bit taller than your 265 as well. If theoretically 3.23 puts me at 2885, then I'm fine with that even if at 2900. I would like a bit more responsiveness than my Supra's 2600 rpm, so this would be a step in the right direction. In reality, I can't find any 3.23 at a decent price range, but have found a few 3.15 (which put me at 2814 rpm). Either works

[The HACK]

Originally Posted by dre99gsx

The other aspect to this is: how do those of you with the standard 3.62 Z4M rear (or higher as some seem to go) live with driving at normal highway speeds? Seems like these cars were not naturally designed for GT / touring based on the gearing.

The S54 doesn't really come alive until 3,000 RPM anyway. Anything below that it sort of putters along like a normal car would. Above 3,000 RPM then it sort of "wakes up" and pulls and pulls to redline like no one's business.

I find living above the 3,000 RPM range at 80mph easier than, say, in my wife's diesel, where at 80mph it's BARELY above idle. On the diesel, at 80mph, you press down on the accelerator, it waits for a split second, drops 2 gears down to 4th, then another split second or so and it takes off like a bat out of hell...At 4,000 RPM. On the M? At 80 a light tip-in on the throttle leads to an immediate brisk acceleration, and if you want to drop 2 gears, @ss pressed to the seat and it will pull and pull until you're way past 100.

Of course, at 80mph the diesel gets about 40mpg while the MZ4 Coupe gets maybe 22mpg on a slight downhill. And once the turdbo kicks in the diesel will go from 80 to land in jail in a blink of an eye (ask me how *I* know), and STILL return 22mpg or better.

No the MZ4 Coupe is not a natural GT/Cruiser and does not fair well at gobbling up miles.

[DanFromTucson]

Originally Posted by dre99gsx

I think it depends, for example:

325xi /touring 4.21 2.49 1.66 1.24 1 3.85 3.23

RealOEM shows the flange on that diff to be LK=94MM/M10 bolt patter, while the Z4 uses LK=86MM/M10, thus that would require swapping some components...

I had a 2004 E46 (330ci ZHP) several years ago, and I swapped in an E85 diff. I had to swap the flanges. I remember finding good information on e46fanatics.com. It wasn't too difficult, but you have to carefully measure the torque when you bolt on the flange.

[GuidoK]

Originally Posted by dre99gsx

Regardless of accuracy, I do know I run 3200rpm right now at 80mph using values from the AEM,

your speedo is accurate too? or did you use gps?

Originally Posted by DanFromTucson

It wasn't too difficult, but you have to carefully measure the torque when you bolt on the flange.

Do you mean the driveshaft flanges or the propshaft flange?
With the propshaft flange you have to feel by hand when the bearings start to create some drag. thats the rigth tension. If you dont replace the bearings, you can use the original crimp bus on the pinionshaft. So there's no real torque setting.
You could measure the drag in force, but there's no data on that.
Maybe use the e36 data. That also requires some very special tooling though.

[dre99gsx]

My speedo isn't any more accurate than what comes OEM with the Z4. My point being that regardless of what my actual speed is, my relative speed on my roads in average traffic reads 80mph. If I'm doing 85 or 75, regardless, I'm still running 3200rpm for minutes on end (long winding straights, wide roads, on NJ roads) and I don't feel comfortable having the engine in that range for that long. Although we are at 55mph state limit, most everyone cruises at 80, even on slow lanes.

Sure, benefits of being in the power band instantly are great for passing others, but sometimes you just want to relax for a bit.