[TwinTurboRacer]

Sorry for this being long, but I've been on a few forums and trying to avoid the obligatory and comments.

Searched for a bit and saw all the MT guys swapping to the AT 3.46 FD gear when doing LSD installs and one comment about doing the opposite. I'm actually trying to go "slower" as I just purchased a FBO/JB4 (sans FMIC) equipped E93. I'm planning on keeping this for a long time and selling the Prius (3rd car). To help justify it, I plan on swapping the FD gear from a MT pumpkin to increase fuel economy.

From research:
07+ AT and MT pumpkins are different sizes and welded so no easy swaps.
Final Drive Ratio (335i): MT: 3.08; AT: 3.46
Reference: http://www.bokchoys.com/differential/GearRatios.htm
Based on my limited math skills, top gear ratios: MT: 2.679; AT: 2.3874; AT w/ MT FD Gear: 2.12
Delta in top gear (really any gear) ratio: ~12.6%.
THEORETICAL MPG (Hwy Cruise) Stock: ~27mpg; Post-swap: ~30.4mpg I realize I may not achieve full efficiency gains so from this point on, I'm using 10% gains as a ROI justification.

I don't (and my wife doesn't) need an 11 second car so no need to comment about all the other mods I have. It came free with the car! I have years of experience tooling around in my previous cars so I'll be doing the work myself (as much as time permits).

To prevent other thread jacking:
I wanted a MT, but got a great deal on this one that I couldn't pass up. I know with the MT I could've already gotten the mpg gains I wanted.
Is it worth swapping the diff gears? Well to justify it, the 10% mpg gains = ~$200 a year (12k miles/yr, $4.00/gal 92 oct., all hwy. More savings with city driving). Figure I'm keeping this for at least 5 years and I'm already planning an LSD swap (to drive in snow) so that $1k+ goes towards the LSD swap.
I'll be putting in a larger FMIC eventually. I'll be cleaning the intake valves/ports after I get a tankful or two of baseline mpg/performance numbers in. I'll be doing all maint./cleaning/checks.
It's out of warranty so no issues there. I look at broken things as opportunities to upgrade.
Speedo picks up signals from output shaft so no issues there.

What I'm looking for from this merry band of Bimmer enthusiasts:
Anyone who has a 335i AT (not DCT) that did the 3.08 FD gear ring swap:
1. Who/what/where/How much was the swap and with or without LSD?
2. Any issues with bogging down with the RPMs being below 1600rpm @ 60 mph in 6th gear when cruising up hills/slight acceleration/cruise control? Stock is around 1800rpm, I believe. (I did the fuzzy math to determine this as I don't have the car yet.) I already know passing (stock) will require dropping down a gear or two from other posts I've read so nothing big deal there. Looking for light input/steep grade drivability.
3. Anyone have one they don't want anymore (with or without LSD) AT small case w/ 3.08 FD gear?
4. Any other comments useful to my pursuit to getting rid of the Prius.

Thanks in advance!

[TwinTurboRacer]

Okay, I'm going to self-immolate myself...

I did a little more research (on another forum) and found out what I needed to know. In short, for those of you with AT and Welded Diffs, the only way you will go to a smaller final drive ratio is to swap out the entire rear end (drive axle, pumpkin, output shafts, etc. from a <08 AT. Exact details of what versions there are located here:
http://www.***********.com/showthrea...No-LSD-really-)

sigh... guess larger tires is the only way to go now.

On that note, anyone with an early model 335i with AT trans and bolted diff looking to part out the drivetrain parts I need?

[4doorbmwpower]

shit i want 3.73s on the AT but i was told it would be hella costly...

[OB]

Originally Posted by TwinTurboRacer

Okay, I'm going to self-immolate myself...

I did a little more research (on another forum) and found out what I needed to know. In short, for those of you with AT and Welded Diffs, the only way you will go to a smaller final drive ratio is to swap out the entire rear end (drive axle, pumpkin, output shafts, etc. from a &lt;08 AT. Exact details of what versions there are located here:
http://www.***********.com/showthrea...No-LSD-really-)

sigh... guess larger tires is the only way to go now.

On that note, anyone with an early model 335i with AT trans and bolted diff looking to part out the drivetrain parts I need?

Use the edit button for the first post..

[Cloud9blue]

Larger tires, heavier rotational mass, less mpg. As with the diff pumpkins, our AT box is actually quite efficient and you also have to think about that engine needs to work harder to accelerate with a shorter final drive ratio... Keep the Prius if you want fuel efficiency.

[TwinTurboRacer]

Cloud, it depends on driving conditions. Don't plan on driving the e93 in the city with lots of traffic. Mostly for drives down the Parkway so the inertial mass will help. Also, with all the mods already on the car, won't really have to boost much to pass. If I can break 30 hwy, I can justify getting rid of the Prius (insurance offsets the gas/maint. savings). Prius isn't too efficient driving 70+ mph... get just below 39 mpg.

OB, leaving the notes in the OP so whoever's thinking of this as well can understand what I was trying to do. Plus, I'll own up to the questions in the post.

I think I'm going to take my time to find a wrecked 335i <2007 AT and grab all the shafts/brackets/pumpkin bits I need. Then buy the MT ring gear as well from someone who "upgraded" to the AT gear (or swap them).

[Cloud9blue]

The math sounds right, theoretically. But you are spending potentially thousands for benefits that cannot be proven at this point... I am all about saving money here and there, but just not sure this makes much economical sense when you are trying to save a few MPG with a 300+hp car. If highway commute is what you do the most, a use Jetta tdi or such would make more sense.

On the other hand, if you are serious about fuel economy, why not just get a JB4 and switch to naturally aspirated mode when you need to cruise and save fuel?

[TwinTurboRacer]

Well, it's to have the "option" of how to drive... I have another 300+ HP 'family hauler" with electronically adjustable suspension for that reason. Comfort mode/soft ride when with the family and sport/firm ride when riding on my own.

Some days, I just want to get somewhere and not interested in racing over. Other days, I like to feel the G forces. Prius was fun for a bit, but not anymore. I don't know about the 335i yet, but I've had turbo cars before and able to drive without boosting. How much power does the e93 335i have without any boost? On a 2 ton car, hate to be as slow as the Prius... :/

I agree thousands wouldn't be worth just for the MPG savings, but the LSD is almost a requirement. I'll see how it does with a set of oversized All Seasons this winter and see if I'm venturing down this path. If I do the LSD, the rest wouldn't be that much more if I can find a yard that would give a great deal on parts that would generally be scrapped (1/2 shafts, drive shafts, brackets, etc...

[Cloud9blue]

I tried out the NA mode a few times, the power feels nearly identical to that of a 328i. I had a 328i E93 loaner a few years ago, it was alright and the folding roof was neat, but I do miss the outright power of my car (FYI the GM autos in the 328i are awful comparing to the ZF in the 335i).

Think you are much better off just get a JB4 and save the money for suspension modifications. To my opinion, the cost of having an LSD installed, particularly on welded MT diffs ($3-4k for parts and labor), are not worth it unless you plan on tracking the car. But again, it's an E93, so you are limited to parking lot type auto-x unless you add a roll cage...

[froop]

Not sure about the JB4 but with the Procede NA map there's no way it's as powerful as a 328 or even a 320i. It's horrible. I've also read that you shouldn't drive it like that for extended periods as it can cause strange adaptation issues.

[TwinTurboRacer]

Dang, just missed out on $500 rear end from an early 2008 e92. Included the driveshaft as well. Going to keep looking though. At that price, it's worth doing the swap and LSD upgrade.