[Scrooge4277]

I did a search earlier and noticed that it's been a long time since the last mention of a diff swap for the e90. Has anyone attempted to stick a 330i step differential into a 330i 6mt?

[ksfrogman]

I likely will in the near future.

[RichP]

I have a step 330 diff in my 325, any questions about it? More torque, more RPM.

[ksfrogman]

Originally Posted by RichP

I have a step 330 diff in my 325, any questions about it? More torque, more RPM.

How did this affect your overall gas mileage? I believe the 330 step diff is 3.64, and the 325 step 3.71 or 3.73. Are you happy with the swap?

[RichP]

Well, at the usual junkyard prices of 500-600, I would be pretty happy. The car has much more umphh off the line, and 6th gear allows you to actually accelerate. Granted, I paid almost 1100 for mine because it was new, so my performance-per-dollar isnt as great.

Gas mileage, hard to say, I dont think it was affected much if at all. 3.63 is a bit short of gearing in a manual, but not bothersome. It rocks in traffic because the car can tear out in front quickly.

Im pretty happy, they dont call a differential swap the "poor mans supercharger" for nothing!

[BMW_E90]

can this be done to a 325i step? im interested..

[Scrooge4277]

How much would it likely cost to have this done professionally? Is this a dumb thing to do to a leased car?

[ksfrogman]

Originally Posted by BMW_E90

can this be done to a 325i step? im interested..

No, you already have the lowest gear ratio available, the 3.73.

The 330i step diff is a 3.64, the 325i 6MT = 3.23, 330i 6MT = 3.15.

[ksfrogman]

Originally Posted by RichP

Well, at the usual junkyard prices of 500-600, I would be pretty happy. The car has much more umphh off the line, and 6th gear allows you to actually accelerate. Granted, I paid almost 1100 for mine because it was new, so my performance-per-dollar isnt as great.

Gas mileage, hard to say, I dont think it was affected much if at all. 3.63 is a bit short of gearing in a manual, but not bothersome. It rocks in traffic because the car can tear out in front quickly.

Im pretty happy, they dont call a differential swap the "poor mans supercharger" for nothing!

Awesome Rich. You've given me even more motivation. I'm surprised you didn't go with the 325i step diff, which would have been a 15% difference and about 500 rpm difference at speed, but your more conservative approach is probably a good middle-road swap.

[///Matt]

why aren't YOU going with a 325step diff???

[RichP]

In my E46, I went from a 2.93 to a 3.38, and I found that a pretty big jump. The only reason I thought I could handle a 3.63 (realize the transmission ratios are about the same in the E46 and E90) is that:

- The E90 has a 400 RPM higher redline
- It has a 6 speed.

If the ratio gets TOO short, 1st gear is going to be useless. I think a 3.63 is about the limit, for me anyway. Beside, you have a really aggressive diff thats open, you have only a better chance for the infamous E90 one-wheel burnout-launch.

[T7R_Sky]

Wait... im confused... whats better performance wise... putting a 325i step diff in a 330 or a 330 step diff in a 325? Would putting a 330 step diff in a 325 make it faster or slower? And what do the numbers mean? (3.64 and 3.73) are those gear ratios or something? Sorry about the noob questions.

[ADHD]

The number are the sizes of your gears!! 3.64 is a shorter smaller gear than the 3.73 which is taller and wider gear giving u more punch off the line!!
the 3.64 would not be as peepy off the line but would probably go better at top speed! hope this makes sense to you!

[T7R_Sky]

Originally Posted by ADHD

The number are the sizes of your gears!! 3.64 is a shorter smaller gear than the 3.73 which is taller and wider gear giving u more punch off the line!!
the 3.64 would not be as peepy off the line but would probably go better at top speed! hope this makes sense to you!

Haha, alright. Thanks ADHD! I think I got it now. So putting a 330 diff in a 325 probably wont make that big a difference. lol.

[RichP]

Originally Posted by T7R_Sky

Haha, alright. Thanks ADHD! I think I got it now. So putting a 330 diff in a 325 probably wont make that big a difference. lol.

Not make a huge difference? Its almost 13% more torque at the ground!

[carbo]

Originally Posted by RichP

Not make a huge difference? Its almost 13% more torque at the ground!

That statement is a little misleading. You are changing the gearing so acceleration can change but the torque increase is only due to engine speed vs road speed. Dyno in 5th gear and you will not see a 13% torque increase at peak with a final drive change.

[ksfrogman]

Originally Posted by ADHD

The number are the sizes of your gears!! 3.64 is a shorter smaller gear than the 3.73 which is taller and wider gear giving u more punch off the line!!
the 3.64 would not be as peepy off the line but would probably go better at top speed! hope this makes sense to you!

Say Tyler, I think you have this reversed. The higher numerically a gear ratio is, the lower it is. For instance, a stock 330i 6MT has a fairly tall gear of 3.15, which means that you don't have to shift as much, and 6th gear feels like it lugs at 55 mph. Going to a 330i step axle ratio of 3.64 makes the gear numerically higher, but mechanically lower; you would have to shift the car more often, and the RPMs would be about 500 rpm higher at equivalent speeds. Look at this calculated table I came up with a few months ago:

[ksfrogman]

One more thing. You probably all remember from physics that Torque = Force x Distance. In a car,

Motor Torque x gear ratio = torque at the rear wheel (RWD car).

In our case, the power would remain about the same, but the car with the lower gear ratio (numerically higher) would accelerate quicker, because it has more torque to quickly move the car from lower to higher speeds. Take, for example, two identical twin brothers, each with equal strength (power). They have identical bicycles, except for the gearing. Bicycle B's 1st gear is more like Bicycle A's 2nd gear. Thought of another way, Bicycle B essentially has to take off in 2nd gear (a taller gear). Bicycle A takes off in 1st gear. The two brothers take off in a race from a dead stop up a hill (under load). Guess who would climb up the hill more quickly?

[ADHD]

NO hate to burst u here u michael but ahh!! 3.73 gears are tAller and will give u more punch off the line compared to 3.64 gears..
the 3.64 will be more sluggish off the start compared to 3.73 gears!
I swapped my trucks gears from 3.53 to 3.73 and I can barely hit 120know, but it takes off like a bat out of hell!! I basically lost my topend sacrificing for my bottom end!

u can get a quick answer from mike at TEknik! call him!


ok example: i race karts!! the taller the gear meaning more teeth the faster it jumps off the line..
the less teeth means its slow off the line but its faster in the straightaways!!

[ksfrogman]

Originally Posted by ADHD

NO hate to burst u here u michael but ahh!! 3.73 gears are tAller and will give u more punch off the line compared to 3.64 gears..
the 3.64 will be more sluggish off the start compared to 3.73 gears!
I swapped my trucks gears from 3.53 to 3.73 and I can barely hit 120know, but it takes off like a bat out of hell!! I basically lost my topend sacrificing for my bottom end!

u can get a quick answer from mike at TEknik! call him!


ok example: i race karts!! the taller the gear meaning more teeth the faster it jumps off the line..
the less teeth means its slow off the line but its faster in the straightaways!!

Right. I agree the 3.73 gears will give you more punch. I think that our discrepancy is in the semantics of the terms taller vs shorter and lower or higher. As far as I know, a "taller" gear is a "higher" gear that allows you to go faster, not quicker. 7th gear is taller than 6th gear for instance.

A taller gear is numerically lower. 3.63 is taller than 3.73.

[ADHD]

Originally Posted by ksfrogman

Right. I agree the 3.73 gears will give you more punch. I think that our discrepancy is in the semantics of the terms taller vs shorter and lower or higher. As far as I know, a "taller" gear is a "higher" gear that allows you to go faster, not quicker. 7th gear is taller than 6th gear for instance.

A taller gear is numerically lower. 3.63 is taller than 3.73.

I see what your saying exactly and we do agreee but your thinking is reversed as far as I know!

A "taller" gear is considered to be a "lower" gear that allows you to go faster off the line resulting in top speed LOSS!!

[ksfrogman]

Originally Posted by ADHD

I see what your saying exactly and we do agreee but your thinking is reversed as far as I know!

A "taller" gear is considered to be a "lower" gear that allows you to go faster off the line resulting in top speed LOSS!!

Hehe. I still disagree with you on the last part.

Let me find something on the web that is more eloquent than how I'm explaining it. Think of your bicycle. 5th gear uses a very short (small diameter) gear than a taller 1st gear. Think of taller tires. They drop your engine rpm but will hurt your ability to take off quickly. Let me do some google-ing and get back with you on this oen.