swapping a 3.46 diff in a 530 - good/bad idea?

[xaxis360]

I have a 2001 330Ci convertible with a 5 speed manual (basically the same drivetrain as my 02 530). It has a lot of pep and high revs which I attribute to the short gearing in the differential. On the highway, I can pass people in 5th gear with ease because of it.

So, I get into my 530, and it feels sooo much more sluggish. the revs are much lower than my 330 and its a heavier car! I am constantly downshifting to 4th or 3rd if I want to get a little aggressive. Although I like the fuel savings and quieter engine, I am seriously considering swapping a 3.46 from the auto but i'm a little hesitant.

I read this post about doing the swap, but I am just afraid a 3.46 is just way too high and will make 1st gear worthless. There's always a 3.15 from a 525 manual, but I haven't heard of anyone who has done that swap.. and if there is even a noticeable difference. The 525 touring has a 3.23 which I would probably like, but are very expensive and difficult to find whereas the 3.46's are plentiful can be had for about $150.

thoughts?

[Cyrix2k]

If all you want is more high gear pep, drive in 4th... Go with a lower diff if you want greater overall acceleration, not just more acceleration in high gear. 5th gear is an overdrive for a reason.

[pbonsalb]

I like a shorter diff, but it does come at the expense of fuel economy and highway noise. Only you can decide whether the benefits are worthwhile. There is a calculator at www.diffsonline.com under technical that you can use if you know your ratios. Assuming you have the same ratios as my 99M3 and that means a 1:1 fifth gear, a 3.46 will be a massive jump from your present 2.93. It is about as much as most people would want on a daily driver. I run a 3.64 but my M3 is seriously hotrodded. The cautious approach would be the 3.15. You would notice the difference -- I first swapped a 3.38 for my 3.23 and could feel a slight difference so your 2.93 to 3.15 would be more noticeable. However, you may end up wanting still more. You could always then buy the 3.46 and sell the 3.15. Once you swap one diff, it is not so hard to swap the next one. I could do it in a couple of hours now.

[MaxVQ]

5th gear is an overdrive for a reason.

Not that it really matters, but on the sixes, 5th is 1:1 so it is not an overdrive.

You V8 guys get the nice 6 speed with overdrive...

[Cyrix2k]

oh, didn't know that. I thought 4th was 1:1 on the sixes. You're right that it doesn't matter as the final drive is selected accordingly.

[newton22]

My 5th gear pulls almost as hard as my 4th gear.

[pbonsalb]

Unless you have a supercharger that works only in 5th gear, your 5th gear does not pull almost as hard as your 4th gear.

[Jackcat559]

Try the 3.46, and if you don't like it for your driving style put a 3.23 in. The 3.46 should be pretty cheap on ebay. I had a 3.23 in my Z4 and loved it, but it was a 6 speed.

[xaxis360]

thanks for the opinions guys. I'm going to think about it. I might start the hunt from a 3.23 from the touring if I can find one cheap, but if I can't i'll probably throw a 3.46 in!

[pbonsalb]

It is a pretty big jump to go from a 2.93 to a 3.46. A lot of people would probably say it is great at first and then get tired of it when they start driving on the highway. Right now with the 2.93, you are running 78 at 3000 and 92 at 3500 rpm in 5th on the highway. With the 3.46, you will be running 66 mph at 3000 and 77 mph at 3500 in 5th. That is a huge jump -- about 500 rpm higher cruise. It is pretty close to running in 4th with your 2.93 (but not quite as bad) so you may want to give that a try for an hour on the highway to see how you like cruising at the new higher rpm but the same old speed.

[newton22]

Unless you have a supercharger that works only in 5th gear, your 5th gear does not pull almost as hard as your 4th gear.

Well alright, not almost as in very closely, but the 5th gear ratio is much closer to the 4th than any of the other gears are to each other.

[pbonsalb]

But there is less torque multiplication and the motor is trying to accelerate from a higher speed. It is not even close, unless your car has some sort of very very weird problem. It is very hard to beat the laws of physics. This kind of difference can be measured in seconds, not tenths of seconds. Give it a try. Use a stop watch to run from 3000 to 4000 in each gear (the longer the rpm run the easier to time manually) or pick a 20 mph or greater speed range like 70 to 90. If you don't have a stop watch, but have a tape recorder, say start and stop and then time the difference on playback later. Ideally, use a friend with a stopwatch.

[xaxis360]

It is a pretty big jump to go from a 2.93 to a 3.46. A lot of people would probably say it is great at first and then get tired of it when they start driving on the highway. Right now with the 2.93, you are running 78 at 3000 and 92 at 3500 rpm in 5th on the highway. With the 3.46, you will be running 66 mph at 3000 and 77 mph at 3500 in 5th. That is a huge jump -- about 500 rpm higher cruise. It is pretty close to running in 4th with your 2.93 (but not quite as bad) so you may want to give that a try for an hour on the highway to see how you like cruising at the new higher rpm but the same old speed.

Great advice. I'm going to try this today and report back!

[JackEatsEmoKids]

I found a 3.15 from a 525i for $375 shipped...I'm thinking about it.