Differential Time, 3.91 vs 4.10 on 6 speed ZCP

[Silbergrau metallic]

You say acceleration gain, but don't forget sometimes it's slower.

E.g. If you're going, say, 90mph. In a stock diff car you'd be in 3rd, in a 4.10 car you'd be in 4th. The stock gear car has more aggressive gearing at that speed, as a result.

People posting videos of gains from diffs like to compares same gear vs same gear at a given speed. In that situation, certainly, yes, a shorter diff is faster. But, it's not a particularity meaningful test.

If you compare optimal gear vs optimal gear... results may differ, depending on the speed you're going. The faster you're going, the more often the stock diff results in a more aggressive net ratio being possible. Plus the shorter ratio slows you down by requiring more frequent shifting.
(the constant shifting was why I nixed the 4.10s on the track car, though it was probably a bit of an extreme case at 2315 lbs. It was annoying and slower because of it)

That makes a lot of sense, I guess I'm just trying to get the most bang for my buck and I was considering gear ratios, since I need the Diff rebuilt and wanted opinions. Going from 3.62 to 3.91 or 4.10 Doubles the price of the Differential on Diffsonline and I'd rather use that money on a better than Stock LSD, like Wavetrac and go with the stock 3.62 ratio like you have done. Did you have the Ring, Pinion and Bearing polishing option added also or is that unnecessary and does the Wavetrac option require different 210mm Output flanges for the carrier. Sorry for all the questions, but I'm trying to make sure I order everything I need the first time.

[Cubieman]

I know nothing else but 4.10s as thats how my vehicle came and have never had the opportunity to drive another E46 M3, I always start in 1st gear from a stop but there are situations such as when rolling to a stop and then the light turns green and then I'll just grab 2nd as opposed to reving it up to 5k and dropping down to 1st.

At residential speeds (25-35mph) you either have to speed a little bit and keep it in 3rd or leave it in 2nd in which case the revs are a bit high. But with the speedo not very accurate and the different gearing I may not be going as fast as I may think.

Just be prepared sometimes for numerous 2/3 shifts in slow spots, especially when caught behind slow moving vehicles, but once you get out of the residential areas no big deal.

Oh yea, 4.10s launch your car pretty well, but I've only done that a handful of times and don't recommend it, that's not really what our cars are all about.

If/when I have to rebuild my diff I may try 3.91's but 4.10's are all I know and they are just fine for most applications imo, and I have never experienced any form of diff whine whatsoever fwiw.

[Obioban]

I have a 10 year/125,000 mile warranty, so I'm hoping the majority of the cost is going to be covered and my friends BMW, Audi repair shop is going to do the work. I'm probably going 3.91 with the Wavetrac LSD. Thank you everyone for all the information and it is sincerely appreciated. I see your car has 4.10 gears, how significant was the acceleration gain, compared to stock, I read somewhere that some people with that Ratio start from a stop in 2nd gear, since it makes 1st so low, is that true. I tried that Gear calculator on Diffsonline, but it wasn't loading.

You say acceleration gain, but don't forget sometimes it's slower.

E.g. If you're going, say, 90mph. In a stock diff car you'd be in 3rd, in a 4.10 car you'd be in 4th. The stock gear car has more aggressive gearing at that speed, as a result.

People posting videos of gains from diffs like to compares same gear vs same gear at a given speed. In that situation, certainly, yes, a shorter diff is faster. But, it's not a particularity meaningful test.

If you compare optimal gear vs optimal gear... results may differ, depending on the speed you're going. The faster you're going, the more often the stock diff results in a more aggressive net ratio being possible. Plus the shorter ratio slows you down by requiring more frequent shifting.
(the constant shifting was why I nixed the 4.10s on the track car, though it was probably a bit of an extreme case at 2315 lbs. It was annoying and slower because of it)

[Feffman]

When I bought my track only M3 it had 4.10 R&P with the Getrag 6-speed (6th gear was never used on track) and I ran out of gear on the larger tracks like Road America, COTA and Road Atlanta. I swapped to a Drexler LSD 🤩 and 3.91 which I feel is fantastic for coming out of corners and those long straights. I believe the EPIC tune raised the red line 200 RPM as well so I wasn't bouncing off the red line on long straights or under hard acceleration. Later came the ZF 5-speed to replace the heavier Getrag.

On the street I'd likely stay with stock or 3.91. My humble $0.02.

Feff

[Silbergrau metallic]

OP you car (ZCP) and mods fully suggest you’re into improving the performance of the car. You’re not going to have a better time to have an improved sportier ratio and also save labor than now. Diff ratio change is known as a “best bang for the buck” for a reason. With that said it’s not for everyone. If you already had a 4.10 and wanted to make car more docile then we’d suggest a 3.62. If you drive with cruise control (that your ZCP doesn’t have 😉 ) on highways for long periods of time over 90 I would suggest not. But for someone with performance in mind I think you’d regret not going at least 3.91.
Overall the best thing for you is to test drive a 4.10 and see what you think. Also look at the gear calculator at Diffsonline and see it’s nothing outrageous at all. But it gives better idea.
If money is a major factor you’re really better off just buying that brand new factory wrapped diff in the classifieds.

I have a 10 year/125,000 mile warranty, so I'm hoping the majority of the cost is going to be covered and my friends BMW, Audi repair shop is going to do the work. I'm probably going 3.91 with the Wavetrac LSD. Thank you everyone for all the information and it is sincerely appreciated. I see your car has 4.10 gears, how significant was the acceleration gain, compared to stock, I read somewhere that some people with that Ratio start from a stop in 2nd gear, since it makes 1st so low, is that true. I tried that Gear calculator on Diffsonline, but it wasn't loading.

[old///MFanatic]

OP you car (ZCP) and mods fully suggest you’re into improving the performance of the car. You’re not going to have a better time to have an improved sportier ratio and also save labor than now. Diff ratio change is known as a “best bang for the buck” for a reason. With that said it’s not for everyone. If you already had a 4.10 and wanted to make car more docile then we’d suggest a 3.62. If you drive with cruise control (that your ZCP doesn’t have 😉 ) on highways for long periods of time over 90 I would suggest not. But for someone with performance in mind I think you’d regret not going at least 3.91.
Overall the best thing for you is to test drive a 4.10 and see what you think. Also look at the gear calculator at Diffsonline and see it’s nothing outrageous at all. But it gives better idea.
If money is a major factor you’re really better off just buying that brand new factory wrapped diff in the classifieds.

[old///MFanatic]

I had a buddy who tried 3.91 with a 6 speed and he said he didn’t like it… def appears to be a personal preference thing. He said he’ll be doing a 5 speed with 4.10’s.

So he didn’t think the 3.91 was short enough improvement over stock and wants a 4.10.

It’s definitely a preference thing.

[bjz!]

Not sure how true this is, but a shop tech said that the 4.10s make more gear whine than the 3.91s because the gears are cut straighter. I think the 3.91s would be nice for a street car that sees a lot of uphill twisty roads. In 2nd gear, the revs can sometimes fall to 2k rpm on those slow, uphill, tight turns.

True.

[maupineda]

Lol. The US is not flat everywhere 🙄

well, is a bit flatter compared to MX, my drive from Michigan, all the way down to Laredo was plain boring, 98% was in 6th gear with not a lot of steering wheel input.

[SandeepM3]

Anybody know if this old superseded part number for the 3.85 is still good to use? The new pt number has no sock but the old still has stock in Germany.

New (no stock) - 33 10 8 311 351
Old (has stock) - 33 10 8 437 888

Just want to make sure I don't get the wrong part as I cant find any info on the old part number when googling and typically ecs/turner/vendors will say the previous SS number, but none do.

Thanks

[bigjae46]

You can also play with your final drive with tires.

I found out that you can to a certain extent. I tried a 245/40R17. Found that the tires heat up too much at higher speeds.

I think the range to stay between is 25” - 26” which doesn’t significantly shift too speeds in gear.


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[Tbonem3]

You can also play with your final drive with tires.

[Obioban]

Question obi, if you were street driving on windy back roads with hills would you still consider 3.91 and 4.10s too short? Or in other words do you believe the stock gearing is best in all scenarios?

That's the one situation where I might prefer a 3.91. 4th is too tall to be useful on twisty back roads, and 2nd is too short for all but the tightest corners with the stock diff. In either scenario you end up in 3rd/4th, so having 3.91s might optimize those a bit more for twisty back road use.

[oceansize]

It was an s50B32 car with the redline set at 8000 rpm, so the result was the same as an e46 m3 with 4.10s.

Question obi, if you were street driving on windy back roads with hills would you still consider 3.91 and 4.10s too short? Or in other words do you believe the stock gearing is best in all scenarios?

[Obioban]

From what I've read, I think I'm going to stick with the stock gear ratio, Curious! you're very knowledgeable about E46 M3's, what was your reason for going to the Wavetrac LSD and is it a much better option to the stock LSD and did you notice anything you disliked about Wavetrack compared to stock, because I want to Buy Once and Cry Once, even if it's more expensive to go with the Wavetrac LSD option.

I haven't had the wavetrac on track (I will in 2.5 weeks), so I don't really want to recommend or recommend against it. That said, the reason I went with it was...
1) never wearing out appeals to me very heavily. I am not selling the car, so an additional part that doesn't need rebuilding/replacing (clutches) is very much in my interest.
2) I tend to use a lot of curbing, which puts wheels in the air-- functioning in that situation is useful for how I drive.
3) it's silent, and I street drive my car (and will continue to do so). Plus, I'm of the opinion that track cars shouldn't make noises. If your car doesn't make noises when it's healthy, if it makes a noise something is wrong-- eases diagnostics. The wavetrac is actually quieter than stock, since no more M clunk.

I do not think it's the best performing diff out there. But, how it's performing now should be exactly how it's performing in 20 years, and it should be doing so silently.