Gear ratio 3.62 vs 3.91

[beta]

For an NA car, in your opinion what gear ratio is best: stick with stock, go to 3.91 or even 4.10?

[IronZ4M]

I say go for a 4.10 becoz it's a huge, difference and a gain of a power feel. thinking of it but it really cost a lot to get a 4.10 or 3.91

[Shipkiller]

This is an open ended question with no right answer.

What do you want to do?
What are your expectations?
How far do you want to go?
How much do you want to spend?

[Incompatible]

Originally Posted by beta

For an NA car, in your opinion what gear ratio is best: stick with stock, go to 3.91 or even 4.11?

What's the car going to be used primarily for? Other mods?

[beta]

I wanted it to be an open ended question to open it up for discussion, see what people think about the stock diff ratio, or changing diff ratios in general. If I had to specify usage I'd say mostly street with a few track/autocross events a year.

[ZStig]

There used to be an excellent thread comparing these ratios, including theoretical acceleration numbers. I can't find it

[Shipkiller]

There are several threads on this subject. Some started back in 2006.
Starscream had a very good one going if I remember.

[beta]

I think you mean this post and this post.

I've seen those and was more interested in seeing if people felt that the stock gear ratio was best and why, or informed opinions from those that have changed ratios or have driven cars with different ratios.

[epbrown]

Originally Posted by beta

For an NA car, in your opinion what gear ratio is best: stick with stock, go to 3.91 or even 4.11?

General consensus is to go with the 4.10 unless you plan to add Forced Induction, which is when most people use the 3.91.

Personally, I've no plans for FI and I went with the 3.91, but the modification plan for my Coupe was specific: I wanted to build the car I felt the M Division should have built, using OEM parts (3.91 final drive, Aero skirts, CSL wheels and PS2s ala M3). My working theory at the time was that they wouldn't have used the 4.10 in a production car as it requires a shift to 3rd for the 0-60 dash, so the time with it is actually slower than the stock 3.62.

Imo, it's perfect for a street-driven M Coupe, but many people with S54-engined cars (E46 M3s, Z4Ms) feel it's too mild an upgrade for the money, especially since the swap to a 4.10 is the same price. I did a couple of lengthy posts on the change at www.m3forum.net, in the M Coupe section.

[RobMCoupe]

My original plan is doing 3.91 ring and pinion. I am considering 4.10 for bang for the buck. That is the reason it is still on my list of mod. Keep going back and forth on this.

[BMW Maniac]

Originally Posted by epbrown

General consensus is to go with the 4.10 unless you plan to add Forced Induction, which is when most people use the 3.91.

I would see an added benefit of going with the 4.10 AND a supercharger...maybe not a turbo, then I'd keep the stock LSD ratio.

[johanness]

Originally Posted by BMW Maniac

I would see an added benefit of going with the 4.10 AND a supercharger...maybe not a turbo, then I'd keep the stock LSD ratio.

Good luck with any traction. 4.10's + SC = bad idea

[Starscream]

I dunno why people always say that about 4.10's,
I have 4.10's and the car is no more skiddish than what it was with the stock diff,

I can also launch the car in 2nd gear but for just driving around 1st gear is still 100% useable,

TBH I've got so used to 4.10's now I don't even feel them lol,
I want to drive a stock Z4M again to see the difference

[epbrown]

Originally Posted by Starscream

I dunno why people always say that about 4.10's, I have 4.10's and the car is no more skiddish than what it was with the stock diff

You're supercharged? The advice against 4.10s on a supercharged S54 comes from the E46 M3 community, which has had a lot more experience with our drivetrain, and they're pretty much unanimous in claiming it's hard to hook up the tires with the 4.10 gears.

I don't think anyone disputes that the 4.10s are livable with the stock S54.

[beta]

I'm surprised no one has made a case for keeping the stock ratio. Anyone else have 3.91 or 4.11 gearing and want to give their opinion?

[mfanatic325]

I went with 3.91s because I have plans for FI in the future
They're nice
You'll be spending around $1k just for the differential rebuild though, just FYI. And that's excluding the cost of a ring & pinion set.

[gtMc]

Originally Posted by epbrown

Personally, I've no plans for FI and I went with the 3.91, but the modification plan for my Coupe was specific: I wanted to build the car I felt the M Division should have built, using OEM parts (3.91 final drive, Aero skirts, CSL wheels and PS2s ala M3).

This is the best summation i've read that accurately describes the way I'd consider modding mine, and with the same reasoning behind it. The gearing makes a lot of sense, and yes its a tough decision. If only we had 1 of each lined up in a row so we could test them back to back in some parking lot.

[gtMc]

Originally Posted by epbrown

My working theory at the time was that they wouldn't have used the 4.10 in a production car as it requires a shift to 3rd for the 0-60 dash, so the time with it is actually slower than the stock 3.62.

do you really need to be skipping 2nd gear with a 4.10? and it really is slower than stock 0-60 as a result? Interesting.

[beta]

Originally Posted by gtMc

do you really need to be skipping 2nd gear with a 4.10? and it really is slower than stock 0-60 as a result? Interesting.

It's not skipping the gear, it's that with 4.10 gears you top out at 50 something mph in 2nd gear. So you have to shift to 3rd to go over 60mph. So 0-60 you'll be slightly slower (since you have to do one more gear change) but 0-100 you'll be faster.

[pokeybritches]

0-60 times don't tell the whole story. It's about distance traveled over time. In an extreme case (to prove my point) let's take two cars with identical 0-60 times of say 10 sec.

1. Car #1 jumps to 10 mph in the first sec and remains there until the last second, where it reaches 60 mph.

2. Car #2 jumps to 50 mph in the first sec and remains there until the last second, where it reaches 60 mph.

Car #2 will appear to be much faster because it has covered far more ground, even though 0-60 times are identical. Without traction issues, a car with a 4.10 final drive will beat a car with a 3.62 in all but the very shortest of straight-line races. The real issue is having to switch gears mid-corner.

[gtMc]

Originally Posted by pokeybritches

0-60 times don't tell the whole story. It's about distance traveled over time. In an extreme case (to prove my point) let's take two cars with identical 0-60 times of say 10 sec.

1. Car #1 jumps to 10 mph in the first sec and remains there until the last second, where it reaches 60 mph.

2. Car #2 jumps to 50 mph in the first sec and remains there until the last second, where it reaches 60 mph.

Car #2 will appear to be much faster because it has covered far more ground, even though 0-60 times are identical. Without traction issues, a car with a 4.10 final drive will beat a car with a 3.62 in all but the very shortest of straight-line races. The real issue is having to switch gears mid-corner.

OK, makes sense very helpful. stupid question here... in the example above, which car would have the 4.10 and which car would have the stock 3.62? I would assume car 2 has the 4.10?

And switching gears mid corner is probably something i'd be more concerned about on a track rather than on back twisty roads?

trying to feel out what gear makes the most sense for the way I use the car....

[epbrown]

Originally Posted by pokeybritches

0-60 times don't tell the whole story. It's about distance traveled over time. In an extreme case (to prove my point) let's take two cars with identical 0-60 times of say 10 sec.

That's true, but I was trying to think WWBMWD? I know car makers care about the stat and shopped accordingly. What really sparked the idea was an article in Car magazine where someone from M Division said the 987 wasn't the Z4M's target, the 997 was. "Yeah, right," I grumbled. "If that were true you guys would have... hey!"

My other "Z4M done right" plan was to order an M Roadster with manual top and seats, swap in the 4.10 gears and chip it to raise the red line until I could still hit 60 in 2nd, and start shaving weight to make it a Speedster but still livable - lightweight exhaust, lighter ac compressor and alternator, lighter battery, lightweight pullies, lightest wheels I could find (I think I had Volks in mind), race buckets... The target was 3000lbs, iirc. It would have been TiAg with black interior. Ah well...