So I know there are a million threads on this but I can’t seem to find some basic answers so here it is.

I want to convert my M6 2004 C5 base to a 3.90 or 4.11. The car (and it’s diff) has 123k miles on it. It’s a daily that I like to romp around on frequently.

What parts exactly do I need, what brands for them are recommended, and how much should I expect a shop to charge to install them?

i know that some places sell an entire differential, which is probably the smart route, but this is predominantly a street fun car that I’m not launching the **** out of. If I can do this on something of a budget without destroying anything, that would be ideal.

How significant is the difference in reliability between the 3.90 and 4.11? I’d kind of like to go with the 4.11 but not if I’m going to be blowing stuff up. I’m currently 363whp with just a tune and full exhaust. Planning to eventually do a cam and flex fuel to aim for 415whp, but that’ll probably be the end of it for this build.

Thanks in advance!

 

On a manual trans you do not have to correct the speedo with a rear end change--But on an auto trans you do have to do a "shift point correction" to make it shift properly
There is not much of a performance gain from a 3.90 to a 4.11 So i would buy one where you can get the best deal
ALSO These C5 rear ends are tricky to do gear only swaps Your best bet would be to buy a rear end that has already had the gear swap done--Just inatall it and go !! Changing gear ratios on your existing rear end is cheaper BUT as I said it is a tricky swap and it is over the head of most rear end companies----If you go to a shop that specializes in C5 rear ends that is different-- but rare to find

 

As covered above, unless you have a reputable shop around you that can regear the case, I'd buy a complete case and have it swapped out. If you're in SoCal, I'd recommend Unitrax. They were the most reputable I could find (through research here in the forums as well) and they were actually the cheapest out of a handful of quotes I got from shops that I didn't feel 100% comfortable with. It's cheaper because they're able to regear the case while it's still on the car, which most people would lead you to believe is impossible haha. I did A LOT of research before finally pulling the trigger. I went 4.10 (same as 4.11) and honestly, I still wasn't as "impressed" as I was hoping to be. At Unitrax, I was able to get the case regeared with shot peened and polished gears, new bellevue washers, and an expensive tranny seal for the price other shops were quoting me for just labor. I bought all of my own parts (except washers and tranny seal) because I had it lined up to go to another shop before they raised their price on me at the last minute. I also had them install a C6 Z06 driver output shaft that I provided. With some upgraded parts and the shot peened/polish service, I spent just under $2,300. Without the upgrades and gear treatment, I could have been done with it for under 2k.

You'll see a lot of people recommend beefing up everything in the diff, but not only does that significantly increase your cost, it makes the next weak link something more expensive. For a daily or weekend car, I choose to just upgrade the driver output shaft.

You'll also find most people (mostly that have never had the gears) complain about not having traction anymore and/or first gear being useless. If you still have street tires, that's probably somewhat true. With cold R888s, I have no problem hooking anytime.

Lastly, the whole "you'll get tired of shifting way more" and "your RPMs will be through the roof on the highway" claims....if I'm just cruising, I now shift less than normal. 1-3-5 is very easy. 1-3 feels similar to the standard 1-4 shift now, but still not as boggy as that. It's useable. And the more I drive it, the more normal it feels to row through all of the gears as well. Your RPM in 6th gear will be a mere 300 rpm higher than with 3.42s (2,100 vs 1,800) and 6th gear is now a lot more usable in general.

 

Okay I’m getting some good advice here. Let’s say I do just get a new rear end. What brand do people consider to be the best or at least the most reputable? I don’t need a racing diff but at least something that can handle up to 500whp and be able to launch here and there from maybe 3k rpm

 

I believe people like DTE and RPM.

 

RKT all the way!

I had a 3.90 built by him about 5 years ago, and have had absolutely zero issues at all! Just make sure to follow the break-in instructions to the letter and change the differential fluid yearly as cheap insurance. At 80 mph you will be turning just a tiny bit under 2,000 rpm and 6th gear really now seems to have a purpose!

It's a good mod, but don't expect it to give you a huge kick in the pants like adding a blower or Nitrous would do - rather, it makes your car now feel "right" as in this is how it should have come from the factory.

Best of luck!


Jeff

 

That's good color on how you shift during commute/daily driving. I will try the 1-3-5.

When I'm driving aggressively, I absolutely love my 4:10. When putzing around in stupid traffic and stop lights, the constant shifting gets a little old. Granted, I had my gear swap and then the car basically sat for a couple years. Am back to driving it and now have several hundred miles so far. Gotta get used to it.

For freeway driving, hell yes 5th and 6th gear are WAAAAAAAAY more usable!!! I can punch the throttle in either and presto,the car goes.

Mileage decrease is de minimis. If you're seriously worried about mileage in a Corvette V8, you're driving the wrong car.

OP: I'm in a C5Z by the way regarding power and usability of 4.10s. Can't say I'd want too much more power than I have now in tandem with these gears. Traction IS an issue if you really get on it. Which makes me smile.

 

Would you do a 3.90 in hindsight?

I've thought about gearing but I figured 4.10 might be too much for the MN12.

 

I really like the 4.1 in my 550whp C5Z.
Yes it's easy to 'light 'em up' in the first couple gears on street tires. yes that is a TOTAL HOOT.
I enjoy rowing the car through the gears and using a lot of engine braking (snap crackle pop!) around town.
Don't try to do it on the cheap ... use RPM, RKT etc.

 

That's great to hear, especially after SO much research comes up with a lot of people thinking 4.10 with an MN6 is too much gear lol. 550whp with 4.10 in an MN12 has got to be nuts. I've actually been kicking myself in the butt for getting 4.10s, so there seems to be hope after all. I got regeared as my "final" decision not to get a supercharger and keep it simple...and two weeks after I got my car back from the shop, I came across a deal on an A&A kit that I couldn't pass up...so here I am. Everyone says I'll hate it, but I had a feeling it wouldn't be THAT bad.

If you're 4.10 in a Z, I'd probably recommend going 2-4-6 instead. 1st gear has to be extremely short for you in general haha. The good thing is, there's no rules...try them both and do what works for you! Sometimes I do 1-3-5, sometimes I do 2-4-6, sometimes I do 1-3-4-5, and sometimes I do 1-2-3-4-5-6 lol. Whatever feels right at the time or is necessary