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Default 3.08 or 3.46 ratio for my new diff? - 09-12-2013, 03:14 PM

Who here has a MT and switched from 3.08 to 3.46 final drive when they got their LSD? Those selling complete pumpkins often list switching to 3.46 final drive as a "feature", but I don't think I like the idea for a few different reasons. I think they try to sell it because they have way more AT than MT cores to use on rebuilds. General opinions welcome, but I'd really love to hear from someone who actually went from 3.08 to 3.46 in their MT. Thanks!


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Default 09-12-2013, 03:53 PM

Also curious


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Default 09-12-2013, 04:15 PM

I swapped for an m3 dct 3.15 lsd, so I can't speak to the 3.46. But just running the math with my tire diameter a 3.46 would put 4th gear right at 131, seems like it would really make the most of that last shift...
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Default 09-12-2013, 04:59 PM

I can totally appreciate the math and if were a drag racer I might be coming at it from a very different perspective. Truth is this is my daily driver, I don't really drag it and only autocross occasionally. I want the LSD because when I do get sideways the car feels unpredictable like it just really doesn't want to drift in a controlled manner.

I don't really see a need to go to the 3.46. It might accelerate faster, but the gears already go by pretty fast and traction is an issue on even good street tires. I think I might prefer to keep the original 3.08 as I'm pretty comfortable with my rev matching and just generally knowing what speed corresponds to what rpm in different gears, shifting without clutching and whatnot. So I'd like to know if people who changed the ratio really liked it or it that's just marketing hype from vendors who have plenty of AT cores and not many MT cores.


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Fernflex Fernflex is offline
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Default 09-13-2013, 06:08 AM

Well in my case I wanted to upgrade from the 3.08 to 3.46 because the gears with manual are too long, Automatic cars with 3.46 rev faster.

If could say negative things about doing the swap, you could maybe get lower mpg and you rpm will always be running higher.

Example

With 3.08 in 6th gear you are at 60mph @ 2,000 rpm but with the 3.46 in 6th you will need to go @2,300rpm to be at 60mph, this is why your mpg go down.


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Craig Craig is offline
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Default 09-14-2013, 08:35 AM

My prior car was an LT-1 Trans Am. At one point I (paid) to switch the rear gear from the factory 3.42 to a 4.10 based on talking to others. My other choice was a 3.73 gear. I always regretted changing the gear and going with such a steep change,....so much so that I eventually paid to have the factory 3.42 re-installed. Here's why:

1) Gear whine: Yes I bought a quality gear (the manufacturer of the OE gear) and yes the shop knew what they were doing. However, quiet running is never the same as factory install. I'm not up on BMW gear swaps but I'd assume the pinion gear still has to be shimmed to the ring gear precisely....unless it's an entire assembly.

2) Traction: True, an F-body suspension is nothing like the BMW, but even with 2 tires spinning, first gear traction became a real mess. Like ice.

3) Short gears and more shifting. When running against friends with factory gear, I'd always be in the next higher gear than them because of the short gear. For instance, on the interstate they could hit a 3rd gear downshift and I'd have to go to 4th. My overall tire size etc. was correct.

4) Gas Mileage: I don't remember noticing that much of a change in mileage. On the interstate sure. But around town it could be said that the shorter gear allows the weight of the car to be moved around easier.

I haven't done the research, but the transmission itself in the BMW auto may have different ratios than manuals so keep that in mind when comparing between the two. You need to multiply the math out for each gear. You can also use online calculators to know where you'll be.

All this is just my personal experience on a different car. Your increase would be .38. Mine was .68 so things would be less magnified for you. I should have swapped to the 3.73 instead. But I love the long gears in this car and I wouldn't want them any shorter especially in a manual. It's ridiculous that we don't have an LSD.


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Default 09-14-2013, 09:28 AM

I have a 6MT and changed from the 3.08 to a auto 3.46 ratio when I did my wavetrac. Freaking love it. The single spools much faster and stays in the perfect range even in 3rd and 4th where it would bog sometimes if I didn't shift ultra fast. Even with the stock twins I am sure it would be very noticeable. I only saw a 2mpg decrease in highway mpg's. It takes your 80mph cruising rpm from 2700 to about 3100 in 6th gear. So I guess if you have drony exhaust then this is not a good mod for you, but then again its easy to fix drone lol.

I did the swap nearly 30k miles ago and would not change back. Yes it did make 1st and 2nd a little more useless without slicks but damn 3rd and 4th pull like a freight train.

BTW I have no gear whine with the wavetrac. The gears and not ultra short, the car still hits 60+ in 2nd, 100mph in 3rd and 130ish in 4th. Yes 6th is a little high but its really not that noticeable. It all up to you though, not a whole lot of people do this so if you want an edge on some guys then this is a good way to do it...




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Default 09-14-2013, 01:54 PM

Thanks for the input guys, keep it coming. A couple different but respectable points of view here so far. In my case I'll probably do upgraded stock housing twins eventually, but not larger.


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Default 09-14-2013, 03:39 PM

Good thread here. I've been on the verge of doing this for a while now as I actually went ahead and bought new PowerFlex diff bushings a few months back. I can't decide between 3.46 or 3.08 either. 6MT here.



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sfvalley335i sfvalley335i is offline
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Default 09-14-2013, 05:16 PM

I bought a diff with 3.46 gear for my MT n54, had the plans to swap it in but ended up selling it to Terry.

Reason for selling was MPG, I get 17mpg as is, with the shorter gears I'm sure I would be closer to 15mpg or less.
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Default 09-16-2013, 09:47 AM

Quote:
Originally Posted by jb's335
I have a 6MT and changed from the 3.08 to a auto 3.46 ratio when I did my wavetrac. Freaking love it. The single spools much faster and stays in the perfect range even in 3rd and 4th where it would bog sometimes if I didn't shift ultra fast. Even with the stock twins I am sure it would be very noticeable. I only saw a 2mpg decrease in highway mpg's. It takes your 80mph cruising rpm from 2700 to about 3100 in 6th gear. So I guess if you have drony exhaust then this is not a good mod for you, but then again its easy to fix drone lol.

I did the swap nearly 30k miles ago and would not change back. Yes it did make 1st and 2nd a little more useless without slicks but damn 3rd and 4th pull like a freight train.

BTW I have no gear whine with the wavetrac. The gears and not ultra short, the car still hits 60+ in 2nd, 100mph in 3rd and 130ish in 4th. Yes 6th is a little high but its really not that noticeable. It all up to you though, not a whole lot of people do this so if you want an edge on some guys then this is a good way to do it...
That's great information. Do you know what diff the DCT cars use? Can the same diff, fit in an Auto, MT and DTC?
Thanks
Mario


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Nugs Nugs is offline
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Default 09-16-2013, 05:56 PM

DCT uses a 2.56.
There is some good info here, including a reference chart with all the ratio, casing sizes and if they are bolted or welded.
http://www.vacmotorsports.com/blog/3...no-lsd-really/
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ApolloTheGod ApolloTheGod is offline
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Default 09-20-2013, 04:10 AM

Boosted cars need lower gear ratios. Remember we are boosted which means we want to stay in the boost longer. Not shorter
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Default 09-20-2013, 06:49 AM

Quote:
Originally Posted by ApolloTheGod
Boosted cars need lower gear ratios. Remember we are boosted which means we want to stay in the boost longer. Not shorter
So turbo cars need TALLER gears to hold boost longer.


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Default 09-20-2013, 02:27 PM

I think he's just saying that if you have shorter gears you'll be shifting sooner and more often as you run it to top speed. Each shift typically results in at least a little throttle closure and loss of boost. ATs and MTs using NLS have less of an issue with loosing boost though the shift. I've personally never been able to get the NLS working without bogging though...


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Blaizon Blaizon is offline
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Default 09-20-2013, 03:05 PM

The lower the gear the more load you create which creates greater spool. So on turbo cars you don't want to go higher in the ratio, especially if you have upgraded turbo(s).


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Blaizon Blaizon is offline
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Default 09-20-2013, 03:06 PM

Quote:
Originally Posted by ApolloTheGod
Boosted cars need lower gear ratios. Remember we are boosted which means we want to stay in the boost longer. Not shorter
I concur.


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ApolloTheGod ApolloTheGod is offline
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Default 09-20-2013, 03:14 PM

You guys guys got what I was trying to say. The 308 diff is literally the perfect ratio. It cant get any better. Perfect power for 40 rolls, digs, etc. And the top speed is amazing with that diff. ask me how I know
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Default 09-21-2013, 08:43 AM

Yep. I think I'm sold on sticking with the 3.08, just needed some validation from you guys. Logistically this is going to be a pain and I'll have to cross my fingers that everything goes smoothly with the install. Wish me luck.


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Default 09-21-2013, 10:31 AM

Just a couple random comments:

When racing gearing selection is normally a function of the intended power band, finishing gear, and 1/4 mile MPH. A stock turbo car target is around 125mph in 4th. So for stock turbos you will want to gear the car so its still in the meat of the power band @ 125mph. For single turbos a good target might be 135mph and 7000rpm. IMHO 3.08 is best for stock turbos and 3.42s for larger turbos.

If you're just talking about general street use, the 3.42s in an MT make it "feel" faster but don't necessarily do anything for performance given the broad power band. You may find it improves traction and cuts a shift by allowing 3rd gear starts @ 40-50mph where you previously needed 2nd gear starts.


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rudypoochris rudypoochris is offline
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Default 09-21-2013, 10:39 AM

On this car don't switch to the higher gear. You want the load to spool the turbos and the powerband is so flat that you're not really getting much besides worse economy and additional shifts. That's my 2c. For an NA car (like my Ford 5.0) I run a 3.70 instead f he 2.7 or whatever it was stock because it makes sense there.


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Default 09-23-2013, 02:50 AM

Quote:
Originally Posted by ApolloTheGod
Boosted cars need lower gear ratios. Remember we are boosted which means we want to stay in the boost longer. Not shorter
Lol...all boosted cars?

So what if you fall out of boost in between shifts with the 3.08? A larger turbo than stock twins will shift the powerband up on the rev range. I agree with Terry.




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ApolloTheGod ApolloTheGod is offline
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Default 09-23-2013, 08:41 AM

Quote:
Originally Posted by jb's335
Lol...all boosted cars?

So what if you fall out of boost in between shifts with the 3.08? A larger turbo than stock twins will shift the powerband up on the rev range. I agree with Terry.
Short gears are always best for boosted cars. 3.08 will do 218 mph with a 6 speed. That is fast. And we all know the 308 gears pull hard as hell. I am about to swap in a 3.62 and I hope it wont be a mistake
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NetEagle NetEagle is offline
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Default 09-23-2013, 12:09 PM

Quote:
Originally Posted by ApolloTheGod
Short gears are always best for boosted cars. 3.08 will do 218 mph with a 6 speed.
Hope you have a couple of JATO rockets strapped to the car to get to 218mph...given the drag limitations and power requirements of those speeds...




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ApolloTheGod ApolloTheGod is offline
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Default 09-24-2013, 01:09 PM

Quote:
Originally Posted by NetEagle
Hope you have a couple of JATO rockets strapped to the car to get to 218mph...given the drag limitations and power requirements of those speeds...
If you had no resistance the car would do 218 yes. However, in the real world no. I have calculated that 206 with proper aero mods is possible. If the gearing were in a different car maybe 211 tops. In a stock 335 body I did 196, but not a mile faster.
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