# Understanding the effects of changing the rear diff ratio (long, with charts)



## KP (Apr 16, 2002)

Moderato said:


> I also think you guys overlooked my previous post. Raising the redline when you go with a shorter rear diff will make up the associated loss of road speed but this obviously brings in a whole new set of concerns.


Could you elaborate on this matter?


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## Nick325xiT 5spd (Dec 24, 2001)

A lot of it relates to shift points as well.

3rd gear is too tall on many (most?) tracks, while 2nd is too short. Changing the rear end ratio lets the 330 come out of turns a lot better, at, say, Summit Point.

So perhaps the 330 would not benefit much from the rear end change in a simple straight line test. But a 330 with a 3.38 would be faster around a lot of tracks.

So when are you going to factor in time and wind resistance?


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## Nick325xiT 5spd (Dec 24, 2001)

KP said:


> Could you elaborate on this matter?


 An M3 goes to about the same speed in 3rd gear as a 330i, even though the M3's redline is 8000. The difference is that the M3 has a 3.64 rear end, while the 330 has a 2.93.


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## Moderato (Nov 24, 2003)

KP said:


> Could you elaborate on this matter?





Moderato said:


> I would like to see what would happen if the stock 330 with the 2.93 was compared to a sharked 330 with a 2.93 and a sharked 330 with the 3.38. So the stock 330's would shift at 6500 rpms and the sharked 330s would shift at 6800 or is it 7000 rpms.
> 
> edit - So do these charts show that the 3.38 is too short for the 330? That's probably why BMW put the 3.07 in the ZHP and raised the redline 300 rpms and what about the 3.15 with a raised redline? That's what Dinan suggests.


We should examine:

1) Stock 330i 2.93
2) Sharked 330i 2.93
3) Stock 330i 3.07
4) Sharked 330i 3.07
5) Stock 330i 3.15
6) Sharked 330i 3.15
7) Stock 330i 3.38
8) Sharked 330i 3.38

All stock 330's will shift at 6500rpms and sharked will shift at 6800 or 7000 (whatever the sharked redline is). By raising the redline in combination with the shorter rear diff you are raising the road speed available in each gear so the loss of road speed with the shorter rear diff gets offset by the raised redline. An M3 has a 3.64, but it also has a 8K redline. There is a reason that BMW raised the redline on the ZHP when they put the 3.07 and that is why.


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## KP (Apr 16, 2002)

Moderato said:


> We should examine:
> 
> 1) Stock 330i 2.93
> 2) Sharked 330i 2.93
> ...


I have been debating going to a 3.38 or a 3.46 (up from 3.15). Most people who have done this swap and who have the shark software (7k redline), have said that the software compensates for the diff, while highway cruising speeds are higher and fuel comsumption down by 1.5-2mpg.


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## Moderato (Nov 24, 2003)

KP said:


> I have been debating going to a 3.38 or a 3.46 (up from 3.15). Most people who have done this swap and who have the shark software (7k redline), have said that the software compensates for the diff, while highway cruising speeds are higher and fuel comsumption down by 1.5-2mpg.


You have a 325 so I'm not too sure about your tranny ratio's but as long as you're raising the redline it will compensate for the diff, but some people might not feel comfortable with changing the stock programing and increasing the redline.


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## adc (Apr 1, 2003)

Nick325xiT 5spd said:


> 3rd gear is too tall on many (most?) tracks, while 2nd is too short. Changing the rear end ratio lets the 330 come out of turns a lot better, at, say, Summit Point.
> 
> So perhaps the 330 would not benefit much from the rear end change in a simple straight line test. But a 330 with a 3.38 would be faster around a lot of tracks.


YES! YES! YES! 

I have felt that at Summit Point on both circuits. While the top end of the 330 ZHP (and maybe better driving :dunno: ) would make me catch up to E36 M3s, I had to work a lot harder and battle wave after wave of impatience and dissappointment as the M3s would pull ahead of me out of the turns...

I used to have a 98 M3 before so I know a lower gearing would help me go faster around those tracks. In many situations on track I found that the ZHP would not be suited for 2nd gear, as it would be in the very upper RPM ranges and it would be too easy to spin the rear wheels or go into understeer. Using the 3rd gear in those situations would make me go faster despite the lower power/torque, mainly because there was no excess power going to the rear wheels to make them spin. A slightly lower 3rd gear would have been absolutely perfectly matched to the chassis, driver and track combination.

I also noticed that I had to use 2nd gear in more situations but that only made me shift (up and down) more, which leads to lost time on the track.

So for me, I think I'll do a 3:15 or 3:23 rear end even without an LSD if I can find a reasonably priced one. I believe 3:23 is a standard BMW ratio, used on auto trannies - so it should be available at a junkyard with a little luck.

I am also eagerly awaiting the Dinan ZHP-specific software, if nothing else for the raised redline.

In the meantime I would like to thank the original poster for the excellent and solid info on gearing, as well as to all the other contributors to this post.

adc
03 330 ZHP


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## dynosor (Jul 15, 2003)

*Calling Rumratt*

I presume Rumratt has not responded because he is too busy fitting his new 3.38 differential....


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## woohoo (May 29, 2003)

Nice write-up. :clap:


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## Moderato (Nov 24, 2003)

dynosor said:


> I presume Rumratt has not responded because he is too busy fitting his new 3.38 differential....


Hey Rumratt, I think that's a good idea. Why don't you just swap the diffs in your car and we can stop all of this speculation and you can give us a real life review!


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## Elwood (Dec 20, 2002)

dynosor said:


> I presume Rumratt has not responded because he is too busy fitting his new 3.38 differential....


 fitting? bah he's probably on a dyno already!


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## wheel-man (Sep 28, 2004)

Rumratt great write-up and evaluation, as well as great additional info from all the contributors.

As an aside, where can one get a 3.38 diff? anyone make an aftermarket with LSD for the 330i?


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## bluetree211 (Apr 19, 2004)

wheel-man said:


> As an aside, where can one get a 3.38 diff? anyone make an aftermarket with LSD for the 330i?


I'll be installing a 3.46 LSD this weekend, it was hand built by an e46fanatics member for a 323. I think that the 3.46 carrier is the only one big enough to fit the kaaz internals. Other lsd types might be able to use other ratios, or if the carrier is custom machined.
Im not a fan of the RPM increase (I only have a 5 speed) but limited slip far outweighs any negatives, both in the icy conditions of the northeast, and autocross/track events coming up next year.


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## Andy (Jul 16, 2003)

wheel-man said:


> ...anyone make an aftermarket with LSD for the 330i?


http://www.performancegearing.com Ask for Jim Blanton and tell him Andy sent you. :thumbup:

You can pretty much get any ratio you want.


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## cwsqbm (Aug 4, 2004)

Moderato said:


> There is a reason that BMW raised the redline on the ZHP when they put the 3.07 and that is why.


The main reason for raising the redline of an engine past its power point is that when you shift into the next gear, you are closer to your ideal shift point. This point is where the horsepower in the next gear equals the horsepower in the current gear. For a street car, they are not worried about what speeds certain corners on a given track can be taken at, since this would vary with almost every track.



cwsqbm said:


> For example, using the above ZHPdyno chart,
> from third to fourth gear: at 95mph
> Third gear, 6800 rpm, 155 lbs-ft: power = 200 hp
> Fourth gear, 5000 rpm, 185 lbs-ft: power= 176 hp
> So, the ideal shift point would be even higher than 6800, which is why aftermarket chips raise redlines.


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## Moderato (Nov 24, 2003)

cwsqbm said:


> The main reason for raising the redline of an engine past its power point is that when you shift into the next gear, you are closer to your ideal shift point. This point is where the horsepower in the next gear equals the horsepower in the current gear. For a street car, they are not worried about what speeds certain corners on a given track can be taken at, since this would vary with almost every track.


Yes but going with a shorter diff from the stock 330 also decreases the top speed of every gear, by raising the redline from the stock 330 you gain back that top speed in every gear.


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## cwsqbm (Aug 4, 2004)

Moderato said:


> Yes but going with a shorter diff from the stock 330 also decreases the top speed of every gear, by raising the redline from the stock 330 you gain back that top speed in every gear.


Why do I care how fast my car goes in 3rd? I just shift to 4th and keep accelerating.

On a given race track, the additional rpms may mean a particular corner can be taken without upshifting, but you are just as likely to find yet another corner that you need still a few more rpms to take. That's why the pros, when possible, change the ratios in the gear box for each track.


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## cenotaph (Dec 19, 2001)

cwsqbm said:


> Why do I care how fast my car goes in 3rd? I just shift to 4th and keep accelerating.


Maybe you do. Maybe you don't, but BMW cares that they didn't add a second shift (to 3rd) for the 0-60 test and therefore reduced that time by 0.5 seconds instead of keeping it the same as the normal 330.


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## Moderato (Nov 24, 2003)

cwsqbm said:


> Why do I care how fast my car goes in 3rd? I just shift to 4th and keep accelerating.


Because in this specific situation, all else being equal if I can hold onto 3rd gear a little longer then you up to a certain point, then my car will be faster.


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## wag-zhp (Apr 8, 2004)

Very nice post and excellent information. :thumbup: :thumbup: :thumbup: 

Now here's the short story. (Note that this is comming from a reformed drag racing addict.) A numerically higher final drive ratio, say for example 4.56-1, equals very quick acceleration, especially when compared to a stock 2.93-1 ratio (or 3.07 for ZHP optioned cars). This quicker acceleration is seen in each transmission gear, is most obvious in the lower gears, and provides the largest improvement in standing starts (this assumes that your tires offer adequate traction to handle the increased torque multiplication).

Note that these benefits, like any other performance enhancements, come with tradeoffs. In this case the tradeoffs are increase engine RPM for any given road speed in the same gear, reduced fuel economy, and potentially lower top speed (provided that the vehicle didn't have a drag limited top speed to begin with).

Now the question that really needs to be answered is "what is the best ratio to use?". That answer depends entirely on what's most important to you and where you drive your car. If you track your car on a race track that doesn't have a straitaway long enough to allow a top speed any higher than 120mph a 4.56-1 ratio would be nearly perfect. However, if you don't use a trailer and drive to and from that same track, note that at 65mph your engine is going to be spinning at about 3400 RPM. (Like any of us really drive 65mph on the highway... At 85mph you'll be spinning the crank at about 4400 RPM.) Yikes, not a very streetable final drive ratio! 

For me, a 3.5-1 final drive ratio would be just about perfect (especially if it came with limited slip). I could get to just under 60mph (57mph @ 6800 rpm) in 2nd gear, could do 144mph in 5th, and still have another gear to play with. 

Let's see, nice long straight, don't see any other cars around, 144mph in 5th, shift to 6th and hit the 155mph electronically limited top speed, ahh ecstasy... (This would be for those days when a couple of hours in the local jailhouse would be a nice break from the daily grind.) :angel:


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## johnnygraphic (Jun 26, 2004)

Wait a minute! Nobody told there would be math on this forum!!!


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## Rich_Jenkins (Jul 12, 2003)

I just wanted to say this thread has the absolutely highest car-related signal to noise ratio I have seen for a VERY long time on this board...let me add my congrats to Rumratt and all the other contributors. :thumbup:


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