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Discussion Starter · #1 ·
Hello to everyone,
There has been some great discussion on the forum throughout the past about the effects of changing the car's final drive ratio. I was finally able to get a nice little comparison run with my good friend's M5, which has equivalent modifications. I'm sorry about the horrible video quality, I will strive to get some decent and more expansive footage soon.

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Here is 'ole blue...
 

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I'm a huge fan of my 3.45's and much prefer them to my 3.15's, but something is wrong on your buddies end. Equal mods does not mean = power. I can see you walking him but that was worse than SC'd vs stock.

BTW, your beast sounds nice! :M5thumbs:
 

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I'm a huge fan of my 3.45's and much prefer them to my 3.15's, but something is wrong on your buddies end. Equal mods does not mean = power. I can see you walking him but that was worse than SC'd vs stock.

BTW, your beast sounds nice! :M5thumbs:
Exactly. There will be little difference over a long distance when both cars are started at a non-optimum rpm (i.e. not sitting on the torque peak), but most importantly, instrument the cars to test them. Why? Since even a 0.2 second difference in hitting wide open throttle makes a continually growing gap between two otherwise identical cars. At 120mph, that 0.2 second difference at the start (assuming both cars are identical, both are shifted exactly and perfectly, etc) equals more than 2 car lengths, and video of these two identical cars will show the "faster one" slowing pulling away from the "slower one". When they hit their speed limiters at 155mph, they will be more than 3 car lengths apart and the "slower one" will never catch the "faster one" since speed is then limited. "Everyone" will claim the "faster one" is the faster car.

As has been shown over and again, just changing the rear gear will not massively increase acceleration. When the shorter geared car is "pulling on" the stock gear one at the top of 2nd gear, it then falls behind the acceleration curve of the stock gear car once it shifts to 3rd, etc. This back and forth goes on all the way up the speed curve. Which one is "faster" is massively dependent on when you measure. If you pick the right speeds, you can prove the stock one is faster; if you pick the second group of right speeds, you can then prove the shorter geared car is faster.
 

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I don't think this is a proper mod. for 400 hp V8 this will take no effect on how often you should change gears, but still... with this mod you get too short 1st gear...
IMHO of course
 

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Gearing is a function of a lot of variables. 3.15 is no doubt a compromise, taking into account a host of those variables.

If drag racing was important, a stock car with deeper gears (3.45 or even 3.62) might shave a .1 or .2 from the quarter mile. But that is it. If you start adding power, 1st gear becomes less and less usable (as noted above), so you have to short shift to 2nd. To me, that defeats the purpose of the shorter gears.

Then consider the cost. Are you better off spending about $2700 for a 3.45 (Dinan website, I know you can do it for less) where you theoretically increase the torque multiplication to the rear wheels by about 10%, or improve both the engine hp and torque across the usable rev range, with say, headers, for similar money?

But everyone gets to make a choice that is best for them. If you check my sig, you will see I don't believe in deeper gears on this particular car, (and that was BEFORE I went ESS) but if you do, and it makes you happy, by all means, go for it! The car will feel faster in each gear, and sometimes how a car feels when you drive it is enough to justify the mod.

Until you can do a true side by side (and the closest would be the same car, with a diff swap, and run on the same day, or one with very similar conditions) to take most of the variables out, runs of similar cars are nice, but not real proof of much. Instrumented testing is a must to reach any meaningful conclusions, at least IMHO.

Regards,
Jerry
 

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The cost of swapping out the gear set is one of the factors that turns me off of this mod. The car would feel faster in 3rd and 4th gear pulls (although I'd probably have to start highway pulls in 4th gear that I used to start in 3rd gear, thus losing a lot of the "extra torque" from this mod) but as was stated before, for the same cost I could add headers and software and improve the power by the same 10% to the wheels but with no negative effect on highway cruising.

That brings me to the second reason why I don't like this mod....I like the fact that I can cruise at 70 mph on the highway and achieve mpg in the mid to high 20's with the stock gears. If I increased the revs by 10% at that speed, I'd probably drop to low 20's on the highway and that detracts from this car's inherent long-distance cruising ability. I take a fair amount of 200+ mile trips in the M5 so a 10% loss in fuel economy and the increased noise from higher revs would probably annoy me.

Still, if you want shorter gears, you certainly, um, got them?
 

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I would say that on a track, for drifting, autocross, whatever then fine and justified, but for the road I can see of no benefit. BUT! It is something different which I do like, and as has been said, with a lot of mods it's how it makes the car FEEL faster, not GO faster, that warrants the mod for the person who's car it is. Why do people get Timmay's Tips put on? It isn't for the power increase!

Also very nice sounding car, what exhaust have you got?
 

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Discussion Starter · #10 ·
Hello again,

I want assure everyone that I completely agree this is a very non-exact/scientific comparison. I recognize there are far too many variables in a match up like this. I figured I would share it with you all anyway. Both cars do have fresh cps sensors, O2 sensors, MAF sensors, new air and fuel filters, new spark plugs, and proper fuel pressure. I maintain both of the vehicles so I am pretty certain that they are healthy examples of the car.

There are many differences in opinion on the benefits of this modification and whether the benefits out weigh the costs. There have been very in-depth debates on the topic in other threads. Definitely worth-while reads available on this forum.

The exhaust is dual magnaflow mufflers with custom cat-back piping.
 

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I can tell you from experience, there is something else going on with those two cars. I have raced, very similarly, 20mph to 120mph and we stayed side by side. His car has lightened flywheel with PC software while mine was stock with a 3.45. The gearing alone would not give that much acceleration difference. It will all depend on where the cars are in the rev range. Each car has to shift. The 3.45 will have to more often thus negating any stronger pull produced by the lower gear.
 

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http://www.m5board.com/vbulletin/e39-m5-e52-z8-discussion/172922-video-m5-3-62-gear-set-v-s-stock-3-15-a-post2016734.html#post2016734

I own a 2003 M5 so I was really looking forward to the video of the 3.45 gear upgrade, to bad it's no longer available... Even so I couldn't help but notice all the expert who weighed in with their opinions. Wow! No doubt these guys don't really have any real experience in the automotive field. Who said a taller gear ratio only feels faster??? That's really shows lack of real life experience, these guys don't have a clue! Hell yeah a taller gear will get you there faster. Don't know what these other people are thinking but simple math tells us more torque = more work in a shorter period of time. Look it up, that's the definition of HP, that is, hp at the rear wheels...

The 2003 M5 with limit removed tops at 180 mph, I just don't see anyone hitting that speed for any real reason and simply unrealistic. So the axle mod makes good sense, makes use of the hp at say 150mph, sounds a whole lot more real to me. So yes you will certainly not increase your all out top end speed or your horse power at the engine, as a mater of fact you will lose some top end. You will however have more Horse power at the rear wheels... After doing the math if you went from a 3.02 to a 3.45, top speed at 7000rpm would be approx. 157 mph, so you would lose 23 mph at top end. So what, who is gonna hit 180 on the street with a stock vehicle??? Top speed of a 157 mph is probably as fast as you would want to go with a street car....

Good call on the axle mod, I know the post is several years old but what the heck....
 

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I do about 2,200rpm at 60mph is this the speed the 3:45 guys are getting.

What I love about a diff change is the "shove" response you get right as you punch it straight after making a gear change.
This is the smile factor that I think really only matters, plus the chance to do more heal toe shifts.

Here's a feel of what I recored a few years ago with basically no exhaust as I was getting a 2' section replaced so warning you might have to visit your doctor after 4 hours.

https://www.youtube.com/watch?v=v5cJA231c-8

Apples
 

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I own a 2003 M5 so I was really looking forward to the video of the 3.45 gear upgrade, to bad it's no longer available... Even so I couldn't help but notice all the expert who weighed in with their opinions. Wow! No doubt these guys don't really have any real experience in the automotive field. Who said a taller gear ratio only feels faster??? That's really shows lack of real life experience, these guys don't have a clue! Hell yeah a taller gear will get you there faster. Don't know what these other people are thinking but simple math tells us more torque = more work in a shorter period of time. Look it up, that's the definition of HP, that is, hp at the rear wheels...

The 2003 M5 with limit removed tops at 180 mph, I just don't see anyone hitting that speed for any real reason and simply unrealistic. So the axle mod makes good sense, makes use of the hp at say 150mph, sounds a whole lot more real to me. So yes you will certainly not increase your all out top end speed or your horse power at the engine, as a mater of fact you will lose some top end. You will however have more Horse power at the rear wheels... After doing the math if you went from a 3.02 to a 3.45, top speed at 7000rpm would be approx. 157 mph, so you would lose 23 mph at top end. So what, who is gonna hit 180 on the street with a stock vehicle??? Top speed of a 157 mph is probably as fast as you would want to go with a street car....

Good call on the axle mod, I know the post is several years old but what the heck....
This is very helpful. Why are people choosing to only upgrade to a 3.45 when 3.62, 3.91 and 4.11 are also available?
 

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I have a 3.62 on mine. at 100mph indicated I'm doing 4K. I should top out at the high 160s I believe. top of 5th goes up to 140 and top of 4th at about 115
EDIT: Entered the data into a calculator (not 100% sure about the diameter I calculated somewhere else):
m5 top speed by josegt5, on Flickr
Seems a little high, but I'm pretty sure I could make it to 170
 

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When I ran 4.10's on my Mustang I needed to recalibrate the speedo. Does the same issue come up here? How did you resolve it. If you don't fix it your speedo will be significantly off.
 

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IMO ultimately this depends on how you use your car. If you do a lot of highway driving, your rpm's will be up and your efficiency will go down. Let's be clear, aggressive gearing does not add any power. It just gets you into the powerband faster. If you get to the sweet spot faster this will obviously lead to increased acceleration gains.

Its a huge difference my Mustang with 4.10's was wicked fast. This will also make 1st gear and 2nd gear insignificant as you will go through them very quickly.

I don't know how many people here drag race enough to justify the mod but its definitely a good mod if that's your purpose.
 

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When I ran 4.10's on my Mustang I needed to recalibrate the speedo. Does the same issue come up here? How did you resolve it. If you don't fix it your speedo will be significantly off.
This is not an issue because the car uses the wheel speed sensors to tell speed. However, if you change wheel/tire size it should change some.
 

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Good to know!
 

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It doesn't affect the speedometer but it can cause issues with the cruise control.
 
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