It seems to me that BMW did a disservice to E34 M5 owners when they quietly changed the gear ratio to 3.73 after mid-1991 from the 3.91 ratio on cars produced prior to mid-1991. They supposedly did so to conserve on fuel economy with the implication that torque was shifted away from lower end at lower rpms. This translates into reduced access to torque when lower end acceleration is needed, and somewhat slower 0-60 and 1/4 mile times. Yes, controlling for all else, an E34M5 built before mid-1991 should out-accelerate E34M5's produced after mid-1991.
Furthermore, even the 3.91 ratio is not ideal because the E34 M5 engine is more of a "horsepower engine" than a "torque engine". If one were a track racer, an ideal "engine" configuration is when peak horsepower and torque approach a one to one ratio. In the case of the E34M5, dyno tests show that the peak HP/TQ ratio is 311HP/266ftlb TQ = 1.17. A more balanced ratio -- from the drag racers perspective -- can be achieved by introducing a higher differential ratio, roughly approximated by the formula (311/266) X 3.91 = 4.57 or (311/266)X3.73 = 4.36, depending on which gear ratio one has now. Acknowledging, however, that the M5 was never designed for drag racing, but more for higher end cruising speed and acceleration, the E34 network advises that, on average, a more desirable ratio for all E34M5 cars would be 4.10.
Since it's torque that counts when we want to get up and go, it therefore becomes apparent that the E34M5's 311 HP isn't as impressive as it might seem. That's because it's torque that really matters when we talk about acceleration, including the distribution of torque throughout the rpm curve. To better understand this, we need only compare the E34M5 with the post-96 540i. I do so here because the 540i is a member of the '5' family, it's also a 4 door sedan, and rumors have it that it out-performs the E34M5. Well, no surprise when we discover that, comparatively, the post 96 and then the post 98 540i's are evolving into "torque monsters" in two ways. With "only" 282 HP, the post 96 540i produces 310 ftlbs of Torque and with the same 282HP, the post 98-2002 540i produces even more, with the highest TQ levels concentrated at relatively low 3500 rpms. (This compares with the E34M5's highest torque at 5000 rpms)
If this information and these comparisons interest you, the good news is that switching to a 4.10 gear ratio will help render real improvements in seat of the pants acceleration without having to change HP or TQ (for example via performance chips or cam sprockets). In other words, the 4.10 ratio won't increase HP or TQ, but it will redistribute the TQ curve to lower gears and rpms when start or rolling acceleration is wanted. Doing so may also extend engine life because it the engine won't have to work so hard at high rpms to generate the power (torque) you need. Yes,fuel consumption may suffer a tad but do you really care?
The bad news is that no one is making a 4.10 differential or even rebuilding the casings with the current 3.91 or 3.71 to be 4.10. Korman used to do so, but then discontinued. The Korman sales rep. told me he could sell fifty 4.10 differentials if he had them. BMW Bimmerworld does differential rebuilds for lots of cars, including a rebuild of our 3.91 or 3.73 gears to conform to stock. So, if I really wanted, I could exchange my 3.73 for a 3.91. But that's too far off the desired 4.10 to be appearling.
Even if available, a 4.10 wouldn't be a cheap upgrade. Prices seem to vary from about $1,200US to $1,800 US for the 3.73 or 3.91 differential rebuilds. On the other hand, installation takes only a 1.5-2 hours by an expert.
Question: does anyone know of anyone who builds 4.10 gear ratio for the E34M5. Does anyone have a 4.10 that I could buy?
Hope this info. is useful and welcome any corrections or suggestions.
Interesting post. Is there any way for me to know which gear ratio I have? My car, I think, was manufactured in June 1991 which is about the time they changed the ratio.
thanks, Samson.
given the availability of options as well as ease of installation, no wonder the chip is probably the most popular modifcation.
I'm curious about the cam gear and how much seat of the pants boost it provides; since that and the 4.10 rear end are the two things that lower the torque peak.
ghost: if you lift up the rear wheels off the ground and put the trans in neutral, mark a reference point on the driveshaft and wheel. then as you rotate the tires, count the number of revolutions until you see the reference point on the driveshaft make one complete revolution.
if the tires go 3 and 3/4 revolutions; then you have the 3.72 ratio. likewise if you count almost 4 revolutions, you have the 3.91 gears.
i WAS aware that BMW changed the final drive ratio,
this i discovered when delving into road test results...
there was no other answer as to why early 3.6 cars
were reving so high at 158mph compaired to later cars.
As for actual diff ratios..(ive looked into this in the past)
520i's run a 4.27:1, however with no LSD !
although i'm sure 260-300ft/lbs would kill that diff !!
It would be interesting to find out just why BMW decided
to change the ratio...i suspect it had something to do
with the introduction of the 3.8 engine (which could
obviously pull a taller ratio) and as the two units (36/38)
were assembled side by side i suspect the factory installed
3.7's until supply was exhausted.
Every UK road test i have for 3.6 litre cars shows a 3.9:1 fitted.
A few years ago i owned a 12v 1990 525i, which had factory
option LSD, my instant observation upon hitting the motorway
was that this thing was running was too tall gearing,
it would not pull beyond 5100rpm in 5th.
I often wondered that if you ordered a 520/525/530 with LSD
you'd get lumbered with an uncomfortable final drive ratio...
For the record, these are all the ratios listed from my road test
reprint book......
Hey y'all...I totally agree with the previous comments about the gearing. In fact, I was just talking with my buddy about the lack of road for me to truly see 165-170mph in my M5. I would much rather have a 4.10 or even a 4.27 and have some around-the-town fun. Of course, I currently have the 3.91 so I can't complain too much At any rate, the point of this post is that I thought I saw somewhere that the E32 cars had a 4.10 or 4.27 diff avaialble. Not sure if this was a limited slip-type, but I'm pretty sure it was offered. In addition, according to the www.bmwe34.net website, the US-spec 535 Auto's had a 4.27 diff from the mid 90's on....could make things interesting!
Thanks for your replies and suggestions, which I've responded to via direct email. For others who might be interested, I made the following points;
-- My understanding is that the differential ratio is always stamped on a metal tag, and can be found on the back plate of the differential, joined by a bolt to the housing. As S stamp at the beginning of the number, eg., S3.73, indicates Limited Slip with 3.73:1 Final Drive Ratio.
-- My understanding is that the E34M5 3.6L has a larger differential CASING than other cars in the 5 series, and obviously in the 3 series as well. I'm exploring this now. If true, one would have to find a used M5 casing to rebuild the M5 differential (ie., the internal gears) or provide a machine shop yr. own M5 case, assuming they will rebuild to 4.10.
-- I'm looking hard to find a reputable company that will rebuild the M5 differential to 4.10. I may have found one and if, so, will post information to this effect, including cost, in case anyone else wants one as well. My understanding is that a used differential can be around $300 with rebuild around $500, vs. $1,200-$1,800 for one supplied by, say, Korman (which the do for M5 3.73 and 3.91, but not for 4.10 ).
-- Regarding the cost-effectiveness of a differential rebuild, vs. a chip or chip plus cam, I agree with most of you that they are complementary performance upgrades rather than replacements for one another. The chip, and then the chip + exhaust cam (if one believes manufacturers claims) do indeed produce incremental increases in HP and TQ. Swaping to a lower gear ratio (eg., 4.10 vs. 3.73) doesn't add incrementally to HP or TQ, but does make use of any increased power (eg., from chip) better by reallocating it to lower gears when improving 0-60 or 1/4 mile acceleration is the goal. I'll try to flesh this out in another post in the near future, again by comparing the E34M5 with the 540i.
After posting my comments obout the advantages of having a 4.10 differential in the E34M5 3.6L, I've been looking everywhere to see who might be able to provide one. Several authorities on the subject -- like Korman, Bimmerworld, and even Turner -- told me I was searching in vain and, therefore, I almost gave up. They said, yes, the M5 differential casing is bigger than others, and yes we can rebuild yours or get a used one and rebuild it for you, but only with a 3.73 or 3.91 gear ratio. The reason they couldn't do a 4.10 , they said, s because NO ONE makes a pinion and ring gear set for the E34M5 (that goes inside the casing) in a 4.10 ratio.
Salvation may now have come my way via a reference provided by Turner motorsport. They pointed me to a guru of BMW differentials and transmissions named Jim Blanton. According to Jim, it's true no one makes a 4.10 ring and pinion set specifically for the E34M5. But, he said, slightly smaller 4.10 ring and pinion sets are strong enough to do the job and can replace the overly strong gear sets they use in the E34M5 casing. He also said a slightly smaller casing can be used on the E34M5 with the 4.10 ratio that can be easily bolted onto the car.
My first question then was, can you build me a 4.10 differential in a casing you choose (it will likely be from a 535i) and ship it to me so I can have it put on. He said yes, and he'd provide with it appropriate flange fittings and arm/bracelets that are needed. My second question was, will it be strong enough to handle the M5 engine with, say, chip and cam sprocket mods. Yes. What about with a supercharger that will put more pressure on parts -- like pistons, differentials, etc. He said it'll be OK, if the car is generally driven on the street, but not likely on the track if you are going to be hammering acceleration & supercharger all the time. My third question, of course, how much. $2,650 if he provides the casing and all, $2,150 if I provide an exchange casing (ie., my M5 casing). How long; 3-4 weeks. And finally, I asked, would you be interested in making several if others were interested. Yes. Well, maybe the price seems prohibitive to some but I'm glad to to make contact with a real expert who had more to offer me than "no way"!
Good work Samson, you desserve a special mention from E34 M5 gurus
My question is : how much time could you expect to gain in acceleration (0-60 and 1/4 mile) when you have a 4.10 on board ? Is it a only a subjective improvement or is it really measurable by a chrono ?
__________________
Y@nn
Last edited by ydelta; 9th September 2002 at 22:58.
Well, I wish I could relate personal experience that it's a real increase in acceleration times from 0-60 but, alas, I can't because I haven't experienced a 4.10 differential in my or any other E34M5. What I can say is, first, that all the performance mod gurus I've read say that if it's increased 0-60 acceleration speed you're looking for, consider going to numerically higher differential. And second, I can summarize the ranting and raving that originates from the gurus at Excluservice in Maryland that services (not sells) the entire range of M cars. And they tell me that if you're single aim is to increase 0-60 times, look to the differential, not to chips and exhaust cam sprockets.
Now, the fellow who can build a 4.10 that will work in our E34M5's is Jim Blanton of Blanton Transmissions. His email is: jblanton@sky.net For obvious reasons, he'd be interesting in putting together several 4.10 differential rebuilds if there was demand.
I have a 4.10 gearset in my M5! I have a '93 M5 that when new had it's rear end sent to Korman and a 4.10 BMW gearset was put in. Korman told the original owner that he was lucky because Korman had only gotten 2 4.10 gearsets in before the door from Germany was slammed shut on exporting them.
A 4.10 is a great ratio, I tach about 3,000 RPMs at 70 mph. I have great acceleration in all gears up to redline in 5th. Only problem is that you are shifting into second as soon as you leave the line. It is a great differential for autocrossing, and I'll have the car on the track this Fall. Mine has the original differential housing, and the 4.10 was verified by me by turning the wheels and counting the driveshaft turns for one wheel revolution.
Contrary to what someone wrote in response to your original article, you don't get 4.10 turns of the wheel for every driveshaft revolution. That would be one hell of an overdrive!
At any rate, the point of this post is that I thought I saw somewhere that the E32 cars had a 4.10 or 4.27 diff avaialble. Not sure if this was a limited slip-type, but I'm pretty sure it was offered. In addition, according to the 
Linear Mode
