Differential gear ratio changes

[kurdi]

Very informative write up. Thank you!
I currently own a 2004 BMW Z4 3.0 Manual transmission with a bad differential. I had a mechanic drain a little bit of the gear fluid and there was metal bits coming out of it meaning my differential carrier bearings is bad and my car is currently making very loud rear tire noise it drives me crazy. I need to swap it to a good differential not necessarily a higher gear ratio, but just a regular 3.07 gear ratio should be fine and if I am not mistaken I think that is the standard ration on my car right now.

Now I have a very important question before I go out and buy the wrong differential. I have been told that a 2.5 BMW Z4 manual or automatic transmission will fit my 3.0 BMW Z4 manual transmission and will bolt in just fine. Is that right?? I can see maybe a differential from a manual 2.5 Z4, but an automatic transmission too?

One more question, If I choose a higher gear ratio such as the 3.46, I have been told that my speedometer will be inaccurate. example, the speedometer says I'm going 40mph, but my actual speed is 35mph. Is this true?

I would appreciate some answers and thank you again for this great post.

With regards,
Danny

 

[kurdi]

Very informative write up. Thank you!
I currently own a 2004 BMW Z4 3.0 Manual transmission with a bad differential. I had a mechanic drain a little bit of the gear fluid and there was metal bits coming out of it meaning my differential carrier bearings is bad and my car is currently making very loud rear tire noise it drives me crazy. I need to swap it to a good differential not necessarily a higher gear ratio, but just a regular 3.07 gear ratio should be fine and if I am not mistaken I think that is the standard ration on my car right now.

Now I have a very important question before I go out and buy the wrong differential. I have been told that a 2.5 BMW Z4 manual or automatic transmission will fit my 3.0 BMW Z4 manual transmission and will bolt in just fine. Is that right?? I can see maybe a differential from a manual 2.5 Z4, but an automatic transmission too?

One more question, If I choose a higher gear ratio such as the 3.46, I have been told that my speedometer will be inaccurate. example, the speedometer says I'm going 40mph, but my actual speed is 35mph. Is this true?

I would appreciate some answers and thank you again for this great post.

With regards,
Danny

Changing the differential gear ratio can be a great performance modification. What follows is a summary for anyone who may be interested.

It’s normally done to increase acceleration. A differential swap takes advantage of the gear ratio in the rear differential to change the wheel torque in a given transmission gear. A higher ratio differential increases the wheel torque. A lower ratio differential reduces the wheel torque.

For example: replacing a 3.07 differential ratio with a 3.46 differential ratio increases the wheel torque by 3.46/3.07 or about 12.7%. That's about a 25 ft-lb increase in wheel troque for a typical 3.0 liter BMW engine...without any engine modifications. Believe me you'll definitely feel that much torque! Non-M BMWs come with a standard differential gear ratio that permits reasonable operation at very high speeds. Those of us who never drive at very high speeds (e.g., on the Autobahn) don’t care much about that end of the performance envelope. What we do care about is the performance available to accelerate through a curve, when merging into faster traffic, when climbing a hill, and so forth.

The differential ratio change is an ideal modification for improving performance in these daily driving situations (or at the track) because it increases wheel torque by the same percentage throughout the entire engine speed range. Mechanically, it provides the same performance improvement as increasing engine displacement by the same percentage, e.g. with the 12.7% torque increase from the example above, a 3.0 liter engine with a 3.46 differential accelerates like a 3.38 liter engine with a 3.07 differential!

Another benefit of larger ratio changes (I drive with a 3.64 ratio that added 18.5% wheel torque) is that they make it quite easy to start off in second gear without slipping the clutch. I can start in 2nd and still comfortably out-accelerate most cars on the road from a stop. I now use 1st gear only for those much rarer situations when I need to get going as fast as possible such as in AutoX events or merging into a tight spot in cross traffic, or when starting off on a steep uphill grade. So, even though the speed ranges of the transmission gears are now shorter, I find that I do less shifting, which certainly helps in commuting traffic.

And, I still get approximately the same fuel economy as before even though my rpms run 500 higher at 70mph and the greater sportiness of the car encourages more spirited driving. When cruising at lower speeds I’m still selecting a gear such that the engine runs between 1500 and 2000 rpm.

In the USA, where 90% of the BMWs (even Z4s) have automatic transmissions, finding a suitable salvaged higher ratio differential like a 3.46, 3.64 or 3.73 is quite easy. Prices generally range from $300 - $500 plus $200 labor for the swap, so swapping a whole salvaged differential is the most common approach. It may be harder to find a higher ratio differential in other countries, such as the U.K., where automatic transmission models are rarer. So, in that case, it may be necessary to order one from the factory or to have a properly trained technician change the gears themselves.

 

[pokeybritches]

Your speedometer will still be accurate. I have a 3.64 final drive in my 3.0i manual. Any E85 Z4 diff will work. Some other models will work too, with varying degrees of modification - E46 3 series, X3, etc.

If you don't want to change the ratio, I have my stock 3.07 final drive that I could let go for $200 plus shipping (probably another $60-100).

 

[Jimminy]

Is there many people doing this in the UK, don't most people already have diffs with highter ratios? (Im only familiar with the coupes and I think most of them have a 3.46 already)

The jump to 3.64 only looks worth doing when sticking in an lsd. The 3.91 from an x3 looks more interesting for the 3.0si cars but does anyone know how it holds up?

Ive just had another look and 3.72 and 4.44 diffs look to be in plentiful supply, 4.44 is probably a little extreme for a road car, can anybody think of any other diffs available inbetween these?

 

[GuidoK]

You have any idea what revs you'll make on the motorway with a 3.91 or higher and what that'll do with your gas mileage?
Probably not the best ratio if you plan to drive in germany :lol:

Also when fitting a lsd and different drive gears, you have to take into account that a 3.64 or 3.91 etc, will not fit a 3.07 diff. So the supplier (quaife or os giken or whatever) must have a diff available specific for that drive gear ratio. So check the specific ratio with your picked supplier.

Also the initial writeup is not 100% correct, or better said, it leaves a part out, and that is if you measure an acceleration from say 0-60 (or 0-100 etc), with a higher diff ratio, you'll have more wheel torque in the same gear, but that gear will end sooner and you have to switch to a higher gear with lower ratio.
So you can reach that 60mph still in 2nd gear with the 3.07 ratio but not with the 3.64 ratio, and you'll have to shift up to 3rd (at maybe 55mph or so), and that last 5mph in 3rd will get very slow compared to that 2nd gear.
Ideally you dont want to play too much with the diff ratio, but with the gearbox ratio's to suit specific needs. But that's way more complicated and much more expensive. I mean when tracking your car shorter ratio's may be a good thing because you are able to use 6th gear on the track, but you don't want to shorten 1st because you may not be able to use its full potential because it gets too short (wheelspin).

 

[cj10jeeper]

+1 on above.

In answer to your question if many are doing this, then I would comment that this is a 2009 thread and since posting I've not heard of a single person doing it, but I have heard of countless people increasing the power of their engine through the various ways.

 

[pokeybritches]

+2

The 3.64 is about as aggressive as I'd ever want to go. If I add more power and get a LSD, I'll move to a 3.46. I'm very happy with the 3.64 though.

The 3.64 pairs well with a centrifugal supercharger, because it allows you to stay in boost in almost any gear. 18.6% more torque is great, and highway rpm is still tolerable (3k or below for the most part). First gear is just right. I can launch the car without excessive wheelspin.

A 3.64 is probably too much for a twin screw. I may make the switch soon, so I'll report back if something changes.

I posted a full review over on zpost.

 

[Jimminy]

with a 3.91 diff at 100 mph I would be doing 4453 rpm instead of 3941 which does not sound to bad to me and as i don't enjoy driving on motorways and I don't travel that quickly on them.
Would top speed actually drop? A Z4C 3.0 si is geared to do 177 mph at 7k and with a 3.91 it would be geared for 157 mph but as its wind resistance that stops you getting to 177 mph would the mechanical advantage of a 3.91 mean it would be able to get closer to the theoretical top speed?
But fuel economy would drop.
I realised it was an old thread but as someone rekindled it and the idea apears to me I thought I would post.
I agree a 4.4 would be to much as you would be doing just over 5000 rpm at 100 mph with a top speed of 138 but you would get there quick

 

[Yorkie Z]

I fitted an lsd into my car last year. I decide to build up the new unit in a 3.23 ratio diff. So I went from a 3.07 to a 3.23 and I can confirm the LSD is fantastic. As for the change in ratio, you would Never know.!!!

I realise I've only made a small jump up but if you went to a really high ratio you wouldn't feel much benefit, and just cock up the revs.

 

[cj10jeeper]

Does make me smile as to what folks deem as acceptable final ratios. I run. Jag XF and it's at 2.73 (the later model runs as 0.67 8th ratio)
99% of driving is <2500 revs. OK it's a twin turbo diesel that lives by torque, but über relaxed at legal speeds.

Still at the other end of the spectrum I run a small block chevy 5.7 driving through a 4.56 final drive. Stick that through the 4:1 transfer box and I redline at about 25 mph in top. I regularly get out and leave it driving in first while I check obstacles on foot -yes really.
Even on dry Tarmac in 2wd I can wheel spin most speeds most gears, such is the effect of a way low gear axle

To me it's all about what compromises you'll accept when you change diff ratios.

 

[sars]

Would top speed actually drop? A Z4C 3.0 si is geared to do 177 mph at 7k and with a 3.91 it would be geared for 157 mph but as its wind resistance that stops you getting to 177 mph would the mechanical advantage of a 3.91 mean it would be able to get closer to the theoretical top speed?

Top speed is governed by power and this does not change with gear ratios

Oh and great thread revival

 

[cj10jeeper]

Sars,
You're the engineer so help me then as to why most cars a achieve top speed in one gear lower than top?