E36 Diffs

[moranor@axis]

thanks FordKoppie that has all the info i need...

simulation says the 540 with the 3.15 has the tinyest of advantage until gearchange then the 2.93 will pull ahead a little and the 3.15 will catch up until the next gearchange... this continues to well over 200kph so essentially no difference in acceleration when driven well...

now I have been giving this some thought because my real world e46 results seem to be somewhat at odds with what the simulation says and have come to the conclusion that is comes down to tractability... yes when driven well the 3.38 diff does not give as big of an advantage, but it makes it way easier to drive the car well because the gears are closer together...

this also reflects the feeling you get from a ratio change... before the 3.38 it felt like my car was always in slightly the wrong gear...

 

[Fordkoppie]

thanks FordKoppie that has all the info i need...

simulation says the 540 with the 3.15 has the tinyest of advantage until gearchange then the 2.93 will pull ahead a little and the 3.15 will catch up until the next gearchange... this continues to well over 200kph so essentially no difference in acceleration when driven well...

now I have been giving this some thought because my real world e46 results seem to be somewhat at odds with what the simulation says and have come to the conclusion that is comes down to tractability... yes when driven well the 3.38 diff does not give as big of an advantage, but it makes it way easier to drive the car well because the gears are closer together...

this also reflects the feeling you get from a ratio change... before the 3.38 it felt like my car was always in slightly the wrong gear...

This was exactly my point, and i am glad the simulator backed it up.
(And I am honestly not trying to say "I told you" :rollsmile: )

But the keyword you used was "when driven well". If you wring every last bit out of the car with the long diff ratio, there really is not that much difference, but it is way easier as you also said to drive the car well with the shorter diff.
Like for instance - if identical cars with the only difference being the diff ratio are rolling in 3rd gear @ 40km/h (+/- 1500 rpm) and simultaneously accelerate flat out without changing gears, the car with the short diff will pull away and the car with the longer diff will never catch up again.

Now if you start in first gear from the same 40km/h, the cars will do exactly what you said the simulator anticipated : "simulation says the 540 with the 3.15 has the tinyest of advantage until gearchange then the 2.93 will pull ahead a little and the 3.15 will catch up until the next gearchange... this continues to well over 200kph so essentially no difference in acceleration when driven well..."

 

[moranor@axis]

lol you can say I told you so... I was wrong...

but the gain on the 2.5l with w 3.38 diff is much larger than with the 540... on the 540 I would say a ratio change is probably not really worth it as the 2.93 seems to be a good ratio for it...

on the 2.5l I would say it is worth it because there is an acceleration gain and it will make the car easier to drive fast...

most of us are not after 1/4 mile or 0-100 times but a car that puts a smile on our face a 3.38 ratio on a 2.5-3l will do that so it makes it worth while even if the gain is not huge :rollsmile:

 

[Loom]

Which I suppose answer my question I asked about mainly driving in town. The other thing is, with a shorter diff the RPM would increase a bit so that might actually benefit fuel economy as one could more easily cruise in forth gear at 60 km/h than with the longer diff, wouldn't you agree?

 

[Fordkoppie]

but the gain on the 2.5l with w 3.38 diff is much larger than with the 540... on the 540 I would say a ratio change is probably not really worth it as the 2.93 seems to be a good ratio for it...



most of us are not after 1/4 mile or 0-100 times but a car that puts a smile on our face a 3.38 ratio on a 2.5-3l will do that so it makes it worth while even if the gain is not huge :rollsmile:

Are you kidding? The 540 FEELS so much better with the shorter ratio which is exactly why I went through the trouble of fitting the 3.15, and it was for no other reason than to put a smile on my face. The car just FELT really sluggish with the long ratio and you had to downshift when cruising uphill just to maintain speed which was really annoying. I mean previously it could do almost 115 in second gear which was just ridiculous. The car is also now a lot easier to drive in bumper to bumper stop start traffic, because with the longer diff, just idling in first gear used to be too fast for the traffic and you constantly had to clutch in, whereas now I can simply let it trawl in 1st on idle in the same slow moving traffic.

We just happened to have access to more than 1 (or 2) 540s to do the before and after performance comparisons with and all in the name of fun. That was where we saw that in practice it doesn't really affect the V8. As was said before - 1 passenger's weight made much more of a difference to the outcome than anything else.