ZF8 Speed Ratio Question

Skilid

Skilid

Skilid

Soldato
Joined
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I've noticed that the gear ratios on the ZF8 speed in my M240i are a bit weird. The difference in engine speeds between all changes seem pretty similar, apart from the 3-4 change which is much shorter. You can see it in this video: https://www.youtube.com/watch?v=-WxZn1_2OBU&ab_channel=AutoTopNL (skip to 1:25)

From the video:
1-2 Shift: Drops ~1500rpm
2-3 Shift: Drops ~1500rpm
3-4 Shift: Drops ~1000rpm
4-5 Shift: Drops ~1500rpm
5-6 Shift: Drops ~1500rpm

Not sure about 6-7 and 7-8 as you can hit the limiter in 6th, but I notice the same thing even at much lower RPMs the difference is still very noticeable.

Do we know why 4th(?) gear is shorter than all the others? My brain can't work out if it's 3rd or 4th that's the shorter gear.
 
teenwolf said:
gearbox map
Click to expand...
:cry::cry::cry::cry::cry::cry::cry::cry::cry::cry::cry:

“Yeah, I remapped the metal cogs inside the gearbox to have a different number of teeth”

Clueless.

As to why it is shorter, most likely either for efficiency (influenced by the testing cycle for emissions) or drivability reasons.
 
DRZ said:
:cry::cry::cry::cry::cry::cry::cry::cry::cry::cry::cry:

“Yeah, I remapped the metal cogs inside the gearbox to have a different number of teeth”

Clueless.

As to why it is shorter, most likely either for efficiency (influenced by the testing cycle for emissions) or drivability reasons.
Click to expand...
I just don't understand why it's that one ratio. How would that help efficiency?
 
Skilid said:
I've noticed that the gear ratios on the ZF8 speed in my M240i are a bit weird. The difference in engine speeds between all changes seem pretty similar, apart from the 3-4 change which is much shorter. You can see it in this video: https://www.youtube.com/watch?v=-WxZn1_2OBU&ab_channel=AutoTopNL (skip to 1:25)

From the video:
1-2 Shift: Drops ~1500rpm
2-3 Shift: Drops ~1500rpm
3-4 Shift: Drops ~1000rpm
4-5 Shift: Drops ~1500rpm
5-6 Shift: Drops ~1500rpm

Not sure about 6-7 and 7-8 as you can hit the limiter in 6th, but I notice the same thing even at much lower RPMs the difference is still very noticeable.

Do we know why 4th(?) gear is shorter than all the others? My brain can't work out if it's 3rd or 4th that's the shorter gear.
Click to expand...


Probably a fudge for WLTP test.
 
Yeah you find some cars with weird rations which just don't make sense in the real world, in our old MX5 4th and 5th were so close together that the wife would call 5th the 'spare 4th' as you never needed to use it, just 4th straight to 6th.

I think for that car it might have been to hit it's top speed in 5th as the 6th was an overdrive or something along those lines.
 
Gear Ratio
  • 1st Gear 5 :1
  • 2nd Gear 3.2 :1
  • 3rd Gear 2.14 :1
  • 4th Gear 1.72 :1
  • 5th Gear 1.31 :1
  • 6th Gear 1 :1
  • 7th Gear 0.82 :1
  • 8th Gear 0.64 :1
  • Final Drive 2.81 :1
  • Reverse 3.46 :1
Seems fairly uniform when you look at the actual ratios
 
teenwolf said:
gearbox map
Click to expand...

8bFSfGg.gif
 
Because I'm boring and this kind of thing interests me, I used an online gearing tool to visualise the ratios. You can clearly see that 3-4 change is shorter than all the others, but I still have no idea why.

ihZfSci.png

According to that 6th - 7th should also be short, but the video doesn't show that.
 
Skilid said:
Because I'm boring and this kind of thing interests me, I used an online gearing tool to visualise the ratios. You can clearly see that 3-4 change is shorter than all the others, but I still have no idea why.

ihZfSci.png

According to that 6th - 7th should also be short, but the video doesn't show that.
Click to expand...

Unsure if it’s the same on the 340i as your car, but I did find that 3rd gear had a very punchy feel to it on A/B roads - not sure if this would be an effect of the shorter ratio?
 
Skilid said:
Because I'm boring and this kind of thing interests me, I used an online gearing tool to visualise the ratios. You can clearly see that 3-4 change is shorter than all the others, but I still have no idea why.

ihZfSci.png

According to that 6th - 7th should also be short, but the video doesn't show that.
Click to expand...
Isnt 4th gear the issue

If you look at the change between each gear ratio

1>2nd 0.64
2>3 0.66
3>4 0.80
4>5 0.76
5>6 0.76
6>7 0.82
7>8 0.78

You would expect the ratio change to be less as you go up higher in the gears, 4th gear really should be between a 1.62 ratio or less.

Maybe ask BMW technical? There will no doubt be a good reason. 3rd gear covers the all important 0-100 time. Its probably purely for that and that alone that the whole gearbox is messed up.
 
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