ZF8 Speed Ratio Question

Skilid · 2 Sep 2022 at 10:54

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.

Skilid · 2 Sep 2022 at 10:57

teenwolf said:
get it mapped

What's that going to do? Gear ratios are fixed.

Skilid · 2 Sep 2022 at 11:25

teenwolf said:
gearbox map

Software does not change hardware. Gear ratios are fixed.

Skilid · 2 Sep 2022 at 11:26

DRZ said:
“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.

I just don't understand why it's that one ratio. How would that help efficiency?

Skilid · 2 Sep 2022 at 16:22

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.

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

Skilid · 2 Sep 2022 at 16:38

MrRockliffe said:
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?

EDIT: Could be that torque is limited slightly in the first two gears.

Any car that uses the ZF 8HP50 should be the same. The M235i used the earlier 8HP45, for example.

EDIT 2: List of variants here: https://en.wikipedia.org/wiki/ZF_8HP_transmission#List_of_ZF_8HP_variants

Skilid · 2 Sep 2022 at 18:18

Greebo said:
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.

Yes. It’s 4th that is shorter compared to everything else, even all of the higher gears. Just seems odd.

It doesn’t affect my enjoyment of the car in any way, but when you’re changing down through the box you can almost skip it!