Between speeds 4-5 and 7-8, ZF uses dog clutches instead of friction clutches to save space.
I think I know what you meant, but just for the sake of completeness...both
dog clutches are used (engaged) in gears 1, 2, 3, and 4. During the 4-5 upshift, one
of the dog clutches disengages, while the other stays engaged...and it stays engaged in gears 5, 6, and 7. Then it disengages during the 7-8 upshift, after which neither
dog clutch is engaged. And then, of course, the reverse is true when downshifting...one of them engages during 8-7 and stays engaged downward from there...and the second one engages during 5-4, and stays engaged downward from there. The dog clutches aren't used just
during those two shifts.
As a point of interest, this transmission defaults to a 2nd gear start in D mode (at least I think 2019 and later does, or an earlier model after the software update has been applied). You can't even tap it down into 1st gear with the left paddle if you're in D. You have to switch to S mode to get 1st gear. So if you're counting shifts in D mode, you'll feel only 7, and it uses 8 of the 9 gears in D mode. As I recall, the overall drive ratio of 2nd gear in the ZF9 is still shorter than 1st gear in the old H6 auto, so it still launches off the line strongly in D mode. In S mode, starting in actual 1st gear, it'll really
The dog clutches are released during upshifts and applied during downshifts which results in a bit of a delay and unconventional feel when downshifting from 8th and 5th.
Yes, and I think Honda's done a remarkable job here with the shift programming to get all shifts to feel just about the same (I think this was largely what the 2019 recalibration brought to the picture, in addition to the 2nd gear default start noted above). The engine speed necessarily has to flare/blip during the 8-7 and 5-4 downshifts...and Honda's sort of matched that behavior across most of the shift range, so 8-7 and 5-4 don't feel so out of place. And, in practice, it's something that feels odd for the first few drives, and then you get used to it. We have one Honda with the ZF9, and our other two have old school Honda H5 automatics, and it's easy to move back and forth without having to change driving style.
I agree, the shifts are nearly imperceptibly smooth with this transmission. Intuitively, you'd think that means it's really tearing up its clutches or working its fluid hard, but anecdotal evidence suggests these transmissions are at least as durable as the H6, especially when it comes to torque converter operation...an apparent weak area for the H6.