cars with 7 or 8 speeds

[supton]

Originally Posted By: Lolvoguy
Originally Posted By: supton
As much as it bothers me, all cars should downshift on a hill. Otherwise it means that the engine is spinning too fast on the flats

what about larger displacement v8's that have ample tq and hp that it doesn't need to downshift?
I've rented 300c's with the 5.7 hemi, and while going up mountain passes with the cruise on, it never needed to downshift to maintain speed while at a low rpm.


Then the final drive ratio very likely could be taller for lower cruise rpm. Maybe it wouldn't matter, they've done wonders with VVT and DBW so as to make engines quite efficient regardless of rpm--but I'd think lower rpm is going to generally eek out more mpg.

I always did like my turbodiesel, that didn't need downshifting for hills. Torque was at cruise rpm.

 

[SteveSRT8]

Originally Posted By: Quattro Pete
Charger SRT has a ZF trans made by ZF.
The regular Charger V6 has a Chrysler made trans based on a ZF design (845RE).

Not sure who's ripping on what, but the biggest thing with these transmissions is the programming which I believe each auto manufacturer is responsible for, not ZF. Chrysler's programming isn't as buttoned up as BMW's or Audi's programming.



Note that IIRC now all ZF's used in Chrysler production are American made since 2013. Only the early production (05 into early 06) SRT8's got German Parts.

My sig car had AMG tags on both the Transmission (not ZF, WA580 Mercedes) and the rear end (215 mm). Engine is signed, too.

Programming on my 14 RAM 5.7 is superb, brake hard and she grabs downshifts like a pro driver, always smooth and always in the correct gear. It seems the smaller engines are the ones with issues, as my friends 3.6 Jeep with the 8 speed is a bit herky-jerky...

 

[Quattro Pete]

Originally Posted By: SteveSRT8
Note that IIRC now all ZF's used in Chrysler production are American made since 2013.
Correct. Chrysler makes them in Indiana, and ZF makes them in South Carolina.

 

[circuitsmith]

Originally Posted By: CR94
"Overdrive" was a meaningful term way back when most cars were rear-drive with a direct 1:1 top gear. Then the overall ratio was the same as the "rear end" (final drive) ratio, unless you had the optional overdrive. Nowadays, it doesn't mean much. The overall ratio still matters, but whether it is achieved with with an "overdrive" transmission ratio ahead of the final drive is of no particular significance. Thus, 0.6×4.0 should perform about the same as 0.8×3.0 or 1.0×2.4.


Somewhere I read it explained thusly:

At some particular drive ratio the engine can just reach its horsepower peak rpm,
"road load" (wind and rolling resistance) on level ground prevents the engine & vehicle from going any faster.
Any taller (higher) drive ratio than that is overdrive.
So the vehicle will have a lower top speed in an overdrive gear vs the lower "Goldilocks" gear.

 

[OVERKILL]

Originally Posted By: circuitsmith
Originally Posted By: CR94
"Overdrive" was a meaningful term way back when most cars were rear-drive with a direct 1:1 top gear. Then the overall ratio was the same as the "rear end" (final drive) ratio, unless you had the optional overdrive. Nowadays, it doesn't mean much. The overall ratio still matters, but whether it is achieved with with an "overdrive" transmission ratio ahead of the final drive is of no particular significance. Thus, 0.6×4.0 should perform about the same as 0.8×3.0 or 1.0×2.4.


Somewhere I read it explained thusly:

At some particular drive ratio the engine can just reach its horsepower peak rpm,
"road load" (wind and rolling resistance) on level ground prevents the engine & vehicle from going any faster.
Any taller (higher) drive ratio than that is overdrive.
So the vehicle will have a lower top speed in an overdrive gear vs the lower "Goldilocks" gear.


I've never heard it described that way

My understanding of it is as CR94 described, which is that any ratio above 1:1 in the transmission, irrespective of final drive ratio, was "over drive", quite literally, above the traditional 1:1 ratio of "D" or "Drive" in the transmission.

This was done for the pursuit of fuel economy primarily but would also allow for deeper rear gears while retaining narrower spacing between the 1:1 and below gears in the transmission, giving better pull off the line, while still allowing for a reduced cruising RPM achieved through the steeper OD gear.

As the number of gears increased, naturally narrowing the difference between ratios, the role of "overdrive" as a significant gear by itself was, as CR94 pointed out, reduced, as the transmission would simply be matched to an appropriate final drive ratio to achieve a decent cruise RPM and thus, the number of gears that were above or below 1:1 became less important.