10 speed Auto Trans - So when does it just become a CVT

A standard planetary set can give you 3 speeds, with one of them being 1:1. If you get creative with how you drive them, you end up with an overdrive, too, so 4 speeds, with one being 1:1. if you put two such units together, you'd end up with 4 x 4 speeds, or 16 gears, though two of them would be 1:1 so in reality it would be a 15 speed box. Or, use a 4 speed, overdrive-capable planetary setup with a 3 speed, non OD and you'd have 12 gears, with two 1:1, so 11 speeds.

Two 3 speed autos would net you 8 total ratios (9 minus the extra 1:1) but no overdrive, which would require a fairly tall final drive ratio. But cruising efficiency in top gear would be better, since geartrain losses at 1:1 ratio are minimal.

Not sure why we haven't seen such beasts yet.

The 545RFE in my Dakota is a revised TorqueFlite 727 with a second planetary in it. It's advertised as a 5 speed auto, but it has 2 different 2nd gears (upshifts vs. downshifts) and 2 different overdrives (normal vs. tow-mode), so it uses 7 out of 9 possible ratios. That's the good part. The part I don't like is the stupid programming for mileage (that doesn't work). If you're rolling along and get into the throttle, it up-shifts instead of down-shifting. If you're in tow-mode, it'll hold overdrive until you're loosing speed at full throttle, then down-shift 3 gears, from OD to 2nd, with the little 4.7 screaming it's lungs out.

In reply to Vigo: Most modern Nissan CVTs do have what they call the "aux gearbox". Basically a 2 speed planetary gearbox to allow a wider range of ratios. Started in 2013 for Sentra/altima/pathfinder, 2015 Maxima and murano. Don't recall if the versa does or not. Pretty sure the juke does not.

Nice, thanks for the info!

As was mentioned, we've actually had two planetary sets ever since 4spd autos came out. Since then, we've just been adding 'holding elements' (clutches/bands/sprags) to hold/release the elements of the two planetary sets in different combinations to 'add' ratios. Some transmissions do have more than two planetary sets, though. The big challenge to getting all the possible ratios out of a set of planetaries is fitting all the holding elements to engage all the different ratios and making all that work. It's basically the concept of Russian nesting dolls turned up to 11 with all of them spinning different directions and speeds inside of each other and hydraulic circuits running through all of them somehow. As bad as auto transmissions are to fully grasp, it's honestly easier to look at all the parts spread out on a table than it is to picture it in cutaway form with all the parts moving in proper relation to each other.

dean1484 wrote: This got me thinking that at what point does a (insert # of speeds here) speed trans really just become a CVT?

As long as I'm picking gears, and able to find the wrong ones, it's a manual.

I think part of the reason for more gears is that most modern automatics keep the converter locked up most of the time and using torque management to keep everything together during shifts. As a result, you don't get the torque converter helping you out for bridging the gap between ratios and smoothing power delivery, a la the previous Powerglide example. Less losses with the tq locked up, but I don't know how much that is offset by more losses in the trans itself.

I don't have a problem with more gears until you end up with a transmission that hunts, doesn't know what gear it wants to be in, or has gears too high to be useful like the chrysler 9 speed that only uses 8 of them, and even 8th is too long to use except downhill or at 80+ mph.

With a manual, 6 gears is nice, and I don't think I would mind 7 if the shifter is good. Keep the ratios tight with a few wide-spaced gears at the top for highway driving. Because you decide when to shift, just don't use the high (or low) gears unless you need them, and you avoid the shifting confusion of the auto. I know I could use a 7th in many of the bikes I've had just to bring revs down on the highway, but I need the low first few gears for off road work.

In reply to Maniac0301:

I really like the Mopar modified ZF 8 speed in my '14 Grand Cherokee. It's the shifter bit that's a tad annoying.

RossD wrote: In reply to Maniac0301: I really like the Mopar modified ZF 8 speed in my '14 Grand Cherokee. It's the shifter bit that's a tad annoying.

Just to keep it in FCA family lines, the rental '15 5.7 Ram 8 speed I had sucked ass. Maybe it's the not flat terrain around here, but it could never find the right gear. Well, I take that back, it could find first while stopped and #8 when cruising at 75mph 50% of the time.....

ProDarwin wrote:

Knurled wrote: Hell, a Powerglide with a proper torque converter is just a CVT except in name. The drag guys set them up with a loose converter and the engine rides the stall speed on the launch and after the shift. Sounds like crap but goes like hell.

Except that setup pours out gobs of excess energy via heat (TC slip). I would say it functions similar to a CVT in that the input speed stays constant and output speed increases, but thats where the similarities end.

Torque converters don't just slip, they multiply torque when below the stall speed, thanks to the voodoo going on with the stator. That's how automatics can get away with much taller 1st gears and final drives.

Generally the ratio is modest, 1.5-2:1 or so. I don't disagree that they can create a good amount of heat (wasted energy) but they function nicely as a variable-speed torque multiplier.

Knurled wrote: Hell, a Powerglide with a proper torque converter is just a CVT except in name. The drag guys set them up with a loose converter and the engine rides the stall speed on the launch and after the shift. Sounds like crap but goes like hell.

In the 50s Chevy and Buick both offered transmissions that were effectively hydraulic CVTs.

https://en.wikipedia.org/wiki/Turboglide

https://en.wikipedia.org/wiki/Dynaflow

When I was really young (like 6), my family had a 1950 Buick with a straight 8 and a dynaflush. Slowest thing in the whole world.

BrokenYugo wrote:

Knurled wrote: Hell, a Powerglide with a proper torque converter is just a CVT except in name. The drag guys set them up with a loose converter and the engine rides the stall speed on the launch and after the shift. Sounds like crap but goes like hell.

In the 50s Chevy and Buick both offered transmissions that were effectively hydraulic CVTs. https://en.wikipedia.org/wiki/Turboglide https://en.wikipedia.org/wiki/Dynaflow

I keep dreaming someone will revisit the 'variable torque converter' idea with magnetorheological fluid, but noone has or will. I just think that would be amazing.

Corvettes had an 8 speed manual in the 80's.. you just had to flip the add on overdrive unit off and on as you ran thru the gears.. never personally driven one, but I hear you can keep the rpms in a pretty tight range once you figure it out..

I don't really understand autos, I must admit, but isn't all this complexity going to be a problem as these vehicles age?