Last post on Mar 28, 2011 at 7:49 PM
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Oct 09, 2007 (7:07 am)
8-speed auto is nothing new. Toyota has been using it in Lexus LS and soon to be put in Lexus IS-F.
When it comes to “speeds”, there is little to no point beyond 6-speed auto unless the losses could be reduced (but with same technology, they could be with fewer cogs too) and gearing span increased. The Mercedes 7-speed auto is a classic example of this. It has an exceptionally tall top gear, perhaps too tall for most practical purposes, a reason it is very close to the next shorter gear (seventh gear is 90% of sixth). Despite this extra tall ratio, the span hasn’t been increased. It is still around 6, which is the norm for 6-speed automatic and CVT. I don’t see an advantage in this case.
Now, if the gear span could be increased, we might be looking at something interesting, as it would allow very short low gears coupled to relaxed tall gear (gear span = first gear ratio/top gear ratio).
#12 of 18 Did I really read this?
by Stever@Edmunds HOST
Jan 25, 2009 (8:07 am)
I'm racking my brain trying to remember if I read an article somewhere about manually shifting a seven or eight speed transmission. It seems like some reviewer complained that matching revs was difficult when downshifting (I'm assuming that was with paddle shifters).
Maybe it's just some sour grapes?
Good article about the power, emissions and fuel economy of seven and eight speed trannies:
A Little Knowledge of Transmissions Can Save You Big Bucks at the Pump
#13 of 18 There already ARE transmissions...
Jan 26, 2009 (11:48 am)
...with more gears, but they're in semi trucks.
Because of the wide variety of loads the "semi" may carry, they usually have a manual transmission to allow the driver to have as much control as possible. However, all truck manufacturers now offer semi automatic transmissions (manual gearboxes with automated gear change) as well as automatic transmissions.
"Semi" truck transmissions usually provide at least nine or ten gear ratios, but possibly as many as eighteen (e.g. Australian Road Trains). A large number of transmission ratios means the engine itself can operate within a narrow range of speeds. The range of speeds over which an engine is expected to perform well has implications for the design - the narrower the range, the more the engine can be optimised for that. Also having so many gears allows fine-grained control of engine braking for better control on downhills and in curves.
A ten speed manual transmission is controlled via a six-slot H-Box pattern similar to that in five-speed cars - five forward and one reverse gear. Gears six to ten (and high speed reverse) are accessed by toggling a selector control - so that first gear becomes sixth, second becomes seventh, etc.
Another difference between semi-trucks and cars is the way the clutch is set up. On a regular car the clutch pedal is depressed full stroke to the floor for every gear shift to ensure the gearbox is disengaged from the engine. On a semi-truck with e.g. Eaton Roadranger series constant mesh transmission (non synchronized) not only double clutching is required, there is an additon of clutch brake as well. The clutch brake stops the rotation of the gears and allows the truck to be put into gear without grinding when stationary. The bottom of the clutch pedal stroke is where the clutch brake activates and as a result only partial or "half" clutch pedal stroke is used when a vehicle is in motion.
#14 of 18 Re: Geeze... [lemko]
Mar 27, 2011 (10:35 am)
the amount of tranny repair is about the same there is no more work on 7/8 speed tranny then a 4/6 speed. its just a bigger trany
#15 of 18 Re: The Point [robertsmx]
Mar 27, 2011 (8:03 pm)
I've got a Mercedes GLK350 4matic with the 7 speed. With the nice flat torque curve the Mecedes 3.5 liter 6 has, the little SUV has no problem holding 7th gear on my interstate runs, even going up fairly steep grades ( I live in WV so there's plenty of them on our interstates). In addition, the GLK is quite quick for a SUV.
#16 of 18 Re: The Point [oldbearcat]
Mar 28, 2011 (4:25 am)
In your opinion, do you think your Benz could get by with fewer gears, or do you feel that they benefit you? Since you live in a mountainous area, I'd imagine that you'd see some extra benefit to those additional gears.
I have to admit, the only thing I've ever driven with seven gears is a 1990 Montgomery Ward lawn tractor! I've driven my buddy's 2006 Xterra a few times though. It has a 5-speed automatic, and I don't really care for it. It seems to have no power in top gear, and when you need to stomp it and downshift, it seems like there's a slight delay, but then it kicks in and almost seems to take off TOO fast, as if it's compensating for that lag.
I'd probably get used to it if I drove it on a regular basis, but most of my driving is done with archaic 3- and 4-speed automatics, so to me that 5-speed just seems like it shifts too much. It doesn't lurch or clunk, but I can still feel it from the change in engine revs.
I used to think that my old 2000 Intrepid would have benefited from an additional gear. Sometimes in 4th gear, it would get a bit gutless, but then when it shifted to 3rd, it almost seemed like overkill. FWIW, at 75 mph, 4th was 2500 rpm and 3rd was around 3750. If they had stuck an additional gear somewhere between 3rd and 4th, I thought it would help a bit.
#17 of 18 Re: There already ARE transmissions... [lemko]
Mar 28, 2011 (7:07 am)
Most drivers of "roadranger" equipped trucks don't bother with using the clutch, except for stopping and starting.......shifting is done 'by ear' and/or watching the tach. Takes some practice.
#18 of 18 Re: The Point [andre1969]
Mar 28, 2011 (7:49 pm)
IMO the 7 speed certainly enhances the GLK's performance. The car I traded for it was a Jaguar S-Type V8 with the 6 speed automatic. Both cars have enough low RPM torque that they don't frequently downshift on hills at interstate speeds. The ZF transmissions shift almost seamlessly as well. I had a 2001 Intrepid with the 2.7 - I know what you're talking about. My business driver is a Honda CRV - with a 5 speed automatic. It makes you crazy downshifting to as low as 3rd while it screams its guts out climbing a hill on the interstate.