Toys4RJill

I don't think there is any evidence that a CVT can't hold up or match an automatics credentials.

The Pathfinder is FWD CVT and the XTerra is RWD 5-speed, both are rated at 5000 lbs.


What is even more interesting is that a Pathfinders payload is 1800lbs while a XTerras payload is just 1061 lbs. (Used Google's Numbers).

An automaker can pretty much engineer anything if they want to.

There is no doubt in my mind that CVTs will eventually make it into trucks and larger SUVs. The performance benefits and MPG gains will be too much to ignore.

Aron9000

The CVT in the newer Pathfinders has been having all kinds of problems. This is the heaviest vehicle they've ever put one in BTW.

http://www.carcomplaints.com/news/20...-lawsuit.shtml

As for the conventional torque converter automatic in the 2005-2010 Xterra, Pathfinder, and Frontier pickup truck, it is a great transmission that is guaranteed to prematurely fail. Problem with those is that Nissan integrated the transmission cooler and radiator all into one unit. Of course they speced the cheapest radiator possible, which is guaranteed after a few years to fail, allowing engine coolant into the transmission, and transmission fluid into the coolant, thus dooming your transmission.

http://www.nissanproblems.com/trends...-transmission/

Moral of the story, don't buy a Nissan. Bunch of French made junk.

Nissans are very popular around where I live in Nashville because their headquarters and factory are here. I've known several people who have been absolutely BURNED by Nissans, usually cars that are 4-8 years old, well maintained, under 100,000 miles on them.

Sulu

I do not believe that Nissan's CVT (or any other similar belt-and-pulley CVT) will serve well in a truck or true SUV that is meant to be used (and abused) in low-traction (snow, sand, mud, off-road) situations, and it is NOT because of the weight of the vehicle. It is because the belt-and-pulley CVT cannot handle a great amount of torque (neither constant nor instantaneous, transitory), and because the CVT does not provide good engine braking.

The belt-and-pulley CVT works by transferring engine torque to the driveshaft by means of a metal belt and 2 variable diameter pulleys. Unlike a fixed-gear ratio manual or automatic (regardless of the design), this type of CVT does not have gears with teeth that mesh together, ensuring torque transfer without slipping. If the torque on the input pulley (from the engine) is too high -- it may be too high constantly, as in a large, high-torque engine, or it may be too high for only one moment such as initial acceleration from slow engine speed -- the belt will slip and torque transfer will be lost or at least very inefficient.

Nissan's CVT, it seems, can only handle about 250 lb.ft of torque (give or take), about the maximum torque of Nissan's popular VQ family of V6 engines. Ever wonder why Nissan does not use its popular CVT on its V8 engines? It is very likely (educated guess on my part because Nissan will never say) that the belt-and-pulley CVT cannot handle the V8's level of torque.

Audi used to use a similar belt-and-pulley CVT but recently announced they are giving them up in favour of fixed-ratio automatic transmissions (transmissions with toothed gears). I suspect that the great instantaneous, transitory torque of turbocharged engines may cause belt-and-pulley CVTs to slip.

I believe that belt-and-pulley CVTs are at their torque limit now. I doubt that we will see such transmissions used with high-torque V8 engines or turbocharged engines (that are tuned for high torque at low engine speed).

Low traction situations (ever tried to rock your way out of a snowbank?) cause great transitory torque as you floor the throttle in the hope of gaining some traction. That is likely to cause the belt in a CVT to slip. A fixed-ratio transmission, however, would not have this problem.

CVTs are also fundamentally different from fixed-ratio, toothed-gear transmissions in their mode of operation. The speed of a vehicle with a CVT is governed by the transmission with its stepless gear ratios rather than the engine. The purpose of the CVT is to maintain engine speed constant, and let the transmission handle the speeding up or slowing down of the car.

On a normal transmission, however, in any set gear, vehicle speed is governed by engine speed -- if you want to speed up (and before the transmission shifts to a higher gear), the engine speeds up, and if you want to slow down (and before the transmission shifts to a lower gear), the engine slows down. In a fixed-ratio transmission, you can also let the engine slow you down -- lift off the throttle and the slowing engine slows you down -- with engine braking. You do not have engine braking on a vehicle with a CVT -- if you want to slow down, you lift off the throttle but the transmission changes its gear ratio to allow a slower speed while the engine stays at the same speed.

In low-traction situations, you want to remain in one gear without the transmission changing gears; going downhill in low-traction situations, you rely on engine braking. You can do that with a fixed-ratio, toothed-gear transmission, but not a CVT.

Because of these 2 shortcomings of the belt-and-pulley CVT (the type that Nissan uses), I believe that a CVT on a truck or SUV is likely to fail. It may not fail by breaking down, but it will fail because it cannot provide the functionality -- high-torque ability and engine braking -- that a vehicle in low-traction situations needs.

Sulu

The shaking and shuddering of the CVT in the Pathfinder may very well be the belt slipping and then gaining traction again on the input pulley. As the heavy Pathfinder accelerates away from a slow speed (or rest), the need for torque will be at its highest, and be very high for a vehicle as heavy as the Pathfinder. This sudden torque load, as the driver floors the throttle and the engine speed suddenly shoots up to the torque peak may be overloading the transmission belt. The belt will initially slip, gain some traction (perhaps as the ECU commands less throttle to compensate for over-torque or merely catching up), only to slip again as the ECM tries to maintain torque peak; this will likely be felt as shaking and shuddering.

I am not sure what Nissan can do to fix it. Perhaps they can reflash the ECU to prevent the engine from hitting torque peak rpm too quickly, allowing the CVT time to catch up; but that may be at the expense of acceleration speed (slower acceleration).

Trade in that heavy Pathfinder for a lighter crossover -- and one without a CVT. Highlander or Pilot perhaps? But the 9-speed ZF transmission in the top-of-the-line Pilots may have problems of their own.

Toys4RJill

Sulu, I'm sure there are waya around the issues that you described. I have seen CVT handle V8s with over 400 lbs of tq. The CVT was not belt and pulley (I think) so it can be done.

mmarshall

If you know of any examples of production CVTs with V8 engines and/or 400+ ft-lbs.of torque, please give us some. (since it may be somewhat off-topic, if necessary, start a new thread). It is one thing to watch an experiment in a lab or under controlled conditions......quite another to mass-produce something in the hands of the public.

nthach

[QUOTE=Sulu;9162047
Nissan's CVT, it seems, can only handle about 250 lb.ft of torque (give or take), about the maximum torque of Nissan's popular VQ family of V6 engines. Ever wonder why Nissan does not use its popular CVT on its V8 engines? It is very likely (educated guess on my part because Nissan will never say) that the belt-and-pulley CVT cannot handle the V8's level of torque.



I believe that belt-and-pulley CVTs are at their torque limit now. I doubt that we will see such transmissions used with high-torque V8 engines or turbocharged engines (that are tuned for high torque at low engine speed).

Low traction situations (ever tried to rock your way out of a snowbank?) cause great transitory torque as you floor the throttle in the hope of gaining some traction. That is likely to cause the belt in a CVT to slip. A fixed-ratio transmission, however, would not have this problem.

[/QUOTE]

Then how come Subaru is bolting a CVT behind the turbo FA20 in the WRX(not the STI) and Forester XT and it's rated at 258ft-lbs of torque? While I do realize it's a full-time AWD car with a center diff and all, and that Subaru depends quite heavily on Jatco thanks to their Nissan linkage in the 1970s-early 1990s - their old 4 and 5 speed automatics were in fact Nissan designs we would hear about Subaru trannies failing left and right. Honda is using a CVT in the new Accord and CR-V, and the latter is no lightweight either.

While a stepped automatic is stronger than a CVT, it doesn't mean it can handle a lot torque - and now that Fiat Chrysler is bolting the ZF 8HP90 behind the Hellcat's blown V8, I wouldn't be surprised if it eats through it - Jaguar used Mercedes' 5-speed automatic for the XKR/XJR since ZF at the time didn't have a sturdy transmission, Rolls-Royce and Bentley used GM's 4L80E for a while - if they wanted to they've could have stuffed an Allison school bus tranny into it :P

Sulu

Yes, please. If you know of a CVT that can handle the torque of a V8, please share that information.

The Subaru's 258 ft.lb of torque would be within the limit of my educated guesstimate of the torque-handling capability of Nissan/Jatco's belt-and-pulley CVT.

The inherent design of the sliding belt CVT cannot handle high torque loads. But the inherent design of the planetary gearset automatic transmission can handle higher torque loads. It is just that ZF may have under-designed or under-built their new 8-speed transmissions. In their effort to make the whole 8-speed transmission unit take up only as much space and weight as a 6-speed unit, some reliability and durability may been lost. But it is inherently the same type of transmission as an Allison truck transmission (a planetary gear design), which we know can handle large torque loads.

ZF's 8-speed longitudinal and 9-speed transverse transmissions are still new so teething problems should not be a complete surprise. Give them another year or two to perfect it.

Toys4RJill

I never said anything about belt pulley vs planetary gear. I omitted that on purpose as we got a lot of opinions and wrong conclusions that the CVT for a V8 could not be done.

The LS CVT is pushing way more torque than the Nissan XTerra needs or most mid size SUVs need. It also weighs 5100lbs and is AWD which tells you that a CVT can be designed to be robust for a high torque application.

Also, the performance will be superior with a CVT vs traditional transmission as well.

Sulu

Sorry, but you cannot compare the Nissan CVT with the Toyota/Lexus (and Ford) eCVT.

The Nissan CVT is a true alternative to a FWD planetary gearset automatic transmission, with a wide gear ratio spread between its lowest gear ratio and its highest gear ratio. It has a low (short) gear ratio to accelerate the car from rest, a high (tall, overdrive) gear ratio to allow for highway cruising with the engine at low, relaxed speeds, and an infinite number of gear ratios in between to drive the car smoothly from rest up to its maximum highway speeds without the jerks of a fixed gear ratio automatic.

The Toyota eCVT, however, cannot be used on its own as a continuously-variable transmission; it can only be used in conjunction with 2 electric motors in a hybrid vehicle. The eCVT has only one, fixed gear ratio, equivalent to a high (highway cruising) gear. Without lower (shorter) gear ratios to amplify torque, the engine and transmission likely would not be able to provide enough torque to accelerate the car away from rest; the torque needed to accelerate the car from rest has to come from the electric drive motor of the hybrid system. The eCVT can only work in a hybrid vehicle, with the electric drive motor providing low speed torque and the engine providing power when the electric motor has run out of steam (and battery power).

Do you have another example of a CVT that will work with high torque V8 engines?