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2015 8-speed Silverado/Sierra fuel economy set at 21mpg highway


Zane

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Looked up the gear ratio and 1 to 1 drive is sixth gear. 7th and 8th are overdrive. I thought the 6L series 6th gear ratio was .67 and the 8th in this one is .65. Please correct me on the 6L series gear ratio of 6th gear if that was incorrect. Just going off memory

overdrive is overdrive there making 8 gears out of 4 combining 2 to make one close ratio gears less rpm change from gear to gear
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overdrive is overdrive there making 8 gears out of 4 combining 2 to make one close ratio gears less rpm change from gear to gear

 

I think they are trying to make it drive like a CVT without just giving us a CVT. If they could get a CVT to not sound like the trans is slipping they would have a winner. Make the torque convertor lock up solid as vehicle hits 15 mph and let the CVT take over. They could even copy the one manufacturer that has their CVT programmed to give the feel of a shift.

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I think they are trying to make it drive like a CVT without just giving us a CVT. If they could get a CVT to not sound like the trans is slipping they would have a winner. Make the torque convertor lock up solid as vehicle hits 15 mph and let the CVT take over. They could even copy the one manufacturer that has their CVT programmed to give the feel of a shift.

not really sure how it would work on a truck I know how it works on atvs but surely it would be nothing like that
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not really sure how it would work on a truck I know how it works on atvs but surely it would be nothing like that

 

CVTs in cars are pretty close to just about all the off road vehicles with. The transmission is basically two variable diameter pulleys, with some form of a centrifugal clutch type device on the edge of both pulleys. If the two pulleys are same diameter, then that would be 1:1 ratio. The device on the edge of the pulley will pull the two sides of the pulley apart, causing the belt to drop down inside the pulley, and on the other pulley, the device will push the two sides of that pulley together. By changing the diameter in opposite directions, will change the ratio. Much like a ten speed bike, if you change to a higher numerical gear on the rear wheel, you can also fine tune that change by using one of the two size pulleys to the larger or smaller sprocket.

 

I do not know what they are using for the belt in the current CVTs. Would need to be strong to keep torque from the engine from burning up the drive belt.

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CVTs in cars are pretty close to just about all the off road vehicles with. The transmission is basically two variable diameter pulleys, with some form of a centrifugal clutch type device on the edge of both pulleys. If the two pulleys are same diameter, then that would be 1:1 ratio. The device on the edge of the pulley will pull the two sides of the pulley apart, causing the belt to drop down inside the pulley, and on the other pulley, the device will push the two sides of that pulley together. By changing the diameter in opposite directions, will change the ratio. Much like a ten speed bike, if you change to a higher numerical gear on the rear wheel, you can also fine tune that change by using one of the two size pulleys to the larger or smaller sprocket.

 

I do not know what they are using for the belt in the current CVTs. Would need to be strong to keep torque from the engine from burning up the drive belt.

so are there no gears as in my atv there's a high and a low gear the faster you go the higher the rpm what would lower the rpm at high speeds would it still shift gears
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so are there no gears as in my atv there's a high and a low gear the faster you go the higher the rpm what would lower the rpm at high speeds would it still shift gears

 

Have you ever looked under the hood of a snowmobile? There is a belt drive that is running inside an adjustable width pulley, as the pulley gets wider, the belt runs in a lower sized diameter pulley. The pulley is a v-shaped pulley, when completely squeezed together, the belt will run at near the very top of the pulley(it is now a large diameter pulley) The pulley does not just "snap" to the next setting, it is applied basically by centrifugal force acting on weights that move levers to either close the pulley up, or to open them apart. There are many images on internet showing the basic functions. Take a look at a snowmobile belt drive to see a very basic system.

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I have a can am outlander it's the same set up but when I'm running 15 20 mph it's about 2k run it on up to 65 70 it's running about 7k rpm that's not going to work for a truck and I know how big the clutch and belt is for my atv how big would have to be for a 6.2 420 hp and 460 ft lb so how else's dose it lower the rpm it has to shift a gear somewhere

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You can be sure that if GM built that, you'd have to remove the cab AND bed in order to change a belt .... at 40k miles ..... :lol: And the belt will be $950 ....

 

I've got an '05 Polaris Sportsman 800 - when you stab the throttle from a dead stop (trying not to flip over backwards ....), it flashes up to around 4,500 -5k and stays there until about 50mph or so, then it climbs to about 6k on the top end around 65-70. Timing retard rev limit kicks in at 6,300 rpm, and fuel cuts at 6,500.

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Have you ever looked under the hood of a snowmobile? There is a belt drive that is running inside an adjustable width pulley, as the pulley gets wider, the belt runs in a lower sized diameter pulley. The pulley is a v-shaped pulley, when completely squeezed together, the belt will run at near the very top of the pulley(it is now a large diameter pulley) The pulley does not just "snap" to the next setting, it is applied basically by centrifugal force acting on weights that move levers to either close the pulley up, or to open them apart. There are many images on internet showing the basic functions. Take a look at a snowmobile belt drive to see a very basic system.

 

On snowmobiles, it's a great system as it's light weight, very tunable (via the clutch weights) and easy to work on. The problem we run into in vehicles is that the CVT's are very torque adverse (ie, have durability issues when dealing with alot of torque output).

 

For snowmobiles that have actual low/high gearboxes as well (Skidoo Skandic for example), they still have the CVT, but the crank case includes an additional gear box that handles the manual gear ratios and reverse if it isn't an electronic system.

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The reference to the snowmobile was intended to give you concept of how a CVT works. It is not done exactly the same way.

The CVT can also be controlled electronically. If you want to accelerate, you just press the accelerator and the CVT will configure itself to put the engine at the best rpm to accomplish what you want it to do. Same for pulling away from a stop. The engine will flash to a certain RPM and hold that rpm until the vehicle catches up. It is an entirely different feeling, and is hard to describe to people that have never felt how a snowmobile or a go cart operates. The snowmobile and go cart are not really cvts in the same sense as cars are, but, they provide an easy way to describe to those that have them.

 

Wikipedia has a lot of information about CVTs. http://en.wikipedia.org/wiki/Continuously_variable_transmission

 

CVTs in automobiles still need some form of a gear type transmission, if you plan on backing up.

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Another couple of examples of Constantly changing Transmissions was the Dynaflow transmissions from the 1950's and 1960's They would never shift a gear, it was all done through hydrolics. Unfortunately they were not very efficient. Another was the 2 speed trans that Chevy used around that same time... the name escapes me but they were popular with drag cars.

 

 

Definitely!

 

Really, the deciding factors for your highway mileage is RPM and aerodynamics. You want the engine to run at the lowest RPM possible, but at the same time, you need to balance the engine torque to make sure that you can actually resist the aerodynamic drag without using excessive throttle. But you only need one gear to accomplish this.

 

You really only need two gears...one to get rolling, and one for cruising. Everything else in between is to help keep the engine at peak hp/tq when driving in scenarios that aren't optimal for those two gears (accelerating, towing, passing, etc).

 

True to a point, any cammed engine has an optimum efficency point, typically they shoot for around 1700 - 2000 RPM. Some of the new engines with V.V.T. have a little better range of power and efficency, but there are still limitations. In an ideal world, you create an engine that will run at a specific optimum RPM for economy that produces enough power, and then you transfer that power to the wheels via electric motors... The down side is every time you take the power from one source, and convert it to another, say from Gasoline into electricity you get losses because of heat. And the price of the vehicle would go up from complexity (just look at the Volt)

 

Another point to be made is that there is a third factor of weight that plays into the mileage, even at highway speeds....

 

I agree the only point that an 8 speed trans would be of benefit would be in the city...

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Another couple of examples of Constantly changing Transmissions was the Dynaflow transmissions from the 1950's and 1960's They would never shift a gear, it was all done through hydrolics. Unfortunately they were not very efficient. Another was the 2 speed trans that Chevy used around that same time... the name escapes me but they were popular with drag cars.

 

 

True to a point, any cammed engine has an optimum efficency point, typically they shoot for around 1700 - 2000 RPM. Some of the new engines with V.V.T. have a little better range of power and efficency, but there are still limitations. In an ideal world, you create an engine that will run at a specific optimum RPM for economy that produces enough power, and then you transfer that power to the wheels via electric motors... The down side is every time you take the power from one source, and convert it to another, say from Gasoline into electricity you get losses because of heat. And the price of the vehicle would go up from complexity (just look at the Volt)

 

Another point to be made is that there is a third factor of weight that plays into the mileage, even at highway speeds....

 

I agree the only point that an 8 speed trans would be of benefit would be in the city...

 

The losses are not due to heat, they are mostly due to friction. The torque converter is not very efficient at transferring power which is why they lock them up as early as they can now. The Dynaflow is a good example of how inefficient they were. Torque converters give a mechanical advantage of somewhere between 1.5 and 2.5 to 1. The advantage drops as the converter nears rotary flow. It can not ever get to be 1:1 without some mechanical intervention.

 

If we were to only need to do 0mph and 65mph, we could do that with just a lock up converter, and some specific rear axle gears. Large locomotives have been showing us how to be the most efficient for decades. Too bad we don't live in that world.

 

Powerglide transmissions are what you were referring to I think. They were popular mostly due to simplicity, and only having to change gears once. They can be built to take huge power, and they use the torque multiplication of the torque converter over a longer timeframe. They also had less parasitic losses. Whether or not that was even an issue I don't know.

 

Something interesting about the powerglide transmission though is that if you wanted an automatic transmission in any pre 68 Corvette, regardless of engine, you got a powerglide. This was not because the transmission was the best, it was simply because there wasn't enough room in the tunnel for the three speed transmission.

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I want a 2-stroke V8 with a 8 speed MANUAL. :D I won't hold my breath on that one ...

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  • 5 weeks later...

 

The losses are not due to heat, they are mostly due to friction. The torque converter is not very efficient at transferring power which is why they lock them up as early as they can now. The Dynaflow is a good example of how inefficient they were. Torque converters give a mechanical advantage of somewhere between 1.5 and 2.5 to 1. The advantage drops as the converter nears rotary flow. It can not ever get to be 1:1 without some mechanical intervention.

 

Technically you are correct, Friction is how the loss occurrs, however the measurable end result of loss is still through heat, Remember Science 101, Energy cannot be made or destroied. For simplicity sake it can be stated that there are four basic forms of energy, 1) motion, 2) heat, 3) light, 4) sound, (Technically I could argue that both light and sound are also forms of motion by waves but I will not go into that as it would confuse everyone) and then there is Potential energy which is the stored energy ready to be used. The end result of the friction is heat, which is where you lose your energy. Friction may cause the heat, but the heat is the measureable loss, just like a combustion engine, what is not used in the combustion process of crating motion in the piston is lost in heat, sound and light, which happens to be 80% of the energy lost IIRC...

 

If you could create a transmission that did not create heat, you would not need transmission fluid to cool it.

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