Someone explain this

[darylzero]

So I saw this story that Automobile Magazine on Saturday named the 2014 Chevrolet Corvette Stingray its "Automobile of the Year."

It's powered by a 6.2-liter V8 engine that can put out as much as 460 horsepower but that can also get 29 miles per gallon in highway cruising. This thing can get 29MPG with a 6.2L V8!! Yet new cars with CVT's get only a few mpg better at say 33mpg @170hp (2014 Rogue 4 cylinder)/ Maxima gets 26MPG.
So wasn't the whole point of the CVT to get better fuel economy?

[krash]

Well, you have 460lb-ft/455hp moving a 3,300lb car vs 175lb-ft/170hp moving a 3,500lb car.

And heres the thing, CVTs are actually not BAD. Theoretically they're awesome. It keeps the motor at its optimal RPM for power and infinitely adjusts gears for that. We use one on our Baja car, its a good application. However, Nissan is not building that CVT for awesome, and CVTs are just not fun to drive at all lol.

[gwoods]

My sis has a 13 Altima 2.5 CVT and she does not drive like me and gets 32-35 mpg driving from West Phoenix into downtown during rush hour both ways. We drove a whole buncha under 20k cars before she bought the Altima and it was the fastest with the best steering. Fastest was gauged by MPH from stop to metering point on FWY onramp. Best steering was gauged by me and a terrified salesman in a abandoned parking lot.

Not everyone is an enthusiest and the CVT does do many things well.... I would not own one

[darylzero]

Well, you have 460lb-ft/455hp moving a 3,300lb car vs 175lb-ft/170hp moving a 3,500lb car.

And heres the thing, CVTs are actually not BAD. Theoretically they're awesome. It keeps the motor at its optimal RPM for power and infinitely adjusts gears for that. We use one on our Baja car, its a good application. However, Nissan is not building that CVT for awesome, and CVTs are just not fun to drive at all lol.

Thanks krash, I understand that CVT's are not bad per se, but they do have their issues and as you said not fun to drive. Nissan CVT's have to use only Nissan NS-2/3 fluid and needs to be filled perfectly otherwise fail safe can happen. It needs to be at a specific temperature or else fail safe happens.
I just cannot wrap my head around how a car with SO much HP can get almost as good if not better mpg with a regular transmission than one with a CVT, which as you said can operate at infinite gear ratios? Doesn't a V8 use more gas than a V4?
I remember growing up and people saying if they could design a transmission that had a lot more gears, or ratio's, cars would get much better mpg and save a lot of fuel.
I assume I am missing something about Torque since you mentioned it...

Reading it on Wikipedia it explains a transmission as:

Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox rotates at a slower rate than the input shaft, and this reduction in speed produces a mechanical advantage, increasing torque. A gearbox can be set up to do the opposite and provide an increase in shaft speed with a reduction of torque. Some of the simplest gearboxes merely change the physical direction of power transmission.

Many typical automobile transmissions include the ability to select one of several different gear ratios. In this case, most of the gear ratios (often simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gears may be "overdrive" types that increase the output speed.

Torque:

Torque is part of the basic specification of an engine: the power output of an engine is expressed as its torque multiplied by its rotational speed of the axis. Internal-combustion engines produce useful torque only over a limited range of rotational speeds (typically from around 1,000–6,000 rpm for a small car). The varying torque output over that range can be measured with a dynamometer, and shown as a torque curve.

I can read those definitions but still don't grasp it for some reason.

[Kompresshun]

Probably the most basic definition of why is this: Torque

The torque and horsepower of a V8 engine take A LOT less effort to move the vehicle along, where a V6 or a 4cyl have to work a lot harder to move the vehicle. Then you combine that with the fact that a V8 engine can also be geared at a lower gear ratio, which causes the engine to run lower RPM's in higher gears and that pretty much sums up the basics of why it does produce better fuel economy.

For example: The 6.2L V8 in the Corvette combined with the 7spd manual(or probably even the automatic) will probably run somewhere between 1,500-1,800 RPM while in it's highest gear cruising. On the other hand a V6 in the same conditions would probably be running around 1,900-2,600 RPM and a 4cyl would probably be between 2,600-3,300 RPM in most cases roughly. So the engine is already working harder, even at cruising speed to keep the vehicle in motion.

It doesn't stop there though, because under those same conditions you could accelerate the V8 engine without a gear change(or a minimal downshift in the automatic) since there is plenty of power/torque on tap to help without consuming much fuel to accomplish it.

On the other hand though, most V6's and 4cyls would require downshifting to accelerate at the same rate because the engine doesn't have the power/torque available in most cases to accomplish it. Which in turn would cause it to consume more fuel because it's having to work much harder than the V8. Even turbocharged engines will consume more in most cases, because while the boost may help it accelerate more easily, the engine will still consume more fuel.

Probably not the best explanation, but that's how I sum it up.

MoD has went on a rant many times about how V8 engines are far more efficient than they are given credit for. Small and boosted isn't always the solution.

Oh yeah and CVT's are dumb. I have no defense for them.

[Looneybomber]

CVT's work great in golf carts!

Torquey engines can run lower rpms. Fewer rpms means less parasitic drag. Less parasitic drag means increased efficiency. Drive that same 6.2L down the highway in 4th vs 7th and MPG will drop a lot. Yes there's a lot more to it than that, for example, engine tune isn't taking into account, but you get the idea.

Now that vette with a CVT might get equal MPG on the highway. It all depends on what kind of losses there are in the transmission (not much in a manual) and gear ratios.

Oh, and the vette is way more aerodynamic than a rogue and also lighter.

[darylzero]

Ok that helps. So why not put a 6 cylinder in something like a Rogue instead of a 4 cylinder? Cost I am guessing? But if the 6 cylinder in a Rogue wouldn't need to change gears as much it should get even better gas mileage correct? Thus offsetting the cost of the 6 cylinder and the added benefit of more torque.

Thanks everyone

[RicerX]

Two things - power to weight ratio and gearbox configuration. The final drive ratio matters in the equation very much. I can't remember the exact final drive ratio, but I remember an Autoblog test of the 3.5 Altima (2013) and its revised CVT allowed them to cruise at about 3,200 RPM at 100 MPH (NUTS). My 6MT 370Z versus my 6MT 3.5 Altima Coupe is another good example. The Z, despite having higher displacement and 62 more horsepower, revved about 500 RPM higher in top gear than the Altima Coupe when driving both at around 75 MPH.

The aerodynamics and actual vehicle weight come into play after that as far as maintaining speeds go. More drag means more throttle to maintain cruising speed, therefore pushing down on the fuel economy.

[PapaSmurf2k3]

Aero and engine/trans/driveline efficiency play the biggest roles in highway MPG. Weight plays hardly any role during uniform linear motion... basically it just slightly changes the friction from the tires to the road, which is quite small compared to air friction.

You also forget that the C7 in "eco mode" cuts down to 4 cylinders. The sad part is, I'm sure the C7 gets even better MPG at higher speeds when the engine is more in its optimal cruising RPM. I know the C6 I took to NICOfest Omaha was getting 32.5 MPG on the highway from 70-80mph. Most EPA tests are done for 55-60 mph.

Yes, the Corvette is just that good. Its also more expensive

[nissangirl74]

You get what you pay for.