CVT Question

I have a basic question about general torque power and CVT transmissions. Does having a CVT mean that the vehicle can have a smaller engine and maintain the same power? For example if car had an
automatic transmission and 3.5 liter V6 engine, and the same model car had a CVT and 3.0 V6, would the CVT compensate for the smaller engine at all? If so, what percentage?
Thanks in advance! JJW
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Sort of. When a manufacturer says that an engine has 200 HP they mean 200 HP at a certain RPM, for instance 6000 RPM. In a non-CVT gearbox, the engine can't be kept at 6000 RPM all the time.
So, for instance, suppose that going from 2nd to 3rd, the RPM would drop from 6000 to 4000. At 4000 the engine would probably produce about 133 HP (to simplify I am assuming the torque would be more or less constant. That won't be generally true, but this is an approximation). When accelerating in that 3rd gear from 4000 to 6000 the engine would produce an average of 166 HP (more simplifications/approximations: (133 + 200)/2 ).
OTOH, a CVT car with the same engine, accelerating (full throttle) at the same speeds would keep the engine at 6000 RPM (this is usually what such gearboxes are programmed to do) so producing always the full 200 HP. If the losses in the transmission are not higher than the conventional gearbox this would mean, in this example, an advantage of about 20%.
The amount of advantage depends a lot on the characteristics of the engine and the non-CVT gearbox. If the engine was very peaky and the gears very separated the CVT advantage would be higher, if the engine was very "torquey" (*) and/or the non-CVT gearbox was "close-ratio", the advantage would be less (a "close-ratio" gearbox loses a bit due to needing more gear changes, but on the whole should be a win since it keeps the engine at higher RPMs).
(* another way of saying that the engine is weak in the top-end)

What are the other characteristics of those engines ? 3.5l is only 16% more displacement than 3.0l, but the increase in power might be much more (or less).

As written above, that percentage would depend on the gear ratios and the torque curve of the engines.
An important point: the above refers to an objective comparision of the CVT versus non-CVT gearbox. In a subjective comparision the CVTs might have a number of inconvenients (since these days their control is computerized, the presence of these problems is a function of the cleverness of the programming):
- one of the reasons why one likes a powerful engine is the ability to cruise at a low rpm with an instant response from the engine when one presses the throttle. CVTs tend to, first, let the engine rev to high RPM and then accelerate. Even if this is faster overall, it might feel slower than the traditional way.
- sometimes one wants to use full throttle at low RPMs. CVTs tend to rev the engine quite high if one uses more than a bit of throttle.
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Thanks to everyone who responded to my questions. I was trying to get a general sense about how effectively CVT's use the HP of a given engine performance-wise versus a traditional non-CVT transmission.
I can see there are a lot of factors to consider.
Best, JJW
On 5/9/2007 1:13 PM, Rui Pedro Mendes Salgueiro wrote:

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Yes, if you want to predict the performance based on the characteristics it is a bit hard. But there is an easier way: just measure it (or for the ordinary costumer, check manufacturer's figures or magazine tests).
There are a number of acceleration figures that are usually published (in Europe, 0-100 km/h, 0-400m and 0-1000m. In the USA I think it is 0-60 (or 62) mph and 1/4 mile.
Also there are figures published for "reprises". That is less standardized but it used to be 40 km/h to 100 km/h, IIRC (I see that Car & Driver tests 30-to-50-mph and 50-to-70-mph). Those are not comparable between automatic (any type) and manual gearboxes, because the downshift done by the auto gearbox is a big advantage, but since you were interested in CVTs versus conventional automatics...
Those figures and a test drive to check if you like the way the CVT works should be enough to form an opinion.
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Depends on whether you truly mean engine horsepower or performance. Engine horsepower does not change with type of transmission. However, a CVT could allow you to use a somewhat less powerful engine and get same average performance.
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To get picky about the wording, it is not entirely correct to say "torque power", as torque is force (measured in pounds if SAE or Newtons if metric) multiplied by a lever arm into rotational force (measured in pound-feet if SAE or Newton-meters if metric). Power is work (horsepower if standard or kilowatts if metric), more like measurements of energy. Its like the difference between measuring how hard you hit something (force or torque) and how much energy is given to something by hitting it over and over again for a period of time (work). One is an instantaneous measurement (torque) and the other is calculated from that and tells how much energy will be produced by that torque over time...
Hopefully that came out halfway comprehensible...
Harry
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wrote:

Another point to ponder in the torque vs HP mindmeld is that you can have torque, such as on the rear axle, even when there is no movement of the axle, but to have power you must have movement.
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Actually, power is work over time. (Or energy over time.) Energy is generally measured in Joules (I don't know if there's any non-metric unit for it.) A Joule is the energy of one Newton applied through a distance of one meter. (Same as one pound through 8.85 inches.) In case you didn't notice, energy and torque have the same units, but very different meanings. (Also, torque is a vector, while energy is a scalar, but let's not go into that.)
A Watt is one Joule per second. One HP is about 746 Watts. This measures how quickly you can produce energy.
Now, for acceleration, what matters is the mass of the vehicle and the net force excerted on it. If we assume zero air resistance, and infinite friction at the wheels, net force is the same as torque at the wheel divided by wheel radius. Let's say A is acceleration, M is mass, F is force, P is power, and S is speed. Then we have A=F/M. Since P=F*distance/time=F*S, we have F=P/S and A=P/(M*S). So the faster you go, the more power you need to maintain the same torque at the wheels and the same acceleration.
So back to the OP's question: I know next to nothing about CVTs in practice, but in theory they should allow you to run the engine at peak power the whole time, which would give you more acceleration than if you could only run at peak power for an instant or 5. Whether a 3.0L with a CVT will accelerate better than a 3.5L with a traditional transmission depends entirely on the characteristics of the engines and transmissions. Basically, for whatever time period you're interested in, take the average HP produced for the car with the normal transmission, and that is how much peak power you'd need from the CVT to get the same average acceleration. (If masses and efficiencies vary, you'd need to account for that too.)
Keep in mind that I said the same *average* acceleration. Whichever car has that acceleration happen earlier will be ahead of the other one, even though they'd both reach the same speed at the same time.
Ivan
Ivan
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