Continuisly variable transmission

[EricZ103]

ok so I read this thread http://www.nissaninfiniticlub....35383 and few others. I think have the whole horspower and torque ideas figured out. I was thinking about it though and tell me if this makes sense, or if I'm just stupid. In a car with a CVT( I know audi makes one) horsepower isn't important at all. Becuase theoretically when you have the car floored the rpms should always be at the peak torque for maximum accerlration, revving beyond that would be pointless and would actually harm performance. There is no jump(shifting) in the gear ratio where the mechanical advantage of shorter gears from the ability rev, because you are always at the ideal ratio. Therefore horspwoer is a worthless nummber for a car with a CVT. make sense, It did in my head I just have a hard time explaining it. I'm just lowly high school student and some times get lost your in guys techinacal nerdy explanations, but I read this board alot, and I think I'm learning from it :thinker also I wonder if they have a tach as an option for cars with a cvt

[MrFox]

Becuase theoretically when you have the car floored the rpms should always be at the peak torque for maximum accerlration, revving beyond that would be pointless and would actually harm performance. There is no jump(shifting) in the gear ratio where the mechanical advantage of shorter gears from the ability rev, because you are always at the ideal ratio.

Correct :thumbup

Therefore horspwoer is a worthless nummber for a car with a CVT.

Not quite...

Power is torque with rotation. Torque without rotation can't move anything. Get a 100 lb kid to stand on the end of a 10 ft diving board, and he has created 1000 ft-lbs of torque at the base of the diving board. The diving board isn't about going to go anywhere though.

We are used to engines where the torque curve peaks before the power curve peaks. The only reason peak torque is lower than peak power in cars is because they must operate with discreet gear ratios, and thus require some kind of power band in order for them to actually have some range of engine speed to accelerate in each gear. In a CVT, there is no need for such a power band. In a CVT, the engine's peak torque and peak power RPMs are designed to be equal. The engine will have absolutely no low end grunt, but CVTs don't care about low end grunt because it never needs to let the RPMs drop for gear changes.

I'm just lowly high school student and some times get lost your in guys techinacal nerdy explanations

Hey... ! What do u mean nerdy?:slap

[maxnix]

Hence why diesels are a good match for CVT. Narrow power band with almost no pumping losses at most efficient rpm (WOT).

[s13sr20chris]

Get a 100 lb kid to stand on the end of a 10 ft diving board, and he has created 1000 ft-lbs of torque at the base of the diving board. The diving board isn't about going to go anywhere though.

an excellent example. i had not even thought of it that way. thanks.

[s13sr20chris]

Hence why diesels are a good match for CVT. Narrow power band with almost no pumping losses at most efficient rpm (WOT).

i was thinking that a two stroke would love a cvt. they have very narrow powerbands.

[maxnix]

i was thinking that a two stroke would love a cvt. they have very narrow powerbands.

And are very dirty, emissions wise. They are much more flexible than diesels.__________________Brian1995 Q45 & Q45t & 2000 Q45

Discover the power of the button!

[EricZ103]

Alright, thanks for the help. and i was just joking about the nerd part. I still not sure I sure comprendo 100% though.

Quote »Power is torque with rotation. Torque without rotation can't move anything. Get a 100 lb kid to stand on the end of a 10 ft diving board, and he has created 1000 ft-lbs of torque at the base of the diving board. The diving board isn't about going to go anywhere though.[/quote]

I know they will still create horsepower, but is it a number that is actually important in how fast the car is going to accelerate? If you already answered this I'm sorry, I must not be understanding any of this at all.

Quote »We are used to engines where the torque curve peaks before the power curve peaks. The only reason peak torque is lower than peak power in cars is because they must operate with discreet gear ratios, and thus require some kind of power band in order for them to actually have some range of engine speed to accelerate in each gear. In a CVT, there is no need for such a power band. In a CVT, the engine's peak torque and peak power RPMs are designed to be equal. The engine will have absolutely no low end grunt, but CVTs don't care about low end grunt because it never needs to let the RPMs drop for gear changes. [/quote]

Won't peak torque alwasy occur before peak horsepower, due to the mathematics .

And one last question.... will a car(with a cvt and all other factors equal) that makes a peak torque of say 150lb/ft at 3000rpm accelerate at the same rate of a car that 150lb/ft at 6000rpm. The way I'm comprending it the second example would accerlate faster becuase it could better manipulate gears. Or am I wrong on this too. Should even compare rpms when talking at about torque, since that would be just like comparing horsepower I dunno I'm confusing myself, any help would be appreciated.:help

[s13sr20chris]

i am no authority here, but let me suppose something for you. if i am wrong one of out engineer buddies will chime in. im thinking that with a cvt peak torque will be the most important factor. it should be more important than where it occurs or what hp you get from it. with a reg trans hp is mega important, but with a cvt i cant imagine that it would play as serious a role as peak torque. i get this because with a cvt you can stay at peak torque all the time by manipulating gear ratios. peak torque producing peak acceleration thereby negating the effect of staying in gear longer and hp in one fell swoop. this is not counting what mr fox pointed out above about torque not doing anything without rotation.

[MrFox]

[quote=" EricZ103 Won't peak torque alwasy occur before peak horsepower, due to the mathematics . [/quote]

You are 99.99% correct... peak torque alwasy occur before or AT peak horsepower, due to the mathematics.

I know they will still create horsepower, but is it a number that is actually important in how fast the car is going to accelerate?

You've figured it out yourself already....

[quote=" EricZ103 ...will a car(with a cvt and all other factors equal) that makes a peak torque of say 150lb/ft at 3000rpm accelerate at the same rate of a car that 150lb/ft at 6000rpm. The way I'm comprending it the second example would accerlate faster becuase it could better manipulate gears. Or am I wrong on this too. Should even compare rpms when talking at about torque, since that would be just like comparing horsepower [/quote]

Nope, you've got it. :thumbup

To put it in a slightly different way:

If gear ratios are constant (our original in-gear acceleration example from the other thread) and torque is variable, then maximum acceleration occurs at peak torque.

On the other hand, if gear ratios are infinitely variable (CVT), then maximum acceleration occurs at peak power. However, peak power in a given engine can be maximized if peak torque occurs at the same RPM.

[MrFox]

im thinking that with a cvt peak torque will be the most important factor. it should be more important than where it occurs or what hp you get from it.

You are neglecting the gear train's mechanical advantage. Lets illustrate by an extreme example:

Imaginary engine Mk1

Peak torque: 1000 ft lb @ 1000 RPM (Which gives it 190 hp @ 1000 RPM)

Peak power: 200 hp @ 10000 RPM(Which gives it 105 ft lb @ 10000 RPM)

This engine drives a car equipped with a CVT and 24" diameter tires. At 60mph, the wheel needs to rotate at 776 RPM.

At 1000 engine RPMThe required reduction ratio = 1000/776 = 1.29Rear wheel torque = 1000 ft lb * 1.29 = 1290 ft lbRear wheel thrust = 1290 ft lb / 1 ft = 1290 lb

At 10000 engine RPMReduction ratio = 10000/776 = 12.9Rear wheel torque = 105 ft lb * 12.9 = 1355 ft lbRear wheel thrust = 1355 ft lb / 1 ft = 1355 lb

[s13sr20chris]

ill be dagon'd.(sp?) i completely forgot about that. it still helps to be spinning faster. thanks bro.

[IvoryJ30t]

actually, if you have two cars with cvt's ,same weight, different engines, say one has 200 lb/ft @ 5000 rpm, and the other has 200 lb/ft @ 7000 rpm the second car will accelerate better due to the fact it has a higher horspower output.

also, if you have two identical cvt cars, say with a torque peak of 5000 rpm, and a power peak of 6500 rpm, if car number 1 holds the rpms at 5000 while car 2 hold the revs at 6500 rpm, the second car will win.

optimally, you want full throttle acceleration at hp peak, not torque peak. in an optimal gearbox layout, you want to shift at hp peak, and land on torque peak.

say, if the second example cars had a manual transmission, you would want the gear ratio spaced so that when you shifted at or slightly later than the hp peak of 6500rpm, you would want the next gear to land the revs at 5000 rpm. obviously this is hypothetical, but what would the advantage of a cvt holding the revs at torque peak be as opposed to hp peak? nothing, unless you were towing something. and then it would still be benificial to hold the revs at hp peak after the vehicle has accellerated to a reasonable speed.

[EricZ103]

Quote »actually, if you have two cars with cvt's ,same weight, different engines, say one has 200 lb/ft @ 5000 rpm, and the other has 200 lb/ft @ 7000 rpm the second car will accelerate better due to the fact it has a higher horspower output.[/quote]

That what I was saying, but I was trying to sya the Hp. number isn't improtant, because you could just look at peak tq. + rpms and see which one was faster.

Quote »also, if you have two identical cvt cars, say with a torque peak of 5000 rpm, and a power peak of 6500 rpm, if car number 1 holds the rpms at 5000 while car 2 hold the revs at 6500 rpm, the second car will win.[/quote]

I'm confident enough to say your wrong on this one. Read the thread I reffered to in my first post. Your feel smarter after it, well at least I did.