If modern engines have flat torque curves...

[Burt]

Why do they still need to routinely downshift during acceleration?

My Logic: Torque is necessary for acceleration, horsepower is required to maintain speed.

So why does my car need to downshift to increase rpm's from 2000 to 2500 during moderate acceleration if it makes the same torque? Wouldn't it be more fuel efficient to use some of the available torque at 2,000 rpm? Is there more wear on the bearings at 2,000 rpm than at 2,500 rpm?

Also seems like these 'sport' settings on auto transmissions, that change the shift points slightly, are pointless, too.

 

[Nick R]

Because even though torque is used in acceleration, horsepower is important too, and an engine makes much less power at 2,000 vs 2,500 vs 5,000. Also as long as your lubrication system is working properly you should have no extra wear on the bearings at 4,000RPM vs 2,000RPM.

 

[Rand]

hp is just a made up number its torque x rpm / 5252


its all about torque and how much is available when.

also while counter-intuitive.. its possible in some situations to have less wear at higher rpm because of bearing load etc.

 

[NateDN10]

There may be some NVH concerns as well, or it may just be unlocking the TC in preparation for the possibility of higher throttle?

ATs have to guess what you're going to do, so their decisions don't always work out with what you actually wind up doing.

Some people complain if an AT is reluctant to downshift, others complain if it downshifts too readily! Best course of action? Get a car with a manual transmission!

 

[1999nick]

More power is necessary for climbing hills and acceleration. Downshifting from a typical .70 overdrive transmission ratio back to a 1.00 multiplies the available torque by about 30% at the transmission output shaft.

 

[TFB1]

When you are talking about a 600-5000RPM operating range, there is no such thing as a flat torque curve, especially in a naturally aspirated engine...

 

[TallPaul]

Originally Posted By: Rand
hp is just a made up number its torque x rpm / 5252

its all about torque and how much is available when.
Yep. I know that from experience. My old F150 4.9L had gobs of torque off the line. Now it is a different experience with the Ranger 2.3L, very slow off the line. Ah but I do like the 26+ mpg over the 4.9s 14 mpg.

If you don't have big displacement (which usually means low end torque) you need to increase the RPMs.

 

[supton]

Horsepower is a measure of work done per time. More HP means more work getting done in the same amount of time. Torque is a measurement of applied force. It is possible to apply lots of torque and do zero work (think of trying to turn a rusted bolt--lots of force but if it doesn't turn then no work was done).

Don't forget, even with a flat torque curve, a downshift means more torque, as the lower gear ratio of the transmission means more torque gets applied to the wheels. If your engine was turning 2k and a downshift caused it to go to 4k, then you (potentially) doubled the amount of torque turning the wheels. The engine torque may be unchanged, but the torque to the wheels changed drastically.

When it comes to discussions about old school motors accelerating off the line faster, yes, they had more off-idle torque. Which means they had more off-idle horsepower too. You don't have one without the other.

 

[bepperb]

Even if it had a perfectly flat torque curve you would still get more acceleration from a lower gear not because of the higher RPM of the engine but because of the torque multiplying effects of the gearing, which vary from gear to gear.

 

[Burt]

Originally Posted By: bepperb
Even if it had a perfectly flat torque curve you would still get more acceleration from a lower gear not because of the higher RPM of the engine but because of the torque multiplying effects of the gearing, which vary from gear to gear.


Ok, finally an answer that makes sense. If I am on a bike and I push equally hard, the bike accelerates more quickly in low gear than in high gear. So the gearing multiplies the torque, just as a pulley multiplies force.

Got it.

 

[mechanicx]

Originally Posted By: bepperb
Even if it had a perfectly flat torque curve you would still get more acceleration from a lower gear not because of the higher RPM of the engine but because of the torque multiplying effects of the gearing, which vary from gear to gear.


+1. Torque at the wheels is needed for acceleration. And more reduction gearing increases torque.

You can also almost think of HP as related to gearing. With the same engine torque, same gearing and 2x engine RPM (double the HP)you would roughly double your output torque.

 

[NHGUY]

My 88 Aries accelerates from around 40mpg to highway 65 mph without a downshift.Of course its a 3spd not an overdrive,but the combination of engine torque and reletively light weight seems to do the job.Chrysler geared 1st and 2nd pretty low anyway so if it did downshift it would rev excessively with little to no gained speed.While that 40-65 is no lightning bolt,it does keep up with the merging traffic ahead of me,so its no slouch or "slug" in that respect.Also,K wagons had 3.02 gearing,not the 2.78 that coupes and sedans had with the 2.2 engine.

 

[Scott_Tucker]

Originally Posted By: Burt


My Logic: Torque is necessary for acceleration, horsepower is required to maintain speed.



You've got it backward. Dropping to lower gear increases torque at the wheels due to the gearing.

 

[Hokiefyd]

Originally Posted By: supton
Horsepower is a measure of work done per time. More HP means more work getting done in the same amount of time. Torque is a measurement of applied force. It is possible to apply lots of torque and do zero work (think of trying to turn a rusted bolt--lots of force but if it doesn't turn then no work was done).


This is the bottom line. Torque is not a measure of work; it is simply a measure of force applied. Horsepower is the measure of work we are interested in, because it includes the element of engine speed (or speed per time).

It is possible for an engine to continue at a constant rate of acceleration as it increases in speed, even if its torque peak may have occurred many thousands of revolutions per minute ago. This is because though the torque may be decreasing, engine speed is increasing, so the measure of work being done (horsepower) may be the same. It's also possible for an engine to accelerate harder as its speed increases. As long as the torque curve remains flat, if engine speed increases, horsepower (and work produced) necessarily increases as well.

Torque means nothing without engine speed. As stated before, you can apply 200 pound-feet of torque onto a bolt and do absolutely zero work. Same is true in a vehicle. Electric motors produce peak torque at zero RPM. They may have torque, but if they're not spinning (IE, moving a vehicle), no work is being done.

This is why downshifting is sometimes necessary, even to simply maintain speed on a long hill. It may have good torque at that speed, but it can do more work at a higher engine speed if the torque curve is flat.

 

[mechanicx]

Another very simplified example of the usefulness of HP and gearing.

For example with a flat TQ curve:

100 lb ft TQ at 2000 RPM 38 hp 1:1 gear ratio=100 lb ft TQ

dropped down to 2:1 gear ratio with the same TQ

100 lb ft TQ at 4000 RPM 76 HP 2:1 gear ratio=200 lb ft TQ


With the same engine TQ, gear reduction doubling engine speed you doubled HP and doubled output TQ. HP, TQ, RPM and gearing are very related.

 

[mechtech2]

A car downshifts to get higher torque from better gearing in the lower gears.