Downshift to accelerate

I have an automatic transmission and I was just wondering why it is necessary for the engine to downshift when I want to accelerate to pass someone on the freeway. Why can’t the engine just stay in the same gear while revving higher to produce the extra power necessary to accelerate? Why does it downshift under hard acceleration?

Because all engines have an RPM range where they produce more power than any other time. The transmission’s job is to try to keep the engine in that optimum RPM range when you’re asking for more power, like when you put your foot down.

Think of it this way - -the car could stay in the higher gear when you accelerate, but it has no way of knowing if you’re flooring it just to get ahead of someone, or if you’re flooring it to avoid the dump truck that’s about to rear-end you. It assumes when you floor it, you want the vehicle to accelerate as fast as it’s capable of, and therefore you need the downshift.

Okay thanks. So what you’re saying is that downshifting will allow the vehicle to accelerate as fast as possible and if it were to stay in the higher gear it would not accelerate as quickly if I stomped on the gas pedal? Why is it that a lower gear is able to accelerate the vehicle faster? I am just a little confused because I thought that a lower gear equals slower speed and shifting down a gear in order to go faster is just a little counterintuitive to me. Thanks for your help.

Have you ever driven a manual?

A lower gear = slower speed at a given engine RPM, but you’re not asking about maximum speed in gear, you’re asking about acceleration.

Almost all vehicles accelerate more quickly in lower gears than in higher gears. They go FASTER in the higher gears, but the rate of acceleration decreases.

I said “almost” all vehicles. That means most normal vehicles you’re likely to encounter on the public roads.

If I’m cruising along in my manual transmission car, say at 55 mph, and I want to accelerate, I can step on the gas and remain in high gear, or I can downshift and step on the gas. If I remain in high gear the car will accelerate, but not very quickly. If I want to accelerate quickly I will have to downshift one or two gears to get into the engine’s power band, which is usually at higher revs.

Your automatic transmission does the downshifting for you. Automatically.

I will add a little more. The engine will put out the most “power” at a specific RPM. It will deliver the best efficiency at a different speed. Those two factors work together. Your automatic transmission tries to pick the best combination for the conditions (your speed, wind, hills, the throttle position (your telling it to speed up, maintain speed or slow down)

Generally modern transmission do a very good job of guessing what you want to provide it for you.

Acceleration is proportional to torque at the driven wheels.
In lower gears the engine torque is multiplied by a larger factor (gear ratio) than in higher gears.
So if maximum engine torque were the same at different RPMs a lower gear could deliver more torque at the wheels.

Many types of electric motors have a virtually flat torque curve across their operating range.
Gas motors generally have less torque at low RPMs then rising to a peak or series of peaks at mid speed and falling at high speed.
So a downshift from low to mid speed can make a great increase in available acceleration because of the double advantage of greater engine torque and higher torque multiplication to the wheels.

Have you ever driven a manual?

Or tried riding a 10-speed bicycle up a hill in high gear?

Many types of electric motors have a nearly flat HORSEPOWER curve. Gasoline engines usually have a roughly flat TORQUE curve across a good bit of their operating range. Horsepower is a combination of torque and RPM, thus HP goes up as RPM rises, until the engine reaches an RPM where it either can’t go any faster or can’t breath well enough to produce sufficient torque.

Horsepower is what is needed for acceleration, and RPM is needed for horsepower…To gain RPM which allows the engine to produce MUCH more horsepower, the transmission must downshift…

"Have you ever looked at the specs of an engine in a magazine and seen something like “this engine makes 300 pound-feet of torque at 4,000 RPM,” and wondered how much power that was? How much horsepower are we talking about here? You can calculate how many foot-pounds of horsepower this engine produces using a common equation:
(Torque x Engine speed) / 5,252 = Horsepower

The engine that makes 300 pound-feet of torque at 4,000 RPM produces [(300 x 4,000) / 5,252] 228 horsepower at 4,000 RPM."

Now to see how important RPM is, in the above example, increase the RPM from 4000 to 5000, leaving the torque the same, and work the equation again…

“Many types of electric motors have a nearly flat HORSEPOWER curve.”

With permanent magnet DC motors used in electric vehicles the output torque is proportional to the armature current.
If the current is held constant (the way most variable motor controllers operate) the torque vs RPM curve will be virtually flat.
Full torque is available at near-zero speed.
How about full torque from 100-6000 RPM? Compare that to a gas engine.

In order to keep the current constant as RPMs rise the armature voltage must increase to overcome back EMF.
Since Power = Voltage X Current the input and output Power rise with increasing RPM.
So the [horse]Power vs RPM curve slopes up with an electric motor driven with a constant current

This might sound radical, but horsepower does not directly cause acceleration.

Consider a car taking off from a standstill.
In that first instant the brakes are released the driving wheels are not turning, but they are tugging at the ground and pushing the car in the forward direction.
HorsePower at the wheels = (Torque x wheel speed) / 5,252 = 0
So in that first instant it’s all about torque, not horsepower.

Newton’s Second Law: F = M X A, Force = Mass X Acceleration, so A = F / M

The Force pushing the car forward is the Torque at the wheels / wheel Radius. F = T / R

So, ignoring rolling and wind resistance A = T / (M X R)

Years ago I was stuck on a ski chairlift when the power went out. Since they couldn’t’ use the normal electric motor (which puts out full torque from a standstill), they had to rely on their backup motor (a gasoline Ford 6-cylinder engine).

I was about 3 chairs from the base station where the 6-cyl engine and operators were. The 6-cyl was connected to the chairlift through a manual clutch (similar to one in a car). It took the operators several minutes of rev’ing that 6-cyl engine to incredibly high RPMs, with the 6-cyl engine stalling each time from the load. Finally, after heavily riding the clutch while rev’ing the engine, they were able to get the chairlift to crawl.

Watching that happen clearly taught me the benefit of having full torque at a standstill from an electric motor.

If you are on the freeway and using it correctly you should not need to accelerate to pass anyone.

Variations will occur depending on traffic conditions, but just get on the freeway. Put yourself in the right lane, pick the speed you want to do and do it (cruise control is very handy). When you are approaching someone going a slower speed, pull into the left lane, maintain your speed, and when you can see both of their headlights in your rearview (rule of thumb) pull back into the right lane … all the while your speed shouldn’t change.

Do this for a while and you’ll notice that a lot of traffic congestion on freeways, when it occurs, comes from people doing weird things like inexplicably varying their speeds, not using the left lane for passing only, etc.

As I said, traffic conditions can change that, but if you do want to accelerate without the downshift just do it a lot more gradually. The downshift comes largely from a change in the throttle position - which is under your control.

As far as why the downshift gives better acceleration, you’ve already gotten that answer.

In addition to all of the good information that’s been provided, I would like to add something.

Drivers of manual transmissions do the same thing, but it usually only makes sense if you are driving something with a lot of torque, like a Ford Mustang. Take a look at how Burt Reynolds drives his girlfriend’s Maserati in the movie The Longest Yard. He appears to downshift a lot.