Shift Points for Maximum Acceleration

[blue2000s]

I've seen a few posts on the board asking at what RPM to shift for maximum acceleration. The answer is, for any given vehicle speed, the engine could be making a certain amount of torque to the wheels in each gear. The goal is to use the gear that's making the maximum torque at that speed. It's not just a matter of the engine torque but a combination of engine torque gear ratio and vehicle speed.

To illustrate, and for you curious folks, I've attached a spreadsheet that does all the calculations for you. You need to input tire information and engine torque.

As you can see, at any given speed the wheels are getting the most torque from the engine in the lower gear up to redline up until 4th gear, where there is a slight advantage to shifting into 5th a little before redline at about 114 MPH. 5th to 6th similarly has a very minor shift point benefit before redline.

download and delete the .txt from the file extension, it's a .zip file.

[986Jim]

Except for the fact that this doesn't acctually work in real life.

Try this at the track with Any car, no matter what it is. If you shift where your torque is at the maximum instead of redline you will always run a slower 1/4 mile time. This is a pretty old myth and one that I fell into myself when I first started drag racing.

I have drag raced about 8 different cars in the last 15 years pretty hard core, right into the 10's in a few of them. They have all been to the dyno, and I have tried shifting them all at peak torque like this graph illustrates. If you shift at peak torque you simply go slower every time.

Your always better to shift at your highest RPM point. As the RPM falls back down, it doesn't matter that it's over the Torque peak when you shift, it's that you land in a higher RPM point when you did the shift into the next gear. The higher you land in the RPM band after your shift the faster the engine will accelerate. If you shift at 5500rpm and fall to 3500rpm you car will be slower than shifting at 6500rpm and landing at 4500rpm because the engine will rev easier and faster to 6500rpm again from 4500rpm faster than 3500rpm to 5500rpm regardless of what your torque numbers are on a graph.

The best illustration of this is with a motorcycle. My CBR did 11.4 @ 126mph. It revs to 15,000rpm but peak HP is 13,000rpm, peak torque was 9000rpm or so. It was significantly slower if I shifted at anything less than absolute redline. The difference was a few 1/10ths and would knock me back to high 11's each time I messed with the shift points. Even in first gear, short shifting the bike made it slower. I did it however because I had to worry about poppin the wheel up hittin second without the clutch at 50mph.

Cool graph, but doens't pan out in real life. Don't take my word for it however, if you want proof hit up some hardcore drag racing forums and ask them this there. Their response will be much the same as what I just told you.

[blue2000s]

Originally Posted by 986Jim

Except for the fact that this doesn't acctually work in real life.

Try this at the track with Any car, no matter what it is. If you shift where your torque is at the maximum instead of redline you will always run a slower 1/4 mile time. This is a pretty old myth and one that I fell into myself when I first started drag racing.

I have drag raced about 8 different cars in the last 15 years pretty hard core, right into the 10's in a few of them. They have all been to the dyno, and I have tried shifting them all at peak torque like this graph illustrates. If you shift at peak torque you simply go slower every time.

Your always better to shift at your highest RPM point. As the RPM falls back down, it doesn't matter that it's over the Torque peak when you shift, it's that you land in a higher RPM point when you did the shift into the next gear. The higher you land in the RPM band after your shift the faster the engine will accelerate. If you shift at 5500rpm and fall to 3500rpm you car will be slower than shifting at 6500rpm and landing at 4500rpm because the engine will rev easier and faster to 6500rpm again from 4500rpm faster than 3500rpm to 5500rpm regardless of what your torque numbers are on a graph.

The best illustration of this is with a motorcycle. My CBR did 11.4 @ 126mph. It revs to 15,000rpm but peak HP is 13,000rpm, peak torque was 9000rpm or so. It was significantly slower if I shifted at anything less than absolute redline. The difference was a few 1/10ths and would knock me back to high 11's each time I messed with the shift points. Even in first gear, short shifting the bike made it slower. I did it however because I had to worry about poppin the wheel up hittin second without the clutch at 50mph.

Cool graph, but doens't pan out in real life. Don't take my word for it however, if you want proof hit up some hardcore drag racing forums and ask them this there. Their response will be much the same as what I just told you.

I don't think that you completely understand what the graph is saying. For any given vehicle speed, the torque to the wheels is shown. You can see from the graph that the best place to shift pretty much IS redline. This information is accurate if it's read correctly.

[insite]

redline is usually best. forget torque; you want to shift just after peak POWER, thus allowing revs to fall at or near the peak POWER range.

torque is nothing without motion (work). you can apply torque and never actually move the car (zero work). power describes the rate at which work can be performed (i.e. how quickly can you move a mass of n pounds a distance of x feet). when your car is at peak power, it is performing work at its fastest capable rate.

this is why jim986's real life experience shows him that best acceleration runs occur when you ignore the torque peak and use the power that the almighty RPM helps us generate.

[BuffaloBoxster]

Gnerally speaking any advantage you think you'd gain from short-shifting is negated by the fact that the gear you're going into is further down the powerband.

[blue2000s]

Originally Posted by BuffaloBoxster

Gnerally speaking any advantage you think you'd gain from short-shifting is negated by the fact that the gear you're going into is further down the powerband.

Actually you can shift into a gear that's higher in the power band but still not have a benefit in acceleration because the torque at the wheels is lower than if you'd stayed in the higher gear.

[blue2000s]

Originally Posted by insite

redline is usually best. forget torque; you want to shift just after peak POWER, thus allowing revs to fall at or near the peak POWER range.

torque is nothing without motion (work). you can apply torque and never actually move the car (zero work). power describes the rate at which work can be performed (i.e. how quickly can you move a mass of n pounds a distance of x feet). when your car is at peak power, it is performing work at its fastest capable rate.

this is why jim986's real life experience shows him that best acceleration runs occur when you ignore the torque peak and use the power that the almighty RPM helps us generate.

It doesn't sound like you're quite getting it either. Power and toque are directly related, you can't neglect one or the other. The statement you've made is not accurate in that the calculations of acceleration use torque becuase it's units are more directly converted to the units of acceleration. Make no mistake, it's TORQUE that's commonly used in the calculations for acceleration.

[insite]

Originally Posted by blue2000s

It doesn't sound like you're quite getting it either. Power and toque are directly related, you can't neglect one or the other. The statement you've made is not accurate in that the calculations of acceleration use torque becuase it's units are more directly converted to the units of acceleration. Make no mistake, it's TORQUE that's commonly used in the calculations for acceleration.

not true, you can apply torque without generating power. try applying 90 ft-lb of torque to a bolt that's torqued to 100. it won't move, but you are still applying a torque to it. you are doing no WORK, and are therefore producing no power.

[just_me]

... because the torque multiplication effect of the gearbox works in your favor, since you get the torqgue multiplied by a ahigher number of revolutions equal to the ratio of the two gears).

Grant

Originally Posted by insite

redline is usually best. forget torque; you want to shift just after peak POWER, thus allowing revs to fall at or near the peak POWER range.

torque is nothing without motion (work). you can apply torque and never actually move the car (zero work). power describes the rate at which work can be performed (i.e. how quickly can you move a mass of n pounds a distance of x feet). when your car is at peak power, it is performing work at its fastest capable rate.

this is why jim986's real life experience shows him that best acceleration runs occur when you ignore the torque peak and use the power that the almighty RPM helps us generate.

[99Boxster]

Sorry guys but WTF are you talking about? Are you saying the harder I press the accellerator the faster my car goes?

[blue2000s]

Originally Posted by just_me

... because the torque multiplication effect of the gearbox works in your favor, since you get the torqgue multiplied by a ahigher number of revolutions equal to the ratio of the two gears).

Grant

I'm running out of ways to explain this folks. Acceleration comes from force, which is directly related to the torque at the wheels and the wheel radius. I'm trying to make it as understandable as possible. Like I said above, you CAN use power, but that's not the way it's traditionally done.

[PDX-986S]

Waiting with baited breath to find out who wins this argument.

[Brad Roberts]

Waiting with baited breath to find out who wins this argument.

Nobody!

In the end: we will shift where we want to shift. I love engineers, and theory, but I live in a real world. What is on paper doesnt always work.

Some of us call it "seat of the pants". I call it "calibrated ass"

Old school: "Bubba" shift the truck just after the valves float!!


B

[blue2000s]

Originally Posted by Brad Roberts

I live in a real world. What is on paper doesnt always work.

In my experience, that's what the people say when they don't understand the theory. Engineering is all about the application of science to the "real world" and using it to make real products. This is relatively simple stuff that is 100% applicable to real life.

[blue2000s]

I'm not sure that this should have turned into a horsepower vs torque arguement. An conversation of energy vs force is really pretty academic. Interesting, but ultimately you get the same answer either way you approach it. The calculation is just slightly different.

What I reallly wanted to show was a way to calculate the best shift points for acceleration. The spreadsheet that I've provided will do that and it's kinda fun to play with. I'm hoping that's what everyone takes away from it.

[denverpete]

Originally Posted by blue2000s

...What I reallly wanted to show was a way to calculate the best shift points for acceleration. The spreadsheet that I've provided will do that and it's kinda fun to play with. I'm hoping that's what everyone takes away from it....

All I know is that I'm NEVER posting a thread about acceleration, I can tell you that.....

[blue2000s]

Originally Posted by denverpete

All I know is that I'm NEVER posting a thread about acceleration, I can tell you that.....

LOL, I'm a glutton for punishment.

[insite]

the bottom line is that for a given acceleration run, you have a static amount of work to do. for instance, if your car weighs 2800 lb and you are running a quarter mile, you must move 2800 lb a distance of 1320 ft. you will be performing 3,696,000 ft-lb of work.

since the object is to minimize the time in which this work is accomplished, we must maximize power. work / time is the very definition of power. shift points should be chosen in such a way as to maximize average power AT THE WHEELS. this is why blue2K is correct in stating that the curves alone will not tell us the best shift points; gearing must be considered. the car with the highest average horsepower AT THE WHEELS over a given distance wins the race.

[blue2000s]

Originally Posted by insite

the bottom line is that for a given acceleration run, you have a static amount of work to do. for instance, if your car weighs 2800 lb and you are running a quarter mile, you must move 2800 lb a distance of 1320 ft. you will be performing 3,696,000 ft-lb of work.

since the object is to minimize the time in which this work is accomplished, we must maximize power. work / time is the very definition of power. shift points should be chosen in such a way as to maximize average power AT THE WHEELS. this is why blue2K is correct in stating that the curves alone will not tell us the best shift points; gearing must be considered. the car with the highest average horsepower AT THE WHEELS over a given distance wins the race.

100% in agreement. From the time it takes to move that distance, you'll get an average acceleration and know what the speed is at the end.

Just to add, not just gearing and torque must be considered, but also the vehicle speed in gear.

[986Jim]

Originally Posted by insite

the car with the highest average horsepower AT THE WHEELS over a given distance wins the race.

This is simply NOT TRUE!!!! Spend some time at the rack track. Your Porsche can make 250whp between 4500rpm and 6500rpm range steady but a car that makes 200whp starting at 4500rpm and makes 300whp at 6500rpm will be faster.

Highest peak HP wins. It doesn't matter if it makes it over a wider range or not. Accelerations is one thing, you can math that to death, but the motors ability to accelerate or rev in a gear is a whole other story. The second car has higher peak power but less power overall will be faster. You simply can NOT argue this, it's a proven fact, goto a drag racing forum and ask, they will math you to death.

Even in the car with 250whp over a steady range will still want to shift at the highest absolute RPM point even if power is dropping off at that point, it will still be faster.

It's not about the most power across a band, is ONLY about the most power. Stop using math, and goto the race track and see what happens. Cars with big long flat power bands can get beaten by cars with lots of peak HP, because thats what wins races.

You guys are argueing with text books in a argument that takes races in real cars. You can't justify it with math, goto the track you'll figure it all out there.

This is as bad as the bench racing at clubsi for god sakes.