How To: On Knowing When to Shift!

[iRoush]

When Should I shift? This seems like a simple question but there really is a lot more behind it then just shifting at redline. Having a better shift point does not make you "make more horsepower" all it does is increase the effectiveness of your rpm band as well as maintain a steady increase of momentum in a foward motion. Realistically, to get an accurate reading for when you should shift you need the following:

Dyno Graph showing the Horsepower and Torque Curve

The RPM drop between a shift from 1-2, 2-3, 3-4, 4-5, or 5-6 if you have a 6th gear.

There's a reason for me listing all of the different gear selections. This is because not all transmission gears nor drivers are created equal, meaning that not each gear is evenly spaced out in a transmission nor does every driver shift the same so RPM drop can very from car to car or driver to driver. Now that we've got this covered, let's dissect a Dyno Graph.

Here's the basic layout of what a dyno graph will look like. Horsepower and Torque on the Y-Axis and RPM on the X-Axis. Let's use this graph as an example.

For this exercise assume a 7500rpm redline (off the chart) and a 2000 RPM drop between gears. If I were to shift at 7500RPM would I be taking advantage of every bit of power available to me? The Short answer is no. See Graph 1 Below:

Graph 1

First off, I apologize for 7500 rpm being off the graph, just assume it goes all the way up to 7500 before shifting. If we have a 2000 rpm drop from say, 1st to 2nd gear, that would put us at 5500rpm in second. Why is this shifting habit not optimal? For one, you're going way out of the RPM range of the car, it doesn't have enough air to be able to go that high so you'd be losing a lot of forward momentum. And secondly, you're completely avoiding the most important part of the car's power. It's Peak Torque. As they say, "horsepower sells cars, torque wins races." Torque is what makes the car accelerate horsepower just maintains that acceleration. Now if you were to shift at 6000rpm, that would have you shifting right where you want it, right? Max horsepower then putting you in the grunt of the Torque curve. Actually no, this is not what you want.... See Graph 2 Below:

Shifting at this RPM, sure you're shifting right before your Horsepower drops off, but by doing so in a 1-2 gear shift, you will experience what is known as bog, which will absolutely kill any forward momentum you may have. I said it once and I'll say it again, Torque is what will accelerate you, horsepower is just there to maintain that acceleration. By shifting at 6000RPM's you are shifting much too early and putting yourself before the full rise of the torque curve.

Now, if you were to shift at 6800rpm's with a 2000 RPM drop, that is your winning ticket of a shift. Are you going past your peak horsepower? Sure, and it may kill some momentum, but of all 3 options you have, this will keep the most momentum going forward and you will see either faster ET's or if you road race, quicker lap times. See Graph 3 Below:

By shifting at 6800RPM's, you are putting yourself right where the peak torque band is at 4800 RPM's. In fact, an engine will operate most effectively, resulting in maximum acceleration, when the rpm is maintained at the torque peak all the way up to the next gear shift that puts the rpm right back at peak torque. In a perfect world, you would have Peak torque and Peak Horsepower perfectly spaced so that as soon as you hit peak horsepower, you would shift and have it put you right back at peak torque. Unfortunately this is often not the case with street cars and oem transmissions built for Miles per gallon versus smiles per gallon. But always keep in mind the following:

An engine will operate most effectively, resulting in maximum acceleration, when the rpm is maintained between the torque peak and horsepower peak

Edited August 30, 2012 by 330CubeGt

[Number Tew]

Nice writeup!

[ninety8stang]

Very nice man. Great info! Shifting is only half the battle. Knowing how and what the car likes and what rpm it performs the best at is the other half. Haha

Edited June 30, 2012 by ninety8stang