Ask ten people at a car show when you should shift a manual car for the quickest quarter mile and you'll get a lot of different answers. "At peak torque." "At redline." "When it stops pulling." Only one of those is close, and the real answer comes from a bit of simple math about how horsepower, torque and gearing work together.
This article explains that math in plain terms, then shows how it applies to The Drag Car Racer's five-speed.
Torque and horsepower, briefly
Torque is the twisting force the engine puts on the crankshaft. It's what you feel pushing you into the seat.
Horsepower is how fast the engine is doing work. It's torque multiplied by engine speed. In the units most drag racers use:
Horsepower = Torque (lb-ft) × RPM ÷ 5,252
That formula explains why the two numbers peak at different places. On a typical engine, torque rises, flattens out somewhere in the middle of the rev range, and then slowly falls. But because horsepower multiplies torque by RPM, horsepower keeps climbing past the torque peak. As long as torque is falling more slowly than RPM is rising, power still goes up. Eventually torque drops off fast enough that power peaks too, and then falls toward the rev limiter.
It's also why every torque curve and horsepower curve on a dyno sheet cross at exactly 5,252 RPM.
What actually accelerates the car
The engine's torque doesn't reach the road directly. It's multiplied by the transmission gear and the rear-end ratio before it gets to the tires. First gear might multiply engine torque by 3.5 or more, fifth gear by less than 1. That's why first gear pulls so hard and fifth feels lazy.
What pushes the car forward is torque at the wheels. So the question isn't "where does the engine make the most torque?" It's "at this road speed, which gear gives me the most torque at the wheels?"
Why you don't shift at peak torque
Say you're in first gear and the engine has just passed its torque peak. It's tempting to shift here, since the engine is starting to "fall off." But if you shift to second, the RPM drops a lot, and second gear multiplies the engine's torque by a much smaller number. Even at the engine's torque peak, second gear usually produces far less wheel torque than first gear does, even with first gear past the peak.
In other words, staying in first is still pulling harder than second would. Shifting early throws away acceleration.
The real rule: shift where the next gear pulls harder
The quickest shift point in any gear is where the wheel torque in the current gear drops to the wheel torque you'd have in the next gear, right after the shift. Before that point, the current gear is pulling harder. After it, the next gear would be.
There's an equivalent way to think about it that only needs the power curve: you want to keep the engine in the part of its range where it makes the most average power. When you shift, the RPM falls by the ratio of the two gears. You want to pick the shift RPM so that the power just before the shift is about equal to the power just after it. For most engines, that works out to a shift point a little past peak horsepower, and short of the rev limiter.
How much does the RPM drop?
The RPM after a shift drops by roughly the ratio of the new gear to the old one. Using The Drag Car Racer's gear ratios (3.5, 2.1, 1.4, 1.0 and 0.8) and a shift at 5,600 RPM, the simulator's tachometer drops to:
| Shift | Ratio change | RPM after shifting at 5,600 |
|---|---|---|
| 1st → 2nd | 3.5 → 2.1 | about 3,760 |
| 2nd → 3rd | 2.1 → 1.4 | about 4,070 |
| 3rd → 4th | 1.4 → 1.0 | about 4,290 |
| 4th → 5th | 1.0 → 0.8 | about 4,680 |
The bigger the drop, the longer it pays to hold the gear before shifting. In the simulator, the point where the power before the shift equals the power after it is about 5,850 RPM for the 1–2 shift, 5,700 for 2–3 and 5,650 for 3–4. The 4–5 shift comes so close to the stripe that its timing barely matters.
Notice that every shift lands the engine above 3,000 RPM, the point where this engine's torque flattens out. That's no accident. A good set of gear ratios and a good shift point work together to keep the engine near its best power after every shift. The gaps also get smaller as you go up, which is why the higher gears feel smoother.
Now imagine short-shifting at 4,000 RPM instead. The 1–2 shift would drop the engine to about 2,800 RPM, below the torque plateau, and the car would bog until the revs came back up.
What about the rev limiter?
The opposite mistake is holding a gear too long. Once the engine is well past its power peak, it's making less and less power per rev. When it hits the rev limiter, the ignition or fuel is cut to protect the engine, and the car stops accelerating almost completely. Every tenth of a second on the limiter is time you're just coasting.
There's also the shift itself to think about. During any shift, there's a moment where no power reaches the wheels. In The Drag Car Racer it's a quarter of a second. That's one more reason not to shift more often than you need to, and one reason drag racers love quick-shifting transmissions.
Shift lights
Watching a tachometer needle at full throttle is hard, especially when you're also watching your lane and the other car. That's why so many drag cars have a shift light: a bright bulb on the dash that comes on at a preset RPM. The driver just waits for the light and shifts.
In The Drag Car Racer, the engine's torque flattens from about 3,000 RPM, power peaks around 5,300, and the rev limiter is at 6,000. Past the peak the power falls off quickly, down to about 60% at the limiter. The shift light turns amber at 5,600, just below those crossover points, and flashes red at the limiter. Hit upshift the moment it goes amber, in every gear, and your reaction lands you right in that window, within a few hundredths of the car's best possible ET. Shifting at the 5,300 power peak costs about a tenth of a second over the quarter mile, and so does waiting for the limiter. Sitting on the limiter for a moment in each gear costs three to five tenths. Both mistakes show up on your time slip as a slower ET, and riding the limiter also costs trap speed.
Finding the shift point for a real car
For a real street or race car, the process is:
- Get a dyno sheet showing power across the rev range, ideally with the car in the configuration you'll race.
- Look up your gear ratios and work out the RPM drop for each shift.
- For each shift, find the RPM where the power before the shift roughly matches the power after the drop. That's your starting shift point.
- Test it at the track, a couple hundred RPM either side, and compare the time slips. The eighth-mile and 1,000-foot times show exactly where each change helped or hurt.
Many racers find the math gets them within a hundred or two RPM, and the track does the rest.