Text by Cameron Parsons // Photos by Cameron Parsons and Joe Singleton
DSPORT Issue #180
Changing GearsThe AUTO-BLiP is designed to blip the throttle during downshifts so that you don’t have to, but why is rev matching so important to begin with? If you ever attend a racing school, at some point the instructors will ask you why you should downshift as you approach a corner. Most students incorrectly answer that it is to help slow the car down. While some people downshift to use engine compression to slow a car on the street, the task of deceleration should only be left to the brake pedal when on track. Changing to lower gears before a corner serves the purpose of keeping the engine RPMs inside the power band for when the car leaves the corner. Although H-pattern gearboxes are the most popularly produced and driven form of a manual transmission, they come with the drawback of requiring extra care when shifting. When you depress the clutch and change gears with the shifter, the timing of your actions affects the entire powertrain from the engine, to the clutch, to the transmission, to the differential, to the wheels and tires. Each of these components spin at high rates when the car is in motion and are tied to each other to spin in synchrony. When the chain is interrupted, you risk damaging components and losing control of the car. For example, if you push in the clutch pedal and shift to a lower gear without any input on the throttle, the engine RPMs fall back toward idle speeds. When you release the clutch pedal, the engine has to spin up to match the rotation of the transmission input shaft, now rotating at a different speed due to the changed gearing ratio. If the clutch is released too quickly, a sudden jolt interrupts the drivetrain, the driveline components wear quicker, and the driving wheels and tires break loose from the road. If a short but significant press on the throttle (“blip”) in between gear changes were added, the engine speed would match the input shaft speed. This smooths out the act of downshifting and thus results in reduced drivetrain wear, less clutch slip and consistent grip on the driving wheels and tires.
Getting in SyncWhile a car drives in a given gear, a whole chain of important components spin in harmony with each other. When the clutch disengages (clutch pedal depressed), the engine and transmission can then spin at different speeds. If the transmission and engine internals spin at different speeds due to excessive throttle blip or not enough, a quick clutch engagement (clutch pedal released) forces the engine and transmission to suddenly match speeds again. This sudden shock can damage parts or cause traction loss.
Human ErrorA problem arises with the process of slowing a car, blipping the throttle and downshifting a manual transmission for a corner: two feet to control three pedals. This is where good timing and coordinated footwork come into play. Heel-and-toe requires the right foot to apply brake pressure while the left foot depresses the clutch pedal to prepare for a gear change. With the brake applied and the clutch disengaged, the driver moves the shifter into neutral, then pivots their right foot for a quick blip of the throttle without backing off of the brake pedal. Depending on the car, the driver can simply roll their foot past the right edge of the brake pedal to reach the throttle, or they can keep the ball of their foot on the brake while swinging their heel over to reach the throttle. With the engine RPMs up and falling toward the matched input shaft speed, the driver can then engage the next lower gear and release the clutch pedal. This process requires lots of patience and practice before it becomes second nature. Even when the timing is right and the technique is smooth, drivers commonly produce inconsistent brake pressure. The motion of using one foot to control two pedals causes a tendency to lift off the brake pedal in order to reach the throttle. This results in earlier braking points to compensate for the poorer braking performance, or wheel lockup if the brake pressure fluctuates too much.
Heel-and-Toe1. Brake The right foot applies the brakes as the car enters a corner or deceleration zone. The le foot is ready to press on the clutch pedal and the driver’s hand is ready to change gears. 2. Clutch and Blip While continuing to apply the brakes with the right foot, the le foot presses the clutch pedal to the floor. During the moment that the clutch is disengaged, the driver changes to a lower gear and the right foot either rolls its right half onto the throttle or pivots the heel over. In most cases, a quick blip of 50-percent or more throttle su ices for a proper rev match. 3. Clutch Release The brakes are still applied as consistently as possible. As the RPMs fall back down a er the throttle blip, release the clutch the moment that the engine and transmission speeds match. The proper timing of this di ers between cars, and only comes naturally with a lot of practice. 4. Brake During the entire Heel-and-Toe process, the brakes were applied consistently. A er the downshi (s) are complete, the le foot returns to its resting position.