How Auto Rev-Matching Smooths Out Manual-Transmission Driving

A GM engineer explains how a car can heel-toe better than you. 

Corvette Rev Match Paddle Corvette Rev Match Paddle

For decades, keen manual-transmission drivers worked to master the art of rev matching, using a “blip” of throttle to synchronize the speed between engine and transmission during a downshift. Back in 2009, however, Nissan introduced a technology to automate the process. Now rev-match tech is included with almost every manual-transmission car sold in the US. 

To explain how this technology works, first we’ll cover some manual-transmission basics. When a car is in gear and moving, the engine and transmission input shaft rotate at the same speed, and everything is all good. Push in the clutch pedal, the engine and transmission decouple, and both are free to rotate at different speeds.  

In an upshift, when a driver lets out the clutch pedal, recoupling the engine and transmission, engine speed decreases to match that of the transmission. Since engine speed naturally falls between gear shifts, as the driver has let off the throttle, this process is quick and smooth.

Downshifting is a different matter. 

Engage a lower gear at speed and the shaft/clutch disc speed rises. When the driver lets out the clutch pedal, engine speed must increase to match the higher speed of the transmission. If the two speeds are mismatched as the clutch engages, it can send a shock through the driveline, resulting in the car jolting. It’s not a pleasant experience, and one that can upset the balance of a car at critical moments during braking and cornering. 

A quick “blip” of the throttle while the clutch pedal is depressed matches engine and transmission speed before re-engaging the clutch, smoothing out the process of a downshift. It’s not too difficult to do, once you get a feel for it. 

Executing a blip while braking is an entirely different matter because, well, most drivers only have two feet. Hence the “heel-toe” technique, where the driver blips the throttle with part of their right foot, while holding down the brake pedal.  

(“Heel-toe” can be something of a misnomer, depending on the pedal setup of the car. Oftentimes, a driver uses the inside of their right foot for the brakes and rolls the right side onto the throttle pedal.) 

It’s not an easy technique to master. Braking hard helps position the pedals for the right amount of leverage to blip the throttle pedal – something you’re not likely to be doing, say, braking from a stop after a highway offramp. 

Sometimes, the brake and gas pedals are spaced too far apart to easily work both simultaneously. Moreover, a perfect heel-toe can be a lot to manage when, say, you’re in a heavy braking zone on track, downshifting through multiple gears before a corner.  

Automating this process makes life a lot easier for the driver, while also bringing the benefits of a good heel-toe downshift. And while Nissan was the first to offer the system in a road car, the 370Z, its first use was in motorsport years earlier. 

Ferrari first employed a “semi-automatic” sequential gearbox with the 640 Formula 1 car of 1989, essentially a manual transmission with automated shifting. But it wasn’t until 1992 that throttle-by-wire came to F1, enabling automated throttle blips for downshifts. But in early sequentials like the Ferrari’s, drivers still had to rev match on downshifts. (Some sequentials also required drivers to use the clutch, as in a conventional manual.) 

The shifter of the Cadillac CT5-V Blackwing. Note the "Rev Match" button at the bottom left corner. 

A 2009 Car and Driver article about the 370Z’s “SynchroRev Match” system expresses surprise that it took so long for an automaker to come up with this technology. Not long after the Z, other automakers developed their own versions. GM’s system arrived with the C7 Corvette, and now, both the CT4-V Blackwing and CT5-V Blackwing have it. Motor1 spoke with Sean Hoffman, a clutch control calibrator at GM, who worked on the Blackwings. 

As Hoffman explains, GM’s system relies on crankshaft-position sensors, gear-position sensors, clutch-position sensors, and output shaft-speed sensors. The engine’s ECU then takes this information and turns it into a torque request to the engine. The ECU calculates a targeted engine speed and then determines how much and for how long to open the throttle.

“We're always calculating an exact number of RPM, and it's based on the transmission output speed and the gear that you’re going into,” Hoffman says. “We know what the gear ratio is. Just multiplying the two together gives you your target engine speed, so the calculation is exact — or to as many decimal places as the software allows — but obviously, the system can’t do it perfectly.” 

An overshoot or an undershoot of, say, 50 to 100 RPM is acceptable. A driver likely can’t feel that. The target RPM constantly changes, too. 

“[O]bviously when you shift, you depress the clutch, and the vehicle is likely to decelerate somewhat, or you might be braking,” Hoffman says. “And so, as the vehicle speed changes, that transmission output speed changes and you're always calculating a new target RPM.” 

While the throttle is the primary device for getting to the target engine speed, the ECU has control over fuel and spark as well. Hoffman says that sometimes, the system might cut fuel once it gets close to the targeted speed to prevent a large overshoot. 

In the Blackwings, the auto rev-match system also works on upshifts, helping smooth the process, reducing clutch wear over time. Typically, on an upshift with auto rev-match turned on, the engine will cut fuel as soon as you put in the clutch pedal, also a boon to emissions and fuel economy. 

The system never looks at wheel speed, so if you change rear tire sizes in your Blackwing – thus changing wheel speed in gear – rev matching will still work fine. While perhaps unlikely, if a customer swaps in a lighter or heavier flywheel, changing the rate at which the engine builds and loses speed, that could affect the system. You might see more undershoots or overshoots, as the system is only designed to work with the engine as it comes from the factory. You may also encounter issues with a severely worn clutch, too, but at that point, you’ve got other problems.  

Obviously, there’s still the fleshy bit lording over the whole equation. They’re the ones moving the shift lever and pressing the clutch, so the system must account for things like the position of the clutch pedal and the rate of travel to make every shift as smooth as possible.  

What’s happening here isn’t too different from the process of rev-matching with an automatic transmission. But Hoffman points out that an auto will have its own ECU talking to the engine’s ECU, making the process simpler.  

There’s also an aftermarket solution of sorts that works for many cars called AUTO-BLiP. Al Vergara, who runs the company, is at pains to say that the device doesn’t do rev matching, as such. Instead, it plugs into a car’s OBDII port and generates a throttle blip after the brake pedal and clutch are pressed in sequence. Vergara explains that it uses simple brake-light switch, throttle-position sensor, and clutch interlock signals to generate a throttle blip. Knobs on the box allow drivers to control the length of the throttle blip, and the delay between after the clutch is depressed. 

It’s obviously not as sophisticated as what an OEM can do in-house, but at $395, it’s an affordable way to add similar functionality to older vehicles, albeit one that has by-wire throttle. The system is most commonly used on track-day cars. 

There’s an argument to be made that systems like this take the driver out of the equation, and the whole point of a manual-transmission car is to increase engagement. But personally, whenever I’m testing a car that has this functionality, I switch it on and off often. There are times where trying to rev-match is impractical, or just annoying. Other times, I want to work a bit more, reveling in the satisfaction of nailing a perfect heel-toe downshift... if I can manage it.  

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