When you manually downshift with the paddle, the transmission is suddenly in a conundrum as the throttle is on the ground, the steering is turned, and the driver is requesting a downshift. Who in their right mind power downshifts, right? Any driver would perform a rev-matched downshift that requires lifting off the throttle before the clutch is disengaged. But because we drive semi-automatics like automatics and the intent is to accelerate, the go-pedal stays on the ground with no response. Under these conditions, the computers in the semi-automatic have to physically intervene by reducing the electronically controlled throttle and wait for the torque conditions to fall within an acceptable window before performing the downshift. While the computer processing time is imperceptible, the execution time isn't. What you feel is a half-second long bog (which feels like an eternity when at the limits) mid-turn as the computer waits for the right conditions. Once the downshift happens, with an abrupt shock on account of the torque difference, you feel a hard jerk, which further upsets the car and weight transfer.
Light braking offers its share of problems too. When light braking to a complete stop, the deceleration comes from the brakes and negative torque from the engine. As a driver, how hard you brake is controlled by brake pedal pressure, which is modulated according to engine torque. But, during downshifts, there isn't any negative torque passing through the driveline, so deceleration can feel inconsistent at very low speeds.
Within the short lifespan of semi-automatic transmissions, technology has grown leaps and bounds. The newest dual-clutch transmissions, like the one found in the GT-R, are almost flawless, even from a manual transmission driver's point of view. The positive shift sensations, almost instant shifting from the time of input to the actual gear change, and the smooth intelligence of an automatic during city cruising really can make the manual transmission seem obsolete, even to us. Even though only offered as hardware on high-end cars today, the semi-automatic will trickle its way down to standard production cars because of its performance abilities, as well for its fuel savings and emissions reductions.
Testing Methodology And Reading The Graphs
Each telemetry graph shown here offers a wealth of information. The data was collected with our G2X telemetry system that was used to record time, distance, speed, and vehicle acceleration. Since each car has different power, weight, traction, gear ratios, and is calibrated for different needs, we weren't really interested in how fast a car can accelerate or decelerate. We're interested in how long it takes each transmission to up-shift at peak power and the shock (or change in acceleration) of each gear change. Our measurements are based on second to third gear up- and downshifts, at each transmission's fastest and most aggressive shifting. These gears experience more wheel torque than higher gears and offer the best shock data. Downshift rev matching at 4000 rpm was also tested as the car coasted under engine braking only.
Each chart shows the car's longitudinal acceleration in g's, as well as the vehicle speed during acceleration and deceleration under engine braking only. Because of the nature of accelerometer data, each gear change is represented as a series of spikes as the accelerometer measures the shock of the shift and slowly settles out. In addition to the random signal noise that comes from the accelerometer mounted in the car, it's impossible to know exactly when the shift starts and ends.
Instead, the duration of each shift is measured by the change in vehicle speed. During shifting, the wheels are not under power and so the slope, or increase in speed, drops off slightly. The drop-off (or shift plateau) is very noticeable for manual transmissions and semi-automatics that have hard shifts as the change in speed is what you feel as a passenger. Dual Clutch transmissions that shift nearly seamlessly have drop-offs that are barely noticeable in the trace. Even though the numbers won't directly correlate to what OEM's officially claim as the shift time, the duration of the plateau is what we're gauging gear change durations by since, ultimately, what we care about is the amount of time that full power isn't delivered to the wheels. Shift intensities are measured by the biggest change in acceleration. Even though the accelerometer registers residual noise, we are only considering the shock of the gear change during the shift plateau. The larger the change in acceleration, the larger the shock, which is good for communicating with the driver but has the drawback of shocking the driveline and breaking traction.
These charts also offer other little comparative tid-bits, like how hard each car accelerates and the acceleration you would feel. The time between shifts also indicates how closely geared each transmission is, like on the Lexus IS-F.