Manual Trans

Advanced Manual Transmission Shifting techniques

This section discusses performance-driving techniques, which may be detrimental to the life of the power train and other components of the vehicle.

Power shifting (a.k.a. speed shifting)

This is an advanced technique that gives a speed boost to the car when upshifting. This should only be used in racing applications, as it is very hard on drivetrain components, especially if done improperly. Even done properly, it generally puts undue stress on the drivetrain, and shortens the lifespan of the driveline components.

To perform a power shift, keep the gas pedal on the floor (instead of lifting) while pushing in the clutch. At the same time, quickly shift to the next gear. Quickly re-engage ("pop") the clutch as soon as the new gear is selected. If done properly, the car will receive a "bump" in acceleration (due to the jerk of the decelerating engine) as the clutch is re-engaged. If done improperly, it can result in engine or transmission damage, sometimes catastrophic.

Heel-and-toe

While decelerating, the technique of heel-and-toe shifting allows advanced drivers to maintain pressure on the brake pedal while simultaneously "blipping" the throttle to aid downshifts.

Double clutching

"Double-clutching" (also known as "double-declutching" in Europe) is an old technique that dates from before the days of synchromesh gearboxes. It can make downshifting (and arguably upshifting) smoother and can reduce transmission wear. It involves depressing the clutch and dropping into neutral, releasing the clutch, using throttle to match revs for the gear to be selected, depressing the clutch again and selecting the new gear. The action (which sounds more complicated than it really is) ensures that the output shaft is running at the right revs before a gear is selected hence creating smoother shifts and reducing wear on synchromesh systems. Some types of unsynchronized manual transmissions require double clutching. These transmissions may be found in some older vehicles and racecars, as well as on large trucks, especially tractor-trailers.

Double clutching on a synchronized gearbox is unnecessary in most driving situations. However, it is possible that an unskilled driver may second-guess something that the machine is designed to do automatically, and can cause more harm than prevent it. A true unsynchronized transmission is unforgiving to mismatched speeds, requiring a great deal of precision from the driver. A synchronized transmission will allow the driver to get away with sloppy double clutching, and an unskilled driver may end up causing more harm by not matching the engine speed properly.

However, even with a synchronized gearbox, double clutching under skilled hands is desirable when shifting down to the lowest gears. The synchronizer, particularly the one for the second gear, receives much abuse, as the speed differential it has to manage is usually the greatest. It is indeed usually the second gear synchronizer that fails first. Some recent Honda manual transmissions incorporate a triple-cone synchronizer in the lowest gears for extra durability.

Many transmissions until recently did not have a synchronizer on the first gear. It is therefore necessary to double-clutch when downshifting into the first gear, though such instances are rare. Some transmissions today still do not have a synchronizer for the reverse gear, requiring that you come to a complete halt before shifting into reverse (see the "Grinding gears" section above for further implications of this). Double clutching is of course impossible, as you cannot reverse the direction of the engine rotation. You must simply wait for the car to come to a halt and engine revs to drop to idling.

Single clutching

A downshifting technique similar to a cross between double clutching and shifting without the clutch. The transmission is taken out of gear whilst the clutch is still engaged. The accelerator is then 'blipped' to bring the output shaft to the correct speed, the clutch is disengaged, the lower gear is selected and the clutch is engaged again. This is a fairly common racing technique.

Shifting without the clutch

It is possible to shift gears without using the clutch at all, by careful throttle manipulation. When transmissions were unsynchronized this was usually the easiest method, and still works in modern cars, although it requires a lot more practice than the usual methods described above.

Although the technique works in synchronized gearboxes, it is inappropriate in nearly all circumstances. A synchronized gearbox has a small clutch called a synchronizer (or "synchro") in every forward gear which brings the input and output rotation to the same speed before allowing them to engage. Like the main clutch, these synchronizing clutches use friction. These synchronizing clutches will allow for a margin of error when engaging without the main clutch. If the engine speed is somewhat too high or too low, the synchro will essentially take on the job that the fully engaged main clutch isn't doing. However, a synchro ring is not intended to do the job of a main clutch. Its job is to spin up or spin down just the input part of the transmission that is assumed to be disconnected from the engine. It is not intended to speed up or slow down the entire engine. Such experimentation done on a regular basis will prematurely wear out the synchro rings, which will have to be replaced.

On cars with a freewheel, such as the older SAAB with a two-stroke engine, shifting without the clutch is standard procedure. But actually, the freewheel is a form of clutch: one that is disengaged whenever the input rotation is slower than then output rotation. One must disengage the freewheel by allowing the engine revolutions to drop, which is analogous to pressing the clutch pedal.

Copyright © 2007-2008 DragTech.com. All Rights Reserved.