Charging up a winding hill with an engine at full noise can be an exhilarating experience that leaves you breathless. But when that thrust of power takes a prolonged absence during a gear change, it's both bloody annoying and potentially quite slowing. Invariably, passenger's heads snap forward as the car's acceleration G-forces are released, and you then have to rely totally on the engine to pull (or push) you back up to speed. Here's how to quicken the gear changing process.
Manual Gearbox Fluid
The viscosity of your gearbox fluid has an effect on how quickly the gear synchro ring engages. When the gear hub is shifted toward the gear during a change, fluid that gets trapped in between must escape. To do this, the inner side of a synchro ring usually has a set of fine radial grooves that enable this flow of fluid to occur.
Therefore, from a performance point of view, a thin fluid is preferable as it can be dispersed more rapidly through these small grooves.
Car manufacturers specify the appropriate viscosity gearbox fluid for their vehicles under normal operating conditions, but it is possible to deviate from this recommendation slightly. Changing to one grade lower in fluid viscosity can yield improvements in gearshift speed without any major sacrifice. However, changing to a too thin fluid can significantly reduce the protection of gears and their associated hardware. Interestingly, we've been told that one Australian carmaker now recommends auto trans fluid (which is much thinner) for their manual gearboxes, as there were problems encountered in the field with extra-slow gear changes...
Synchro Modification
Synchro rings are frictional surfaces incorporating small teeth that are designed to match the rotational speed of the gear to engine revs. The standard synchro mechanism that comes fitted to most vehicles can be improved by modifying the existing synchro ring or fitting an aftermarket one that has larger radial grooves. This lets the gearbox fluid flow more quickly out of the sandwich situation we've mentioned above. Therefore, this modification is similar in principle to using a thinner gearbox fluid, and the result once again is a quicker shift.
Smaller gains can be found via the simple process of lapping. In this procedure, the inside of the synchro ring is coated with a special paste and it is then rubbed back and forth over the centre engagement hub of the gear. This wears away any possible high spots on the inside of the synchro, giving it a smooth inner surface to provide the optimal contact area on the centre hub. The better grip the synchro has on the gear, the better it can equalise the rotational speed differential - which, in-turn, improves gearshift speed. Note that this process should be performed whenever a used gear or synchro ring is being re-used. Lapping shouldn't be necessary when installing a new gear and synchro ring together, but it is advisable to check how well they conform prior to assembly.
Furthermore, in a drag racing-only application, the number of "teeth" on the synchro ring can be nearly halved, which (we're told) gives an even quicker engagement of the gear cog. However, the obvious problem associated with this is vastly increased synchro teeth wear, necessitating the need for frequent gearbox inspections or rebuilds.
Short-Shift Linkages
There are many short-shifters available on the market to suit most high-performance cars. The purpose of these devices is to alter the geometry between the gear knob handle, the pivot ball and the bottom of the selector rod. Usually, the distance between the fulcrum and the bottom of the actuation rod is increased, thus giving increased selector rod travel for any given amount of gear knob movement. On average, the distance of the gear throw can be reduced by around 50% - but we've seen one that cut the throw by what we'd guess would be a massive 80%!
On the subject of linkages, it is also important to ensure any rubber or poly bushes in the gear selector system are in good condition. This gets rid of any "pea soup" that might otherwise impede the swiftness of gear swaps.
Short-Shift Gear Knobs
Another item readily available, an aftermarket short-shift gear knob is designed to be shorter in height than an OE knob. This, like the short-shift linkage, gives a different geometric relationship between the gear knob handle, pivot ball and bottom of the selector rod.
However, this variation takes the opposite approach of shortening the top section of the gear knob handle, to give a higher ratio between the amount of handle movement and selector movement. This again gives a shorter gear selection stroke, but generally only by around 5-10%.
Drivetrain Factors
Also see
The Clutch of the Matter
Although not directly related to the shift sequence itself, the rotating mass (inertia) of drivetrain components can effect the duration of a gear change lull. If the inertia of the drivetrain is low, the faster the synchro ring can equalise the rotational speed of the gear to engine revs. This means the shift is completed more rapidly. Lightweight driveshafts and other such exotic items help this cause but more relevant to us, we've discovered that the Australian 2-litre Super Tourers often run a different gear set during qualifying than they do for the race. These qualifying gears have a series of holes drilled through the side of the gear near the centre. This reduces the mass of the gear slightly to give faster gear synchronisation - but please note that for the sake of reliability, solid gears were always used for the race. An alternative to cross-drilling a gear is to have one purpose-made from lightweight materials to suit you particular car. This might not be a cheap exercise, but it may be a more sound engineering solution.
The "bite" of the clutch plate against the flywheel is also an important factor in the quick-shift equation. Paddle-type clutches offer a faster transition against the flywheel, which enables the release of the clutch pedal to be more closely followed by the application of throttle. On the subject of clutches, the actuation system can usually be adjusted so that there's a minimal amount of pedal movement between the friction point and the pedal's static position. This enables the clutch to respond faster to the driver's input.
Driving Style
For those intent on losing an absolutely minimal amount of acceleration during an up-shift, it can be possible to "flat-change". This technique involves holding the accelerator pinned to the floor during the whole time the clutch is engaged and the gear is being selected. But as you could imagine, the accompanying stress on the driveline can be potentially destructive, and there is also the possibility of over-revving the engine. It's great if you're driving a racecar - or better yet, someone else's!
Turbo Aids
The torque of a turbo engine can be severely lacking immediately after a slow gear change, where the driver has released the accelerator for an extended period. In this instance, the volume of exhaust gas that's needed to spin the turbo up to boosting speed can be lost. There is a choice of two ways to combat this: a throttle-closed blow-off valve, or a competition-only anti-lag system.
A blow-off valve vents some of the air that gets trapped between the turbo compressor and the closed throttle butterfly (as occurs during a gear change). This, in theory, is to prevent stalling the rotational speed of the turbo which means boost is readily available for the re-application of the throttle. The approach of anti-lag is quite different. Here, whenever a closed throttle is detected, fuel is introduced into the exhaust manifold and ignited downstream of the exhaust valves. These small explosions occur prior to the turbine and create enough exhaust gas to maintain sufficient turbo speed to create boost pressure. This ensures torque immediately upon re-acceleration.
Footnote
Given these above upgrades, the so-called dog box is still regarded as the ultimate gear system for use in an extremely demanding application. A dog box does away with any form of gear synchros, and uses stepped cogs to alternately engage the gears to the transmission mainshaft. While this design does enable rapid engagement of gears, its operation requires the driver to closely match engine rpm to the gear that's to be engaged. So for every gear change, you need to blip the throttle to ease the gear into position - especially on downshifts. And that isn't really an appropriate style for an everyday roadcar.
Stick to the other quick-shifting tricks we've covered and you'll see significant improvements without such a massive driving compromise. Then you can say "bye-bye gear change woes"!
