Gearbox setup & selection

Gearing up!

Gearbox – Lower ratios give better acceleration times and lower fuel economy. Six or seven speed boxes are better matched to modern engines. Most street applications are closer 1st and 2nd gear ratios for that off the line 0-60 boost or for the fast exec a lower 2nd and 3rd ratio give improved overtaking flexibility and 30 – 70 mph acceleration with a high 4th and 5th gear to give motorway fuel economy. A specialist can set up the perfect box to suit your application and driving style.

The addition of a 6th gear means that the box is best suited to the wide power band of the modern engine and as all of the other gear ratios have less of an overlap the driver has to make a careful selection of the right gear. An overdrive gear was fitted to older cars and this is quite different from a 5th or 6th gear as it altered the final drive ratio. Acceleration was not easy in overdrive but it did make the car more economical and was more of a motorway cruiser.

With powerful cars the gearboxes can break up so you have to ensure that the correct types of metal are used. One cheap but annoying performance mod is to get a lower ratio box from a similar model of car with a smaller engine for example an Astra 1.3 box into a 1.8 or even the 2.0 engine and you will have very good acceleration (your top speed will be lower as will economy but you can't have everything). Be warned though the gearbox will not last long! A Carbon Fibre driveshaft can help prolong the life of a gearbox as it twists and flexes to absorb the initial shock wave from the engine as the revs increase or decrease whereas a steel driveshaft takes the shock wave directly to the box. Thicker oil can help reduce a whine alternatively use a good quality oil additive.

A straight cut dog tooth gearbox is a little more crunchy at times as the shape of the gear teeth is like a dogs tooth (surprise surprise) but this minimises transmission power loss as the gears intermesh a lot more efficiently. Dog tooth gears allow for faster gearchanges, while a straight cut gear gives more durability and strength. Highly tuned engines often have narrow power bands and a dog tooth box with low ratio gearing can maximise the output. As much thought needs go into the selection of gear ratio and type as went in to the rest of the engine tuning.

Gear cuts to choose from range from a straight cut, through Semi helical and helical. An example of gear ratios for big powered V8 sports cars upto 450bhp :-

Limited slip differential controls the power output to the wheels especially beneficial on low grip surfaces, in wet conditions, or with a large powerful engine that causes wheel spin and can introduce a torque steer effect which can, if you adjust your driving style, give better handling on fast bends. A good LSD can also help minimise torque steer (the sideways motion under heavy acceleration.) Quaife are one of the major players in the transmission and diff market and have an application to suit almost any vehicle including backwards gearboxes to handle motorbike engined car conversions.

It is also possible to convert a FWD to 4WD with some transmission upgrades a little imagination, and a lot of determination, or even a FWD to RWD if you like a challenge!

Car specific Tuning tips: