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Truly turbocharged

We learned last week what gears in a gearbox do — first gear multiplies the engine’s power but at the cost of speed, and overdrive is at the opposite end of the spectrum.

india Updated: Aug 27, 2009 16:04 IST
Grease Monkey

We learned last week what gears in a gearbox do — first gear multiplies the engine’s power but at the cost of speed, and overdrive is at the opposite end of the spectrum. We have also learned that the engine produces a lot of twisting force at certain engine speeds, and it needs to be kept in that particular range for the best compromise between performance and economy.

Have teeth, will whine
Gears are like toothed wheels in a bicycle, only they’re a lot stronger. They differ in a few other ways as well. For example, if you think of gears, you’ll think of teeth that stick out radially. Gears with teeth that run straight across the side of the wheel are relatively easy to engineer and engage readily, but the gearbox consists of at least two gears engaging at any point of time. When the two gears’ teeth engage with each other, they make a little sound.

Since many teeth engage and disengage every second, it multiplies and you end up with a characteristic whine.

Engage reverse gear and accelerate slowly — you’ll hear the gearbox whine. However, this noise is absent while using the forward gears, because manufacturers have spent thousands of hours perfecting gears with teeth that are cut at an angle. This eliminates the whine, which aids driving comfort.

Bridging the turbo lag
The number of gears a vehicle has usually depends upon its application. A small-capacity city bike has four gears, because you need good acceleration and not much speed in the city. The Boss Hoss bikes, which are V8 engines slung between two wheels, have only two forward speeds — they don’t need any more! Cars usually have five gears — some cars like the Hyundai Sonata Embera diesel have six-speed manual gearboxes. To explain why, we’ll have to revisit power and torque outputs of the engine. A flat torque curve is desirable, since it is a predictable increase in actual turning force going to the wheels. Cars with turbochargers typically have a jump in torque at the point that the turbo starts getting effective — this is especially pronounced in cars that make a fair amount of power with the help of a turbo. The perfect example of this is the Mitsubishi Lancer Evolution Vlll MR

FQ400, which was a special-edition Evo that claimed a 0-100 kph time of 3.5 seconds. That is what Nissan claims for the GT-R! However, this madness is accessible from 5000-7000 rpm only, and there was a definite ‘before 5000 rpm’ and ‘after 5000 rpm’ difference in the torque. This is why many sportscar manufacturers prefer naturally-aspirated engines, but variable-geometry turbos have taken some of the sting out of the turbo-lag.

How racing cars differ
The cars that the normal person buys are tuned for lots of torque at low rpm, because this makes the car drivable.

This means that you don’t have to shift gears too much in traffic. A racing car, on the other hand, will have to keep the engine revs high because it sacrifices torque at low revs for more pace near the redline. This is why they have six gears — a motorcycle like the Yamaha R15 has six speeds for this very reason. Racers love keeping the engine on the boil. It isn’t easy, but they have to perfect being quick yet smooth if they are to be competitive,

which is why they learn racing techniques like heel-and-toe and double declutching. These aren’t things that you and I will use to beat that traffic light, though!