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Fuzz Townshend demystifies your car’s rev counter

It’s tacho time with our rev revolutionary, Fuzz

B efore the advent of the engine rev counter, or tachometer, judging the stresses inside a car’s engine was more down to sound and feel than science, meaning that less intuitive drivers risked over-revving and causing serious damage.

Many different forces are at play within an operating internal combustion engine and these will generally increase under the effects of load and speed until a point is reached when something will break.

It was quickly realised in competitive motoring circles that a more reliable method of determining engine speed was essential if the ever more highly tuned machinery was to be prevented from throwing itself apart in a centrifugal tantrum. Under race conditions, the rev counter became the driver’s number one feedback instrument, removing the speedometer’s sometimes handicapping ‘fear’ effect and allowing the machine to be driven to within a hair’s breadth of destruction, giving a vital link between the engineering design team and the person behind the wheel.

Fitting rev counters to roadgoing cars quickly gained popularity, especially in post-war years, allowing drivers to better understand an engine’s sweet spot and govern use accordingly. Naturally, this extra knowledge had many knock-on effects, as people began to realise that cars of the day were being thrashed to pieces at fairly mundane road speeds, leading to the trend of manufacturers offering gearbox overdrive units and, eventually, five- and six-speed gearboxes in combination with higher ratio rear axles. The science of the rev counter also helped car owners to ascertain at exactly which engine speed a flat spot may be occurring and so alter fuelling accordingly.

Of course, boy and girl racers now had a visual means of more accurately judging gear changes for fast getaways and knowing when the all-important red line was about to be reached. Joking apart, the rev counter perhaps ranked third in terms of importance of the classic dashboard dials, behind the oil pressure and temperature gauges.

Earlier rev counters used a mechanical means of transmitting engine revolutions per minute to the gauge, usually via a drive from the camshaft, using a cable in a similar fashion to a speedometer drive. In this case, the counter was similar in construction to an eddy current speedometer. Electrically operated rev counters traditionally took their engine speed information from either the contact breaker side of the coil or, as with some aftermarket installations, from a sensor placed around one of the spark plug leads. In both cases, the rate or interval of the electrical pulses gave the information necessary to determine engine speed.

‘Boy and girl racers now had a visual means of more accurately judging gear changes for fast getaways’