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Fuzz Town­shend de­mys­ti­fies your car’s rev counter

Classic Car Weekly (UK) - - Living With Classics - FUZZ TOWN­SHEND CCW’S MAS­TER ME­CHANIC

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’

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