VJMC’s Steve Cooper appears again…
As early as June 1977 rumours started to swirl across the GP paddock that Yamaha had a technical ace up its company sleeve and the mechanics helped ramp up the puzzlement by protecting their bikes each time they have been parked up. Given the usage of gas injection on the prototype GL750 highway bike a number of years earlier, many thought this was what Yamaha was engaged on. In actuality nothing fairly so subtle had occurred and if an outsider had been in a position to get a peak beneath the covers they’d have come away somewhat upset. The one apparent change had been a transfer to utilise 4 separate cylinders quite than the earlier two units of dual cylinders – hardly ground-breaking and, arguably, probably so as to add weight. However there was a technique on this obvious insanity. Yamaha was evaluating the 4 separate cylinders in readiness for one more technological leap.
The filling and emptying of any combustion chamber determines the ability and effectivity of mentioned engine, however on a two-stroke the consequences are extra profound and helpful or detrimental relying upon the top use. Getting gas and air into the cylinder(s) and thru the switch ports is simply a part of the conundrum – finally how a two-stroke engine performs is considerably dictated by the form, place, dimension and orientation of the exhaust port. The early OWs had proved to be exceptionally thirsty as Agostini discovered when his TZ500 ran out of gas at Imola in 1974. Because the race group had discovered, it was additionally doable to provide a massively highly effective engine (as indicated on a dynamometer at the very least) however find yourself with an influence supply that’s ‘all-or-nothing’ and horrendously peaky. Finally the important thing to energy and adaptability needed to be an exhaust port whose top may one way or the other be various.
Yamaha initially investigated bodily transferring the cylinder wall in relation to each the piston and the exhaust pipe, however swiftly deserted this concept because it was just too difficult and precipitated extra points that it solved.
The answer was as elegant and easy because it was impressed genius – a small cylindrical valve like a cotton reel partially located inside the exhaust port tract close to to, however not at, the cylinder wall. By rigorously profiling the horizontally-positioned valve after which rotating it in keeping with the RPM degree, it was doable to ‘persuade’ the engine that the exhaust port had a variable top. On the TZ500 motor’s all 4 valves have been linked collectively and actuated by a small servo motor that took its directions by way of the tachometer.
The preliminary race outcomes for the 1978 season would have a tendency to point there have been nonetheless issues nevertheless it had been tyre put on points after which sticking carburettor slides that originally hamstrung Kenny Roberts, Johnny Cecotto and Takazumi Katayama. Come the Austrian, Suzuki’s Barry Sheene was complaining the Yamahas had a 20mph benefit over the RG500! Roberts gave the season all the pieces he had and took the title from Sheene by 10 factors with Cecotto third and Katayama fifth, Suzukis filling the interim two locations. Yamaha may need again within the recreation with its renamed OW35Ks, however the massive TZs nonetheless had profound dealing with points.
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