Multis Multiply

MULTIS MULTIPLY

GORDON JENNINGS

MOTORCYCLE RACING'S QUEST for more and more power always seems to lead us along the path to more and more cylinders. The factories have been moving in that direction for a long time, and there are even a few private constructors building "multis."

In the first category we have Yamaha, with a new V-4 scheduled to replace their very successful twins. Yamaha's four is arranged in a 70-degree V, with two separate crank assemblies, one located above the other and geared together. Cylinders for the lower crankshaft are horizontal; those for the upper crank are inclined forward 20-degrees from the vertical. Induction into the crankcases is through the usual disc-type rotary valves. The carburetors are also located one high/one low, and matching blisters on the fairings give clearance at the intake bells.

Rumor has it that both airand watercooling have been tried, and even mixed cooling with air for the lower cylinder, water for the uppers, and vice versa. However, at the Yamaha four's first outing, in Phil Read's capable hands at Monza, straight water cooling was used. Pre-racing trials, conducted in England at the Snetterton circuit with an air of great secrecy surrounding everything, had Read and Duff both trying the air-cooled version.

No details have, as yet, been released by Yamaha, but the four would presumably have the 250cc twin's short-stroke, big-bore architecture. Also presumably, it would have the direct-feed pressure oiling system introduced on the 250 and included on the new 125 twins. In all likelihood, the water-cooled four has an engine-driven pump to circulate coolant, as is the case with the 125 twin. There is a new frame, to house the much-taller four, and it is said that the handling is improved compared with the 250 twin.

One fact is very clear: the four, despite early-development carburetion bothers, is faster than the twin, and that would indicate a power output circa 60 bhp. That, in turn, would seem to imply that the Yamaha four is probably the fastest 250cc road racing motorcycle in the world. The "old" twin has already demonstrated a sheerspeed capability on parity with the Honda six, and the Yamaha four is even faster — with development work only just having begun. Yamaha racing engineer Takehiko Hasegawa, who designed the machine and brought it from drawing-board to testtrack in only eight months, has brought honor to his house.

In some respects, an even more remarkable achievement is represented by Tauno Nurmi's home-built 350cc V-8. This Finnish engineering instructor has used Honda 50 pistons, connecting rods, valves and carburetors, but the main cases, cylinders and heads are castings made specially for the job. The four chain-driven overhead camshafts have ignition contact-breaker cams on their driven ends. Ordinary coils, drawing current from a 12-volt wet-cell battery, provide the spark. There is a separate carburetor for each cylinder, and a separate exhaust pipe complete with megaphone.

Nurmi estimates the power output at 47 bhp, which seems entirely within reason. The power peak is at 12,000 rpm. The question of the engine's mechanical limit was settled when a missed-shift allowed the revs to zip over 15,000 rpm. One of the connecting rods promptly broke, and that ended Nurmi's first appearance with the machine at the Finnish Grand Prix.

With 47 bhp, Nurmi's back-yard 350 isn't going to cause the Honda four any problems, but the Norton and AJS singles should be fair game. The power is sufficient and Nurmi has assured himself of good handling by installing the engine in a "Manx" frame. If the clutch and gearbox problems can be sorted out, the Nurmi V-8 should do very well next season. We hope its example will encourage others to try something similar. •