TRUTH ABOUT GENESIS?
Q:Cycle World journalist Kevin Cameron wrote in his book Sportbike Performance Handbook that early Genesis engines had combustion problems, the chamber “was tight and compromised.” Referring to spark lead, the engines had, as he stated, “a disgraceful 45 degrees in the case of the old Yamaha FZ750.” Yet well-known tuner Jerry Branch, who also assessed the FZ750 engine, described the combustion chamber as having “potential” and said it was “shaped just about right with most of the charge in the middle, under the plug”—so which one of them is right? I am in the process of writing a book about the FZ750 and was greatly taken by Nick Ienatsch’s article about Brad Dirnberger’s race-winning FZ. I’d be much obliged if Kevin could take the time to clarify the above, as his detailed Tech Analysis of the FZ750 back in March 1985 is at odds with what he states in his Sportbike Performance Handbook. MICHAEL BOYLE NORTHERN IRELAND A: Saying a cylinder head “has potential” is what you say for public consumption when it doesn’t. Otherwise, why did Yamaha’s chief problem-solver, Masao Furusawa, offer Valentino Rossi four different testbikes before the 2004 season? Two had five-valve heads, two had four-valvers, and both 180-degreefiring and 90-degree-firing cranks were offered. Rossi liked best and went quickest on the four-valve 90-degree engine, and it was on an M1 of that type that he won the championship that year. To my knowledge, no one has used a five-valve in Motogp since. Yamaha's R1 1,000cc production sportbike was itself converted from five valves to four valves, which it now features. The piece by Nick Ienatsch regarding a restored vintage racebike does not describe a bike that was competitive in US national AMA Superbike racing but rather a nicely prepared nostalgia bike, capable of winning vintage events. One American builder who had a lot of experience with the FZ, Steve Johnson, told me with the five-valve the builder had a difficult choice. If he wanted strong acceleration, he could raise the compression, but because that slowed combustion on top-end, peak power was poor. If he built the engine for peak power, with a combustion chamber open enough for combustion-accelerating turbulence to persist all the way through combustion at peak revs, its lower compression ratio reduced acceleration. In his experience, the advantages of the two could not be combined. —Kevin Cameron