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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

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