FZR400/600

 

 

When the aluminum framed Yamaha FZR400 made its debut in 1988, everyone raved about the bike's ultraquick and precise handling, its light weight, the crisp brakes--but they weren't as excited about the FZR's motor. Despite revving to a then-astronomical 13,500 rpm, the little 399cc powerplant couldn't crank out more than 65 or so horsepower in stock form, leaving some riders wishing for a bit more in the propulsion department.

Northern Californian Jeff Short was one of those people. After winning the AFM 450 Superbike-class championship in 1994 (as well as setting the lap record for that class), Jeff decided that bigger would be better. "The FZR worked so well with a 400 motor, I figured with 30 more horsepower I'd have myself a mini-superbike," says Short. The problem was that getting those extra ponies from the little FZR motor would take some heavy duty work, and Short didn't want to have a two-race time bomb between his legs.

The solution? Short replaced the stock 400 engine with an FZR600 powerplant. Since the two engines basically use the same cases (the 600 is basically a bored and stroked 400), installation was a snap; only some slight modifications to the cylinder head motor mounts were necessary.

Of course now that Short had the solid foundation for his "mini-superbike," he immediately commenced work on building it. Down in the motor's bottom end, a lightened crankshaft spins a set of Carillo rods, attached to YZF pistons sliding through a YZF cylinder block (the YZF600's shorter stroke means a larger bore than the FZR, which translates to an engine displacement of 661cc). Up in the G-Force Racing-ported cylinder head spins an unusual combination of cams: a Megacycle X1 exhaust cam is used on the intake side, while a Yamaha 400 factory kit instake cam sits on the exhaust side, both actutating valves through FZR400 single valve springs. This combination evolved after many hours of dyno experimentation: "We were losing midrange and top end due to a loss of intake port velocity; this combination gives much crisper throttle response," says Short.

All this trickery inhales through a rack of 35mm Keihin flat-side carbs, while spend gases are expelled through a Mark McDade hand-built stainless exhaust sysem. Undercut gears transfer the power to a custom Falicon clutch basket: "A notorious weak point on the FZRs," says Short, and with our experience with Yamaha clutches, we'd have to agree.

On the chassis side, Lindermann ngineering-revalved GSX-R1100 forks and triple clamps handle the front suspension duties. A Technomagnesio rim measuring 3.5 X 17.o inches shod with Michelin slick spins up front, with a deceleration responsibility entrusted to Brembo billet GP four piston calipers clamping on 320mm Kosman rotors. Out back, a Lindemann-revalved Fox Shock is attaced to a stock 400 swingarm heavily braced by Rossbaron Racing's Ed Toomey. The pirated rear rim was from a TZ250 widened by Kosman Racing to 5.75 inches to fit the wider big-bike-size Michelin rear slicks.

The stock steel gas tank was discarded, and a three-gallon aluminum tank fabricated by Toomey replaced it; Toomey also applied the strengthening gusset on the frame's steering head section.

The shell for the aluminum tank, laid out by Clay Carrier Composites, is made of carbon fiber. The rest of the bodywork is by Air Tech; the front fairing is a Honda RVF750 Replica, while the taild piece is fashioned after an Aprilia RS250.

And how does the finished product measure up? Short claims his FZR400/600 hybrid tips the scales at a measly 310 pounds, while also cranking out a consistent 110-112 peak horsepower and 56-58 foot-pounds of torque on a rear wheel dyno. Of course, the race results would seem to bear those figures out: Short finished third overall for 1996 in the AFM's Formula One class, running low 1:47 lap times at Sears Point against the bigger 750 and 1100 superbikes.

Short says he would've finished even higher, but for a couple of midseason mistakes. "The thing accelerates really hard off the corners; maybe too hard, come to think of it." Nevertheless, it's obvious Short's FZR400/600 hybrid is a combination that can do serious buisness; and he's offered to give Sport Rider a chance to sample it at an upcoming Willow Springs club race. Who are we to argue? Stay tuned for an in-the-saddle race report.