Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/126801
FZ7S0 r IT IS RIDE. (CUII/lll1lt'd/,,,ot 1)(l,l.!,t I FZ750 PENTROOF COMBUSTION CHAMBER Yamaha says the FZ750's combustion chamber shape at TOe produces ideal flame propagation compared to pentroof chamber. Christmas is for kids. And for the kid in us all. So, to make it easier than ever for kids of all ages to get in on the fun of riding, we're offering great deals on a variety of fun bikes. And while you're here, browse through our accessory section. We have gifts for every member of the family.. HONDA THE LIAOER FOU.OW ***************** ttl. K:eiz AND SON STORE HOURS Tuel.-S.t. 8:30 to 9:00 MOTORC;~:~~~I~~. 417 E. Garvey Ave.• Monterey Park, CA 91754 (818) 280-8244 (213) 283-4277 Cycle News Ad Hotline 213-427-7433 80 This cutaway of the FZ750 engine shows the shape of the ports for the three intake valves and the narrow, at-the-top-of-the-cylinder-liners-only water jackets between each cylinder_ The intake tract is nearly vertical in the narrow, high-revving engine. as fuel level decrea es. With almost 50% of the machine's weight on the front wheel, steering caster can be steeper, trail longer and a l6-inch front wheel can be used without causing oversensitive response to steering inputs that reduces high-speed tabilily. Handling respon e is excellent because a shor.ter whee1ba e can be used without the instability caused by too much rearwheel weight bias. In shon, the FZ750 chassis geometry achieves both light handling and good straight-line stability that had been thought to be incompatible goals. Thesmallerdiameter front wheel al 0 reduces frontal area for improved aerodynamic efficiency. Caster on the FZ750 is 25° 30', and trail is 3.7 inches. . With the carbs and air cleaner out of the way the area beneath the seat and tank can be much narrower and lower for an ideal spon riding position. The FZ750's engine is unquestionably the narrowest 750cc inline-four on the market. By liquid cooling only the tops of the otherwise-dry liners, a closer cylinder bore spacing is achieved. Mounting the generator behind the cylinder instead of on the crank end greatly reduces overall engine width. Another Yamaha first. the sensors for the electronic ignition are mounted in the crankcase halves and tbe trigger is set into the outer crankshaft flywheel. With an overall engine width of only 16.3 inche ,the FZ750 engine is 9mm narrower than • the XJ4QO a~d as srrp as clf re~~ly- available V-4s offered by the competition. The FZ750 engine thus has the advamages of a V-4 in narrow frontal area, with none of the disadvamages of the heighl, weight and overall bulk that come from having toaccommodate a second pair of cylinders behind the from two. In conventional engines, the shape of the com bustion cham ber is dictated by the size of the imake valves. Using larger valves to increase intake area puts limitations on the shape of the combustion chamber, making it difficult to achieve the high compression ratios necessary for high power output without detonation. But with the FZ750 the intakes aTe smaller'than the exhausts. Using more valves gives a larger intake area and additional induction flow without requiring large degrees of valve lift. The outer intake valves are set at 17.25° from the venical, with thecentral one at 11.5°. Exhausts are set at 13.75°. Four-valve engines use near 20° angles, and two-valve engines around 30°. The narrow angle allows a lensshaped combustion chamber with a smaller volume than conventional cylinder head configurations. Improved £lame travel from the centrally-located plug means compression can be increased without fear of detonation. The compression ratio of the FZ750 is 11.2: I. The pistons have slightly concav~ crowns. giving the cl;lamber its lens shape and assures adequate clearance between piston and valves and ideal-