Hot Cams!

HOT CAMS!

Inside the Honda Interceptor, Suzuki GS750 and Kawasaki GPz750 are the secrets of performance.

Comparing the valve timing of the Honda Interceptor, Suzuki GS750 and Kawasaki GPz750 is a good way to understand how cam profiles determine engine characteristics. These three motorcycles all have similar peak power, but one has substantially less power at low engine speeds.

Cam profiles can explain these differences, not because one has a better cam design than another, but because the cams are designed to make the best use of other engine characteristics.

Look at the charts on this page. The lines show the cam profiles of the three sporting 750s. Obviously the Kawasaki starts opening its valves earlier, and closing those valves later. This makes for not only more duration, but more overlap, when both the intake and exhaust valves are open at the same time.

The standard explanation is that cams with long duration and overlap make more peak power and less power at lower engine speeds. Certainly half of that is true for the Kawasaki, it doesn’t make as much low-speed power as the other 750s. Why1?

This is the result of the Kawasaki’s eight-valve head. With half as many

valves in each cylinder, those valves are bigger than the valves in the Honda and Suzuki. Big valves require more force to push open and close. Because of the limitations imposed by valve spring design, big valves can't be opened and closed as quickly as small, lightweight valves. So the Kawasaki’s cams need more time to open and close those big valves. This explains the long duration, high overlap figures.

Maximum horsepower developed by the Kawasaki isn’t any greater than the other bikes, despite the extreme cam profiles. This is because the Kawasaki has less valve area. Those two valves, when open, can’t flow as much air as the four valves of the other bikes.

The Suzuki and Honda have similar cam profiles, but the Honda opens its valves farther, and the Suzuki has a little more duration than the Honda. Because the Honda has slightly larger valves than the Suzuki, and opens those valves more, it can make more power than the Suzuki. The Suzuki trades a little bottom end performance for some peak power by opening its valves sooner than the Honda.

As the charts show, when measured at

1mm lift, the Honda has 14° of overlap, and durations of 230° intake and 228° exhaust. The Suzuki has 17° overlap and durations of 235° intake and 225° exhaust. The Kawasaki has 30° overlap and durations of 241° intake and 242° exhaust.

These cam figures are accentuated by the differences in valve area. Here, too, the four-valve designs have an advantage. To start with, at low valve lifts, the circumference of the valves is most important to horsepower, and four valves, even with the same cross-sectional area as two valves, would have more circumference. Also, four valves fit into a round combustion chamber better, so the overall area of those valves is larger. In this instance, the Honda had 1.53 times the circumference of the Kawasaki while the Suzuki has 1.47 times the intake size and 1.4 times the exhaust size.

This large effective area of the fourvalve designs makes shorter duration cams work even better. The combination of quick opening and large effective area makes for even greater low and midrange power, which is exactly what the Honda Interceptor has. 9