CORRECT MIXTURE

Back in the thrilling days when major factories duked it out for the promo bennies of winning the Daytona 200, it was fascinating to walk along pit lane during practice, seeing the bikes and their technologies close-up.

- Kevin Cameron

Here's something interesting: each of the four header pipes on this Superbike has a threaded position for an oxygen sensor, but there are no sensors and the holes are plugged.

Farther back, on the collector pipe, is a single oxy sensor.

The story this told was that the four sensors were fitted as this engine's fuel mixture was trimmed on the dyno. There are always small (usually!) cylinder-to-cylinder differences. The four individual sensors were being used to find best mixture as a technician with a laptop adjusted each cylinder’s fueling to optimum. Once that was done and stored in the ECU memory, a single sensor back in the collector pipe sufficed to detect the usual mixture variations arising from air temperature and pressure changes.

This problem of getting the same fuel-air ratio to all cylinders becomes larger in engines with single-point fueling - such as most of the large air-cooled radial engines that powered so many aircraft in WW II. A single carburetor added fuel to the airstream heading down to the inlet to the fast-spinning supercharger. To enter the supercharger, the air, fuel vapour, and unevaporated fuel droplets had to make a 70-degree turn. The fuel droplets, being 600 times denser than the air, headed for the outside of that turn. Forget any hope of equal fuel mixture for all cylinders.