SPICA Today – Part 3

We’ve mentioned that the SPICA fuel injection pump uses a three-dimensional cam, traced by a mechanical stylus, to control how much fuel is delivered under different circumstances. The stylus, through a mechanical linkage, rotates the pump plungers to expose more or less of their helical slots to the fuel supply.

The cam itself is fundamentally a lumpy cylinder, which is rotated about its axis by the throttle linkage. Pushing the throttle pedal down rotates the cam; its profile moves the stylus and so rotates the plungers to the right fuel supply position. The cam also moves horizontally as engine speed and load vary. This is achieved through six steel balls in a cup-shaped housing; as engine speed rises, the balls are flung outwards with ever-increasing force, acting on the curved inside face of their housing to push the cam sideways and so move the stylus along another varying profile. Other adjustments are applied for altitude (via an aneroid bellows) and temperature.

The end result is exceptionally precise fuel metering across
the full range of engine needs: more fuel at higher rpm, less at
idle and zero fuel during deceleration. “Precise” in that the
cam has over 3,000 specific metering points – Wes and Herb
have mapped its profile on a test bench!

However, as racers began to tune these engines with bigger valves and lumpier cams, improving their breathing at higher speeds beyond the road cars’ 5,800 rpm top end, they began to find that the fueling was off; they suspected that the SPICA pumps couldn’t keep up.

In fact, the reverse was happening. The SPICA units, being positive
-displacement pumps, increase their output linearly with increased speed, and were over-delivering fuel, beyond the engine’s ability to breathe well enough to use it. In addition to some painstakingly-developed custom grinds of the cam for modified engines based on Wes and Herb’s detailed mapping (e.g. bigger cams need more fuel where maximum torque occurs, 4,400-4,800 rpm), they found a way to use a supplemental spring to slow down the fuel increase at higher rpms.

As a result, SPICA remains a very viable choice for both
road and race cars, with precise fueling at all speeds.

Engine Restoration Specialists