Rewind an epoch or two (okay – maybe not quite that long ago). You only had two choices when it came to induction on a racecar: A carburetor (or two, three, four, six or maybe more) or a mechanical constant flow fuel injection system. Carbs were inexpensive (relatively speaking – more carbs equaled more out of pocket cash). But carbs required plenty of tuning savvy and strip time to set up correctly. And the more carburetors you had, the more difficult the tuning task. On the other hand, the set up for a constant flow fuel injection unit was pretty close right out of the box. Of course, the fuel injection system was certainly more costly. At the time, fuel injection offered another advantage: You could run exotic fuels such as alcohol or nitro easily. In the formative years, that certainly wasn't the case with carburetors.
Vintage Mechanical Fuel Injection Systems...
Kinsler Fuel Injection notes that most vintage mechanical injection systems were of the constant flow design (which Kinsler points out is still one of the most rugged and reliable systems): “A constant flow system uses a mechanical fuel pump to increase/decrease the supply flow to the injection unit directly related to engine rpm. This variable flow creates pressure against the fixed orifices of the main bypass jet and the nozzles. Using a barrel valve assembly the idle and the part throttle fuel rate is controlled. We can supply additional bypasses and enrichment circuits to give added flexibility.”
According to Kinsler, a constant flow system has one serious design flaw: "The basic fuel metering is done by sensing throttle angle and engine speed. The throttle rotates a spool with a tapered ramp inside a metering block. Engine speed is sensed by using a positive displacement fuel pump coupled to the engine, so when the engine speed doubles, the output of the pump doubles. The problem lies in the fact that the fuel pump often doesn't suck in all the fuel it should, especially with gasoline, which causes a lean out condition. This happens because the light ends in the gasoline boil at about room temperature, or even lower if you have any vacuum at the inlet of the pump caused by a restriction in the inlet line from the tank. While alcohol doesn't boil as easily, it still requires a carefully plumbed inlet to prevent the lean-out (this is particularly important in non-drag racing events such as road racing)."