How to Maintain the Carburetor and Fuel System (2)

In oval track applications, a Belt Drive or Hex Drive Fuel Pump is preferred where use of a mechanical fuel pump is specified. These pumps offer the highest fuel delivery volume of any mechanical pump yet maintains low fuel pressure at low engine speeds. This feature alleviates "loading up" of the spark plugs. The BG Six-valve and Super Speedway mechanical fuel pumps will also deliver ample fuel volume when used according to recommendations.

For drag race cars, a BG400-2 Electric Fuel Pump is the best way to guard against fuel starvation. If a car is lazy or lays down at mid-track then pulls well in a higher gear, the engine may be experiencing intermittent fuel starvation.

Why? Typically, the carburetor bowls are full at the starting line so the car leaves hard but in the process, drains the bowls dry. In the lower gears, the car accelerates rapidly with the engine picking up rpm very quickly. This rapid acceleration increases the demand for fuel. When the float bowl fuel level drops, the car lays down because of fuel starvation. In high gear, engine speed increases more slowly allowing the bowls to fill again.

The Fuel Can Test

There's been no shortage of well-researched and well-written articles and books explaining the workings of the fuel system. However, many racers, both novice and experienced alike do not fully understand the physics of fuel flow and horsepower. To produce torque and horsepower requires a mixture of air and fuel. To produce 1-horsepower for 1-hour requires approximately .5-lbs of gasoline. If you ran a single-cylinder engine, like the one in your lawnmower, under a load of 1-horsepower for 1-hour and weighed the fuel tank before and after, the tank would weigh approximately .5-lb (five-tenths of a pound) lighter. Therefore the equation for fuel flow is 1-H.P. = .5-lb of fuel, per hour.

This is expressed on a dyno sheet as B.S.F.C. (Brake Specific Fuel Consumption). Highly-tuned racing engines can sometimes be more efficient, yielding B.S.F.C. figures of around #.40 which means 4-tenths of a lb. of fuel, per h.p., per hour. Incidentally, the formula for Alcohol is approximately 1-lb of fuel, per h.p. per hour which, as a consequence, necessitates the running of a belt-drive pump, but that's another story.

Typically, a 600-HP engine will require 300-lbs of gasoline per hour and, by the same formula, an 800-HP engine needs 400-lbs per hour. Remember, these quantities of fuel have to be delivered past the needles and seats and the fuel pressure regulator. Consider also, the fuel delivery system has to combat 'G' Forces: loadings that are so formidable they can threaten to stall the fuel in the line (this may also give a clue as to why a fuel line that is too large in diameter can be as harmful as one that is too small). This leads us to the area that is least understood.

When you have only one carburetor it should be easier to feed than two, right? Wrong, in an engine with a tunnel-ram layout, both the needle and seat area and the float bowl capacity have doubled! Whereas the single four-barrel car that is most prevalent today, has a much harder task in keeping the fuel bowls full! A 700-HP tunnel-ram engine needs 350-lbs of fuel per hour which equates to a little over 85-lbs per float bowl. A 700-HP engine running a single four-barrel (not so uncommon these days) needs 175-lbs per float bowl, compared to a 1200-HP Pro Stock engine with demands of 600-lbs max, 150-lbs per bowl.

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