Tuning Your Pontiac For Gasoline Density And Performance

Once the specific gravity of the fuel is identified, a decision for jet tuning can be made. As with determining and using air density ("Weather or Not You Go Faster," April '06 issue), good notes and a baseline need to be determined. For example, if you ran your Pontiac at the track with 0.712 fuel and the best performance was obtained with a certain jet, use that as a baseline. On your next visit, the fuel checks at 0.736, so a jet change is in order or the engine will be richer than required for peak power. The calculation is:

Using a carburetor manufacturer's jet-area chart (Demon Carburetion has one in the company's catalog), decrease the jet area by approximately 3.25 percent.

If your Pontiac requires dedicated race fuel, it is also important to acknowledge the specific gravity of the gasoline. Upon request, racing fuel dealers provide an information sheet on the fuel they sell. Due to the small-batch production of race gas, along with a higher level of quality control, huge differences in specific gravity on a load-to-load basis are unusual. If you're a traveling racer, then it's important to check and adjust for variations in specific gravity of different brands of race fuel. For example, if you race in New Jersey using Sunoco Cam 2 and travel to an event in Ohio where only VP is offered, it's good practice to check the density of the unfamiliar gasoline.

The actual fuel-check procedure is better explained with images, so follow the photos and captions as HPP shows you another trick to keep your Pontiac ahead of the competition.

Carburetor Icing
Carburetor icing occurs when the intake air is chilled below the freezing point of water by the vaporization of the gasoline. The ice forms on the throttle blade(s) and in the venturi and can cause the engine to stall as soon as the gas pedal is released to idle. This ice formation restricts any air from passing by the throttle plate(s), choking the engine.

Icing can be acute when the air is moist (70 percent or higher relative humidity) and the ambient temperature is between 35 degrees F and 55 degree F. These weather conditions are common during the fall, winter, and spring in many parts of the country and can last into early summer in coastal regions. Carburetor icing is lessened when the intake air is much below freezing since it's common for the humidity to be less.

The extent of carburetor icing doesn't depend on the weather alone. It also involves carburetor and vehicle design and the mechanical condition of the engine, in particular, the components that affect warm-up time. Thermostat, choke, intake-air heaters, and heat risers all help to eliminate carburetor icing. Part of the solution also involves gasoline volatility.

A good index of the tendency of a gasoline to cause carburetor icing is the 70-percent evaporated-temperature specification in the distillation profile. The lower this temperature, the more severe the icing.

Carburetor icing is not as big a problem as it used to be. For emission control reasons, most carburetor engines built since the late '60s are equipped with intake-air heating systems, which generally eliminate carburetor icing. But keep in mind that many aftermarket intake manifolds and headers eliminate these components. Also, race gasoline, which is not designed for cold-weather startability, has a tendency to create more icing than street fuel.

The symptom of carburetor icing is the engine loading up shortly after its started, much in the same manner as with a faulty choke pull-off or stalling when cold and the throttle is released.