Improved fuel conditioning builds power and economy

Revised fuel circuits help BIG97s meter a high quality, atomized Air/Fuel mixture for excellent combustion efficiency. With fuel supply capability close to 90lbs of fuel/hour and a BSFC (Brake Specific Fuel Consumption) of just 0.414 lb of fuel/hr/hp, the BIG97 Tri-power has the potential to support significant horsepower.

We achieved this through new power valve circuits for improved fuel conditioning and retuned boosters for perfect signal.

BIG97s also come with new emulsion tubes and revised jetting, compatible with their greater airflow capability. Along with a bigger bore, the new emulsion tubes have a revised tip angle, which improves the signal curve and stops the airflow ‘curtaining’ off the tube discharge hole, helping prevent high end lean out.

The BIG97 Tri-Power is factory jetted to suit a typical Small Block Chevy/Ford crate motor. Jetting for Primary and Secondary carburetors is different, as marked on the carburetor box. Alternative jetting may be required for altitude, other and non- standard engines; different multiple carburetion; forced induction or special fuels; and local and seasonal differences in fuel formulations.

As with a regular 97, idle air bleeds and high speed air bleeds are fixed, though for the BIG97, a wide range of main jets and power valves is joined by a small range of alternative idle jet sizes for more sophisticated tuning.

The BIG97 Primary carburetor has a new power valve. Located in the fuel bowl and activated by a new lever off the accelerator pump, it feeds top end enrichment directly into the emulsion tubes, significantly improving fuel conditioning at higher rpm.

Why is this good? Because the more it emulsifies the fuel, especially at high rpm when manifold pressure is low and vaporization is harder, the easier it is to atomize, distribute and burn. In a regular 97, the power valve simply pours raw fuel through the accelerator pump discharge tubes into the booster venturi, so when it opens (at around half throttle) total fuel conditioning gets worse because you’re using progressively less of the good emulsified ‘main jet fuel’ and more of the raw PV gas.

There’s no air bleed in the pump/PV circuit, either, so the mixture just gets richer as airflow increases. Worse than that, all this is happening at higher rpm when there is less intake manifold vacuum to help vaporization. It’s a triple whammy that can only hurt horsepower at higher rpm. By directing that power valve fuel into the emulsion tubes, the BIG97 Primary improves the condition of the total fuel delivery, meaning more top end HP on less fuel. BIG97 Primaries retain a valve under the accelerator pump, but it shuts off once the pump has discharged. And BIG97 Secondaries also boast excellent fuel conditioning, as all of the fuel goes though the emulsion tubes.

Careful retuning of the BIG97 venturi booster shape and position has produced a very stable signal, whatever the cfm flow, with a curve that follows the mathematical equation to near perfection.

A strong signal ensures the correct Air/Fuel ratio throughout the power band with good atomization even at low rpm. It can also boost fuel economy by allowing smaller jetting, especially in the Primary carburetor.

The point in the venturi shape where the lowest pressure occurs (the vena contracta), is traditionally where the bottom of the booster is placed, but raising it higher in the venturi, with a radiused entry shape, helped air flow and made it easier to achieve a high quality booster signal curve. In addition, the mixture exiting the booster was then free to expand in the area where pressure is lowest, aiding fuel atomization and air/fuel distribution.