The HP Ultra billet metering blocks come with five interchangeable emulsion jets per circuit (making 20 overall for all four barrels) that create an almost limitless tuning arrangement for optimizing the main fuel delivery curve. Tuning these emulsion jets should be left to the experts, but the basic premise is that these emulsion jets introduce air into the main metering circuit. The amount of fuel in the main well is determined by the main jet. But this can be further trimmed throughout the entire fuel-delivery curve (from low engine speed to peak rpm) by adjusting the size of these emulsion holes. Making an emulsion jet larger adds more air and leans out a particular portion of the fuel curve. Conversely, reducing the size of the emulsion jet adds less air and richens the circuit. Air is introduced into successive emulsion holes from top to bottom uncovered as the float level drops during a sustained wide-open-throttle run such as at the dragstrip. From what we can gather from the carb tuners, these are the basics, but the circuits do not always respond in simplistic terms. Before the days of adjustable emulsion holes, this type of tuning required permanent modifications to the metering block. The beauty of aftermarket adjustable metering blocks is that the emulsion holes can now be easily tuned by replacing tiny jets. And if you get lost, you can always return to the original emulsion package and start over.
Blow-Through Carbs
When centrifugal superchargers really began to push major air a few years ago, the demand for blow-through carburetors matched the demand for blowers. Regardless of what you might have heard, it takes serious modifications to create a good blow-through carb. Because of the massive amount of compressed air that the blower moves for a high-horsepower combination, the carburetor has to meter a bunch of fuel. While a standard Holley downleg booster carburetor will work, the most popular and successful blow-through carbs use annular discharge boosters. These boosters flow more fuel for the same signal and introduce the fuel into the engine in a more atomized form to make more power.
Blow-through carburetors like this Quick Fuel 750 use annular-discharge boosters to delive
But there's more to this than just using an annular-booster-equipped carburetor. According to Quick Fuel Technology's Marv Benoit, increasing the main well capacity of the metering block and using smaller emulsion holes increase the volume of fuel. Quick Fuel also increases the diameter of the passage leading to the booster in an effort to feed more fuel. This approach even extends to the carburetor bowls. With low-boost combinations under 10 psi, a stock Holley needle-and-seat assembly is sufficient. However, with the mondo blower packages capable of moving and pressurizing huge volumes of air, just delivering enough fuel through a single four-barrel carburetor becomes an issue. Benoit says he replaces the typical 0.110-inch-diameter needle and seat with as large as a 0.150-inch assembly that radically increases the volume of fuel delivery. This also requires attention to the fuel-delivery system because excessive fuel pressure can do more harm than good by aerating the fuel in the bowl.
The next step in this direction is dual-needle-and-seat float bowls, such as the C&S Super Bowl, that are appearing on some of the more radical blow-through carbureted applications. We saw this idea on two engines in the DynoMax shootout last year fueled with E85. One of the first to champion this idea many years ago was McClintic Racing from Albuquerque, New Mexico. Expect to see several versions of these dual-float-bowl designs in the near future. Benoit says he thinks that adequate fuel delivery is critical on any engine making 500 hp or more and especially important on a blow-through application.
Another selection tip is to undersize the carburetor slightly. As an example, Car Craft tested a 540ci big-block Chevy on the dyno with a blow-through F2 ProCharger in the Oct. '05 issue ("Blow-Through Superchargers"), making a repeatable 976 hp on pump gas with what most enthusiasts would consider a too-small 750-cfm annular-discharge Quick Fuel carburetor. While power was limited with the factory iron heads, the carb and fuel-delivery system performed flawlessly.
Bowl Side of Metering Block
(A) Timed spark port:
This outlet supplies ported manifold vacuum for distributor vacuum advance only after the throttle is opened slightly.
(B) Vent whistle:
This plastic vent piece vents the float bowl area and also prevents fuel from splashing into the primary venturi under hard acceleration.
(C) Idle mixture screw:
This adjuster screw meters the amount of fuel and emulsified air delivered to the engine at idle.
(D) Accelerator pump entry point:
This is where the fuel from the accelerator pump enters the metering block, traveling up that adjacent diagonal port to the center hole on the opposite side of the metering block.(E) Main jets:
These are the replaceable main jets used to trim the main metering system.
(F) Power valve:
Fuel enters the power valve enrichment circuit from the float bowl.
(G) Other idle mixture screw:
This adjuster screw meters the amount of fuel and emulsified air delivered to the engine at idle.
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