The power valve (PV) is a fuel valve that provides additional fuel flow at higher rpm in a Holley-type carburetor. It operates on vacuum, and various power valves open at different levels of vacuum. Low-speed open throttle yields less pressure (meaning a more powerful suction inside the intake manifold) compared to atmospheric pressure than high rpm at open throttle. The vacuum in the intake manifold is reduced as the rpm increase and the power valve opens at a preset pressure level to provide added fuel for a more efficient air/fuel ratio.

If the PV opens too early, the air/fuel mixture will be too rich. If it opens too late, the engine will starve for fuel. Both results bring a loss of horsepower. Before trying to work with the PV, it is best to know the amount of manifold vacuum you have.

If the PV closes at 5 inches of Mercury, it will open when the engine accelerates and the pressure in the intake manifold reaches that level. The engine might start out drawing 10 inches of vacuum on corner exit, be down to around 5 inches at the flagstand when the PV opens, and be at 2 inches or less at the end of the straightaway at the engine's highest rpm.

It is best to try different PVs, especially while the engine is on the dyno, to find the maximum efficiency and horsepower. Monitoring the exhaust gas temperature (EGT) would be a great way to judge the fuel burn efficiency at various rpm. Is this done on a regular basis? In reality, no.

Air Filter Tech
Most of us don't think much about the air filter as it relates to horsepower, but there are some tuning tips that can help produce added horsepower. We need to think about how the air is routed to the carburetor and how we can improve the flow.

Four-barrel carburetors were designed originally to draw air from the sides. If we direct air from the base of the windshield through a tunnel to the carb, or from any one direction, we could be disturbing the function of the carburetor and not getting an even distribution of air/fuel mixture to each cylinder.

The entire surface of the air filter should be pressurized by the incoming air-not just the side facing the air inlet. Many engine tuners tape off a portion of the air filter or build a wall in front of it to force the carb to draw its supply of air from all around the air filter in equal amounts.

Improved air flow is critical to the carburetor. That is why racers are particular about the design of air filter housing. Care should be taken to make sure that the air coming into the carb is flowing in from all sides.

Adequate air flow is equally important for the fuel bowl vent atop the carburetor. A minimum of 31/44 to 1 inch of space must remain above this vent so that the fuel bowl will be properly ventilated. These vent tubes can be cut shorter to provide more space between the top and the air filter box.

Spark Plug Tech
The heat experienced by your plugs can dictate how efficiently your engine is burning the air/fuel mixture. One easy way to tell if your plugs are getting hot enough is to observe how far the heat has penetrated the threaded portion of the plug. We need to make sure that we are using the correct length for the spark plug's threaded portion. The threads cannot extend into the combustion chamber or end up short of the chamber.

Many top engine tuners have made a science of inspecting the tip and insulator of the plug. Most short-track racers don't have the time or the patience to get that technical. If you remove the plug and visually observe how many rings of the threaded portion of the plug are discolored, you can tell if the plug is hot enough for your application. Usually, if two to three threads from the end of the spark plug are discolored, it means that there is sufficient heat to provide complete combustion.