One trick everyone wants to try is a smaller blower pulley that will spin the blower faster to make more boost. While this can be a benefit, the more common result of spinning the blower faster is more heat. This creates a situation where more boost results in less power because the added heat creates a reduced inlet-air density in the engine despite the greater pressure. A good rule of thumb is 1 percent more engine power for every 10-degree-F reduction in inlet-air temperature. As an example, if a centrifugal can reduce its discharge air temperature by 50 degrees, that's worth 5 percent more power. At 600 hp, that's 30 "free" hp.
Blow-Thru CarbsThe easiest way to make a carbureted package work is in a blow-through application. While it would seem you could use any old carburetor, experience has proven there are significant advantages to using a carburetor designed for blow-through applications.
We talked with Marv Benoit of Quick Fuel Technologies. Benoit has extensive experience with blow-through carbureted packages, and his comments were especially interesting. While you might think a big-volume carburetor would be ideal, Benoit suggested a 750-cfm mixer with annular-discharge boosters for our big 540 blower combination. He even has seen where a smaller 650-cfm carb worked best for a boosted blow-through Pontiac when nothing else would work. According to Benoit, the small venturi helps the carburetor maintain a safe, rich air/fuel ratio at higher engine speeds. The annular-discharge boosters are also more pressure-sensitive, which greatly improves fuel flow.
For blow-through carburetors, there are several ways to get there. Drop-leg-booster carburetors can be made to perform well in blow-through applications if extensive modifications are performed. If converting a typical 4150-style Holley carburetor, for example, Benoit recommends increasing the power-valve channel restrictions and reducing the size of the high-speed air bleed. Both of these mods will increase fuel flow at higher engine speeds. Other recommendations include applying epoxy to the main well plugs in the top of the metering blocks to prevent boost pressure from pushing those plugs out under high boost conditions. There are several carburetor companies that we found in our research that offer specially modified Holley carburetors for blow-through centrifugal-blower applications. Talking with these companies and doing research on your specific application will help you avoid the typical problems that can trip up the uninformed.
The "hat" or adapter into the carburetor is another crucial power component. For low-boost combinations, the hat and inlet plumbing is not as critical. But for you barnstormers running double-digit boost numbers, the hat design and inlet piping become crucial. Increasing the volume of the tube by adding pipe diameter from the blower to the carb can help slow the inlet-air velocity to give the carb a chance to work. Benoit's even seen applications where flow diffusers have been used inside the feed tube between the blower and the carb hat to stabilize inlet-air speed in an attempt to make more power. What can happen with excessive inlet-air speeds is the air "stacks up" on the backside of the carb hat and the net result is a lean air/fuel ratio. The large air boxes that encapsulate the entire carburetor, such as Paxton and Vortech kits, can be beneficial when dealing with high-boost-pressure applications. The difficulty with these boxes is that they make access to the carburetor cumbersome.