Boost-Referenced Fuel SystemsTo make power, you have to have fuel-it's that simple. But when you apply blower boost to the top of a carburetor, that pressure works against the fuel pump. With 10 psi of boost pressure pushing on the fuel in the float bowl, there's no way 7 psi of fuel-pump pressure will be able to push its way past the needle and seat. The solution is easy with the addition of a fuel pump capable of both high flow and high pressure matched with a boost-referenced fuel-pressure regulator. This style of regulator employs a small reference line routed from the carburetor hat to the regulator diaphragm that controls fuel pressure. As boost pressure increases in the intake manifold, this automatically "references" the fuel pressure. Most boost-referenced regulators are designed to maintain a 1:1 relationship, so if the regulator is tuned for 7 psi of fuel pressure normally aspirated, and the supercharger makes 10 psi of boost, then the fuel pump would have to generate 17 psi going into the carburetor to maintain a 7-psi fuel pressure difference. Often, the fuel delivery system will fall short of delivering at max demand, which is why keeping a careful eye on fuel pressure is ultra-important for blow-through supercharged systems.
A big-horsepower system needs a good fuel delivery system. This means a fuel pump with plenty of delivery capacity along with a large line size and no restrictions. There are many companies building high-capacity pumps and regulators that will do the job. We talked with Aeromotive president Steve Matusek to gain some insight for our project. Besides the Aeromotive pump, filters, and regulator he recommended, Matusek strongly encourages building a full-flow or return-style fuel delivery system. In this type of system, the regulator diverts excess fuel back to the fuel tank. The advantage to this type of system is that pressure does not "dead head" against the regulator and create large pressure fluctuations determined by whether the needle and seat is open or closed. A return system creates a more stable delivery of fuel to the carburetor, which is ideal for creating best power, and it appears that less fuel pressure minimizes foaming and aeration in the float bowl. Since we're trying to make 1,000 hp with a single four-barrel carb, this design should make a measurable difference. Our dyno-cell education on fuel delivery with our blow-through 540ci Rat has drilled home that even "mild" 500-600hp blow-through systems require an excellent fuel delivery system. Without it, you'll never come close to making the power.
Cam SelectionConventional wisdom contends that one key element to making blower engines work is a wide lobe-separation angle. This has to do with the overlap period that encompasses exhaust-valve closing and intake-valve opening. Most high-power, normally aspirated cams rely on overlap to improve cylinder filling. With a supercharger, we bank on blower pressure to push the air and fuel into the cylinder. With more overlap (where the intake tract is open to the exhaust side of the engine), intake pressure will push the boosted air and fuel right out the exhaust rather than filling the cylinder. So, the smart move is to employ a cam with wider lobe-separation angle to make more power.