Since superchargers push air and fuel into the engine, valve lift is also an important consideration. Roller cams tend to offer increased lift compared to flat-tappet cams, but also cost more money. Ideally, a mechanical-roller cam will offer the most lift for the degrees of cam duration compared to a flat-tappet cam. You will also want to complement that high-lift cam with a set of quality roller rockers and good valvesprings. Good springs may not seem like a priority, but especially with big-block engines that use large-diameter valves (like our Rat), these components quickly become very heavy. Even at 6,000 rpm, this extra mass is tough to control. Consider, too, the boost pressure working on the inlet side of the intake valve. As an example, let's take a big-block 2.250-inch intake valve producing an area of 3.97 square inches. With 20 psi of boost pressure with the intake valve on its seat, we have 20 psi pushing against almost 4 square inches, which equals about 80 pounds of force trying to open the intake valve against a spring pressure of perhaps 130 pounds. That leaves only 50 pounds of spring pressure to keep the valve closed at high rpm when valve bounce is a common problem. On the exhaust side, typically the exhaust valve will be required to open against higher residual exhaust pressure in the cylinder, making the job tougher for the rocker and pushrod. As you can see, stuffing a blower on an engine makes it harder on the valvetrain components for reasons far beyond merely increasing cylinder pressure.
Boost Retard And Race GasThe simple act of specifying exactly when the spark plug lights is just as crucial on supercharged engines as it is with their normally aspirated cousins. Since we're talking street blower engines, we need something a bit more sophisticated than OE energy. The idea is to design a system to deliver an aggressive advance curve for excellent part-throttle performance while retaining the ability to retard the timing when the blower begins to make serious cylinder pressure. There are several ways to go about this, including a digitally programmable distributorless ignition from MSD or the equally trick Electromotive system. These options are pricey, but there are more affordable alternatives. MSD offers the combination of excellent spark energy with the affordable MSD-6A along with the ability to adjust timing based on boost. MSD calls this unit the 6-BTM, or Boost Timing Master. A dash-mounted control knob allows you to adjust the amount of ignition retard in degrees per psi of boost up to a maximum of 15 degrees of retard.
As an example, let's say we've determined that our blower motor prefers 42 degrees of total mechanical advance at WOT for best power normally aspirated on pump gas. With the blower, we know this much timing will create a major rattlefest with detonation on pump gas. So let's add the MSD 6-BTM and dial in 2 degrees of retard for each 1 psi of boost (e.g., 5 psi retards the timing back 10 degrees, from 42 to 32). The engine is happy, and we can now experiment with total timing with the simple turn of a dial. If you already have an MSD-6A box, you can purchase a standalone BTM module with the retard knob.
The advantage of a system like this is it allows you to run the engine with a stronger timing curve when the engine is at part-throttle on the street. It also allows you to quickly advance the timing if you decide to drop in a load of race gas that can handle more ignition timing without incurring the wrath of the detonation monster. Also remember that a powerful ignition is only strong if the plug wires and spark plugs are doing their job. Always use low-resistance plug wires to ensure all that spark energy is actually reaching the plugs.