Pump Gas Secrets

In theory, the formula for making a boatload of forced-induction horsepower on pump gas is actually quite simple. Since the detonation threshold of pump gas is largely dependent on boost pressure, the trick is to move as much air through an engine as possible at the lowest boost pressure possible. In essence, boost is merely a measure of backpressure inside the intake manifold, so the most effective way of accomplishing big airflow at low boost pressure is with a lot of displacement and a set of high-flow cylinder heads. Greater engine capacity allows packing more air into the cylinders before boost pressure starts building up in the intake manifold. Likewise, reducing the restriction that the incoming air charge must work against—by improving cylinder head airflow—effectively reduces the boost pressure for any given volume of air. This explains why 16 psi will barely produce 200 hp on a four-banger, while that same boost pressure makes over 900 hp in the supercharged Mast 427.

Big-time cubic inches are easy to come by these days, but it takes some astoundingly good cylinder heads to support serious horsepower in a supercharged pump-gas motor. Manufacturing state-of-the-art cylinder heads is one of Mast's specialties, as it was the first company to release aftermarket rectangle-port castings for the GM Gen IV small-block. For this build, Mast plucked a set of its 12-degree LS3 castings out of its catalog that feature CNC-machined 280cc intake ports and big-block–sized 2.200-inch intake valves. While the factory GM LS3 heads are some mean little dudes in their own right, by revising the port design and flattening out the valve angle from 15 to 12 degrees, the result is a substantial increase in low- and mid-lift airflow. At .700-inch valve lift, the Mast heads flow 387 cfm, but just as importantly, they're already moving 300 cfm by .400-inch lift. While the heads used on the 427 require a minimum bore size of 4.125 inches, Mast also offers its LS3 heads with 2.080/1.570-inch valves that are compatible with 3.900-inch bore motors.

Perhaps the most obvious piece of the pump-gas compatibility puzzle in a forced-induction motor is the compression ratio. Generally, running less compression and more boost trumps running more compression and less boost. According to Mast, this explains why late-model EFI motors set at 10:1 compression or higher from the factory can only handle 8-9 psi of boost on pump gas, but dropping the compression ratio down to between 8.5:1 and 9.5:1 allows raising the boost to 14-16 psi on the same grade of fuel. "Some people run as high as 10.5:1 compression on pump gas, but the only real benefit is that you'll make a bit more torque at slightly lower boost. Beyond 9.7:1 compression, the horsepower benefits are negligible, but the margin for error in terms of tuning the ignition timing and air/fuel ratio becomes much smaller," explains Perry Kiritsy of Mast Motorsports.

Less obvious factors contributing to the pump-gas–friendly disposition of the Mast mill are the camshaft and combustion chamber design. "The cam in this motor is ground on a 117-degree lobe separation angle, which is 3 degrees wider than what we use on our naturally aspirated applications. Since there's a 20-degree duration split between the intake and exhaust lobes, there's still a fair amount of overlap, which in turn reduces the potential for detonation," Kiritsy says. Additionally, the efficient combustion chamber of the Mast cylinder head contributes to the pump gas cause as well. "LS motors have very efficient combustion chambers from the factory, which means that they require very little ignition timing advance. We have been able to improve upon the chamber efficiency even more with our Mast LS3 heads, and as a result, the supercharged 427 needs just 22 degrees of advance on the dyno."

Screwed Over

Granted that a 427ci short-block and 387 cfm cylinder heads provide some serious horsepower potential, much of that potential would go to waste without a supercharger efficient enough to get the job done on pump gas. While centrifugal, Roots, and twin-screw superchargers all have their pros and cons, the need for immediate off-idle boost and compressor efficiency makes the 2.9L Whipple twin-screw unit the ideal piece of hardware for the Mast 427. Unlike a Roots blower that traps air between the internal lobes and the supercharger case, a twin-screw design compresses the air between the rotors themselves. The benefit of this approach is that less heat is added to the intake charge as it's being compressed. In fact, some twin-screw supercharger manufacturers claim an 80-degree reduction in inlet air temperature at 12 psi of boost compared to a Roots-style blower of similar capacity. While some may argue the legitimacy of those claims, a cooler intake charge equates to a lower potential for detonation, and there's no question that the twin-screw–equipped Mast 427 makes big-time power on pump gas.