Son, you better get yourself some tubs. It's not often that you open up an engine build story by talking about chassis modifications, but with the way Mast Motorsport's supercharged 427ci LS small-block goes about delivering its 917 hp, seriously fat tires and a well-sorted suspension are an absolute must. Before we delve into the specs, we suggest hopping in your car and cinching down that five-point harness. At 6,200 rpm, a stock 427ci LS7 produces 505 hp. In comparison, at that same rpm the Mast LS3-based 427 kicks out a staggering 907 hp. That equates to an 80 percent advantage in power, and the only way to accomplish that given a similar displacement and rpm range is by making 80 percent more torque. The supercharged Mast 427 comes pretty darn close to doing just that, straining the dyno with 817 lb-ft compared to a stock LS7's 475 lb-ft. At the risk of sounding redundant, son, you better get yourself some tubs.

As impressive as the peak horsepower and torque figures may be, what's even more shocking is how much of that output is on tap throughout the entire rpm range. At just 3,100 rpm, the Mast 427 is already producing 743 lb-ft of torque, which equates to 91 percent of its peak output. That's enough to trounce a healthy street/strip big-block. For example, the 532ci big-block Ford in PHR's now-retired Fox Mustang project car—a combo that peaked at 775 hp—was good for just 546 lb-ft at 3,500 rpm and 673 lb-ft at its torque peak. Furthermore, the Mast 427 averages 789 lb-ft between its 3,100- to 6,400-rpm operating range, whereas PHR's big-block Ford mustered a measly 607 lb-ft average torque output. That's the kind of torque that renders half the gears in a six-speed transmission irrelevant, and makes dealing with the size and weight penalty of a big-block a highly questionable proposition.

Make no mistake, this isn't some unstreetable one-trick pony. Mast Motorsports' design philosophy is all about achieving OE levels of driveability and reliability in addition to maximizing power output. Street cars don't run on Q16, so by keeping the compression ratio reasonable and using Mast's own cutting-edge 12-degree LS3 cylinder heads, the supercharged 427 makes its 917 hp on 93-octane pump gas. For a smooth idle quality and street-friendly low-rpm driveability, the engine's 234/254-at-.050 hydraulic roller cam is relatively mild given its commodious displacement volume. Likewise, the reasonable cam duration maxes out the power curve at a moderate 6,400 rpm, minimizing stress on the valvetrain and enhancing durability. So while the supercharged Mast 427 might run like a race motor, it's perfectly suited for a street car—make that a very fast street car. The best news is that anyone can call up Mast Motorsports and order this exact engine combo as a turnkey crate motor.

Big Power, Small Package

Many of today's engine builders report that compared to years past, fewer and fewer hot rodders are opting to build big-blocks. Considering the tremendous power potential offered by modern small-blocks like the LS-series platform, this trend makes a lot of sense, especially in Pro Touring machines where removing weight off the nose of the car substantially improves handling. Not only do aftermarket LS blocks allow for displacement exceeding 500 cubes, there's no shortage of aftermarket heads that can keep pace. Throw a 2.9L Whipple supercharger into the mix, and the result is an insane amount of power in a package not much larger than a wimpy German straight-six. As no surprise, the supercharged Mast 427 takes advantage of the latest in aftermarket block, cylinder head, and power adder technology.

Every motor needs a solid foundation to hold everything together, and this is particularly true when horsepower figures start approaching quadruple-digit territory. Iron is usually the material of choice for engine blocks at this power level, as the distortion associated with aluminum blocks can lead to compromised ring seal and reduced power output. Fortunately, thanks to RHS's LS engine block, fans of the GM Gen III/IV platform can enjoy the durability of an iron block in a lightweight aluminum package. The rugged RHS unit is cast from A357-T6 aluminum, and has proven reliable in 2,000-plus horsepower forced induction combinations. Nonetheless, at 119 pounds it weighs just a hair more than a production LS7 block. For these reasons, it was the block of choice for Mast's blown 427. Mast also builds motors based on the virtually indestructible Chevrolet Performance LSX block with great results, but for this particular build, the nod went to the lighter RHS unit.

A durable rotating assembly is essential to the longevity of any supercharged engine combination, but the parts don't need to be as exotic as you might expect. The beauty of a forced-induction motor is that while they subject the rotating assembly to severe compressive loads, tensile (stretching) loads—which are far more detrimental—are kept in check by keeping peak rpm at a reasonable level. That means that even at over 900 hp, the off-the-shelf Compstar 4.000-inch forged 4340 steel crank and 6.125-inch connecting rods used in the Mast 427 are more than up to the challenge. Billet components would simply be pricey overkill. The same logic applies to the off-the-shelf Mahle 9.45:1 pistons, which are forged from a 2618 alloy aluminum and feature a -25cc dish.

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.

BY THE NUMBERS
427CI MAST MOTORSPORTS LS3
Bore: 4.125 inches
Stroke: 4.000 inches
Displacement: 427 ci
Compression ratio: 9.45:1
Camshaft: Mast hydraulic roller
Camshaft duration: 234/254 degrees at .050-inch tappet rise
Valve lift: .648/.676 inch
Rocker ratio: T&D 1.8:1 ratio
Piston rings: Total Seal; .023, .025, .015
Piston: Mahle -25cc dish
Block: RHS standard deck
Crankshaft: Callies/Compstar forged steel
Rods: Callies/Compstar 6.125-inch H-beam
Cylinder head: Mast Motorsports LS3
Intake valve diameter: 2.200 inches
Exhaust valve diameter: 1.600 inches
Induction: Whipple 2.9L supercharger
Throttle body: Nick Williams 102mm
Engine management: Mast Motorsports M-120 EFI
Header: Mast 2.000-inch primary
Ignition: GM coil packs, NGK spark plugs
Damper: Innovators West

ON THE DYNO
427CI MAST MOTORSPORTS LS3
RPM: TQ: HP:
3,100 743 439
3,200 742 452
3,300 744 468
3,400 750 485
3,500 758 505
3,600 765 525
3,700 773 545
3,800 779 564
3,900 785 583
4,000 792 603
4,100 800 625
4,200 805 644
4,300 808 662
4,400 809 678
4,500 810 695
4,600 811 710
4,700 811 726
4,800 814 744
4,900 817 762
5,000 815 776
5,100 812 788
5,200 812 804
5,300 814 801
5,400 813 836
5,500 810 848
5,600 805 859
5,700 801 869
5,800 795 878
5,900 786 883
6,000 779 890
6,100 774 899
6,200 768 907
6,300 761 913
6,400 755 917

SOURCE
Mast Motorsports
330 NW Stalling Drive
Nagodoches
TX  75964
866-551-4916
http://www.mastmotorsports.com
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