The heart of the strategy was the construction of a super-strong LT1, bored and stroked from 350 to 421 cubic inches, and nitrous'd halfway to hell. Why 421? A simple, one-word answer suffices: Torque!
Torque is a measure of instantly available twisting force, the kick in the ass that makes a powerful car fun to drive, the force that explodes a drag car off the starting line and through that critical first 60 feet of launch, making the difference between winning and losing. Engine size definitely counts when you want instant, neck-snapping thrust as you mash the loud pedal in a road race. As they used to say, "Ain't no substitute for cubic inches." Translation: It's big-block time, or in this case, big small-block time, like over 400 lb-ft on tap from 2,400 rpm upwards. Instantaneous big torque that's available in the fraction of a second that it takes to slam the throttle wide open. Yeeeeehaaah!
The bottom line is that "four-twenty-one" is about the biggest you can get with a late-model, 350 LT1 small-block, and it takes some nifty tricks to get that big. The CXI design philosophy is to avoid exotic solutions in favor of the best available conventional-type super-duty parts, i.e., off-the-shelf. Then the engine is blueprinted and assembled with great attention to detail, producing straight, stress-free internal engine geometry and, therefore, excellent longevity under extreme conditions.
Building The Beast
Pete Klemm, of Conversion Xtras, researched and conceived the 421 small-block strategy and design. Klemm and Jay Schuster assemble the powerplants. G&G machines the blocks, and Gallant Technical Performance reworks the heads and intake manifolds. The gestation of a 421 stroker begins in the normal manner of any engine blueprinting-disassembling an engine down to a bare block, followed by a thorough hot-tanking and baking. Prep for a 421 continues with the special machining required for the replacement of the three stock center cast-iron main caps with super-duty, splayed-four-bolt center main caps. With the new billet main caps in place and the crankshaft centerline precisely established, the balance of the machining operations use the crank centerline as the basis upon which the geometry of all other surfaces and bores are fixed.
Perfectly straight engine geometry is critical to improving the overall structural integrity of this large a small-block. If lifters bind in their bores as the camshaft rotates, it creates stress risers in the engine. A crankshaft that flexes through various static and dynamic harmonics as the reciprocating assembly rotates produces additional stress risers, stress harmonics, and vibration frequencies, sending shock waves throughout the block. When engine geometry is absolutely straight, there's less internal friction and stress. The block is effectively stronger because forces that would otherwise be working against reliability in normal operation have been alleviated.
CXI's Corvette 421 uses a 4.125-inch, 4340-alloy, zero-balance stroker crankshaft. The 4.125-inch stroke is essentially the longest stroke that can be made to fit a late LT1 while maintaining excellent structural integrity. The crank is extensively machined for reliability, balance, and clearance, including welding and lathe work on the counterweights. The block requires extensive metal removal in the main support webbing, removal of material in the oil pan rail, and corresponding reshaping of the specially selected (for crankshaft clearance) oil pan. It is bored .030-over (to 4.030-inches), and honed with a torque plate to simulate the distortion produced by fully torqued head studs. It then is decked and fitted with ARP head and main studs, ARP rod bolts, and Grade 8 fasteners are used throughout the powerplant.