On those rare days when CXI Racing's 1,000-hp, 421-inch LT1-powered '95 Vette gets a break from the grind of commuting in Houston's traffic and goes racing, Jay Schuster is the driver. Today we're doing a photo shoot and Jay is following tight on my pickup's rear bumper as I lean out the back, shooting the car at 55 mph on a two-lane highway. Further behind, in Conversion Xtra's other test mule-a 2000 C5-owner Phil Twardowski is running interference to keep other traffic safely out of the way. Traffic is beginning to thicken behind the C5, and one of the trapped vehicles is a black Porsche Turbo Carrera, whose driver is showing definite signs of impatience
I snap another photo, freezing the C4's image at 1/30 of a second with a nice road blur through my Canon T-90's wide-angle lens. Suddenly, above the roar of the wind I hear the unmistakable shriek of a high-output engine, and through the lens see the black Porsche leaping out over the double yellow lines to pass the traffic. I signal my driver and our three-car convoy exits the highway into-of all things-the entrance to the biggest outdoor shooting range I've ever seen. This must be the Wal-Mart of gun ranges.
My driver makes a U-turn around an island, followed by the two Corvettes. Without warning, the Porsche flashes by with a nasty blare of fiercely revving German engine crackle and turbocharger whistle, accelerating toward the shooting range-and the driver raises a fist with a single digit pointed skyward. Man, this guy must need a bigger gun, or something! I glance back at Jay in the red C4, with its 650-horse-on-the-motor LT1 under the hood and the two-stage nitrous system's bottles hidden under the hatch, and catch the look on his face as the jerk in the Turbo escapes unscathed. Jay's expression is like something you might see on a big tomcat that hasn't eaten in a long time as it stares at a fat pigeon on the wrong side of a window. The look a predator has when its brain is jacked full-on to a setting way beyond kill-maybe torture to death-and you know that pane of glass is gonna melt away any second. There's a promise, a mission, in Jay's expression, and I figure that the clown in the Porsche better join a Witness Protection Program, or trade the black Turbo on a Geo Metro and take a five-year road trip to Siberia, because there's gonna be a 1,000-horse street Vette looking to blow him into the next dimension if they ever meet at a stoplight! This has to be the first time I've ever felt sorry for someone in a Turbo Porsche.
The ConceptAmerica's real sports car, says Conversion Xtras' main man, Phil Twardowskij, is the Corvette. Okay, there's a thing called a Viper, but where do you see one except occasionally on the cover of a magazine? Daimler-Chrysler has only sold a few thousand of them, and there are only a couple of shops in the country that even know how to hot rod one of 'em, much less make a decent living at it. No, Phil figures that the Vette is still the Real American Sports Car, and CXI's strategy is to focus on tuning '92-and-newer Corvettes and their LT1 and LS1 small-block V-8s with performance parts and packages, and building turnkey street racer tuner cars. The red '95 C4 is a CXI concept car, designed to showcase the company's abilities to build an ultimate street car out of any '92-and-up Vette. Says Jay Schuster, "We give the customer a Corvette where basically he can go out and carve up a 'built' Viper."
The idea behind the 421 C4 was to preserve the Vette's basic appearance, character, and independent rear suspension, while stacking on plenty of big stroke, a ton of nitrous, and all of the key suspension and drivetrain upgrades needed to make 1,000 reliable rear-wheel horsepower in a balanced package that gives up nothing in the way of street manners, and has the cojones to kill anything else on the pavement, be it in traffic, on a road course, or down thequarter-mile.
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.
Camshaft-to-connecting rod clearance is critical with this long a stroke, requiring that the cam bores be align-honed and fitted with oversized bearings. The cam is machined from a special reduced-radius, 1-inch base circle lobe designed to clear the rods as they swing much further outward on the massively stroked crank. Valvetrain components include Comp Cams' R-series T-bar hydraulic roller lifters, super-duty valve springs, and 1.6-ratio roller rockers, a package that, in concert with the CXI cam, will resist valve float to the 421's 7,500-rpm redline. The cam specs out with .598-inch gross lift on both intake and exhaust, while the duration at .50-inch lift is 244 degrees on the intake and 246 degrees on the exhaust. Lobe separation is 114 degrees, befitting a big-inch engine intended for streetability and for kicking ass when the occasion arises. The billet steel connecting rods, 5.850-inch standard length Olivers, feature special undersized big ends and relocated rod bolts, to reduce the proximity of cam and rods, and are symmetrically machined for added clearance.
Each 421 small-block is assembled by Pete Klemm and Jay Schuster, using custom JE forged low-silicone aluminum pistons. These pistons utilize short skirts to clear the throw of the crank, feature reverse domes, and are set up for full-floating pins. The top rings are plasma coated, while the second rings are of standard, cast-iron design. The ring groove spacing is wider than stock for additional ring-land strength under heavy nitrous applications. In concert with the lightly milled cylinder heads, the compression ratio works out to a stout 12.0:1. Thanks to an artful GM engine management recalibration by Eclipse Engineering in Whittier, California, the 421 runs happily and without detonation on 93-octane pump gas.
The heads start off as LT4 castings, thoroughly worked over by Gallant Technical Performance for serious maximum airflow and velocity, which is essential to making maximum power across a broad powerband-in this case, from off-idle to 7,500 rpm. Maximum flow, as measured on the GTP flow bench, is 300 cfm on the intake side, and 220 cfm on the exhaust. GTP also hand-ports the LT4 intake manifold, which is first cut apart by CXI, from plenum to the head ports, and opens up the throttle body bores to match a dual-throat Arizona Speed and Marine 58mm throttle body, which flows 1,000 cfm. A set of Ford Racing Technology (formerly SVO) 30 lb-hr injectors and a high-volume fuel pump provide both reliability and 600-700 horsepower worth of fuel, and an MSD 6A ignition with nitrous timing retard handles the combustion requirements.
A McLeod dual-disc clutch and lightweight steel-faced aluminum flywheel provide a surprisingly light touch for a setup capable of reliably transferring 1,000-plus nitrous-assisted horses rearward through a beefed-up ZF six-speed. The C4's four-wheel independent suspension has been tweaked with Koni double-adjustable "yellow" shocks and Suspension Engineering urethane bushings. A six-point rollcage meets both NHRA safety requirements and adds a needed dose of extra stiffness to the Vette's chassis, which has been lowered an inch at the front and 11/4-inches aft. ZR-1 wheels and BFG Comp T/A R1s are used on the street, while drag radials struggle to apply some of the prodigious power during the CXI 421 C4's drag strip forays. Seriously needed braking capabilities are on hand, thanks to huge 13.5-inch Baer brakes with four-piston calipers planted at all four corners.
The reality of the car is that there's no longer a need to look for power, but how to deal with too much power. In other words, "What stock part breaks next?" CXI's Jay Schuster relates that he's broken both halfshafts, shattered the differential case, and twisted the Dana 44 housing sideways and open about half a foot. By now the CXI folks have learned exactly how far you can go with a stock-type C4 suspension under almost any conditions.
And how much power is too much? On CXI's Dynojet chassis dynamometer, the '95 churns out slightly over 575 rear-wheel horsepower-on the motor only-and exceeded 1,000 horsepower at the tires with the multi-stage nitrous system activated! As you can imagine, launching this wild C4 on street tires takes a fine touch, and is not for the fainthearted. Even with the exceptionally limited traction, CXI's '95 Corvette has run the quarter in 11.40 seconds, reaching a phenomenal terminal velocity of 142 mph! There's nothing around that's a legitimate street car-including those overpriced factory hotrods that are named after a venomous snake-that can give this Vette any trouble.
It's a sauna outside as we drive around Houston in the late summer. The city is a massive, steaming pile of heat and humidity, but CXI's C4 idles nicely, without the slightest hint of engine cooling problems, and we're cool as the proverbial cucumbers with the A/C blasting away. With the Corvette's removable top clamped in place, the stereo sounds great, and the sound of the engine is a distant, rolling thunder. Wretched excess my ass, this thing's perfect!