Though Vinci Hi-Performance is popularly associated with sensible but nasty street Corvettes, Roger Vinci emphasizes that the modification ratio is two trucks or SUVs for every Vette. Trucks are sleepers, Corvettes are not. Get the picture? VHP works closely with Crane Cams on the development of products for the LS engine, including the Powermax handheld tuner, camshaft phasing, and field R&D. Vinci also tests for Borla, JBA, Holley, CNC Cylinder Heads, Random Technology, and Ecklers. VHP is always looking for a few good trucks.
The 325ci V-8 in our Silverado 1500 has responded well to Vinci's minimal changes and now we're setting the stage for a large increase of go-fast. To recap, the engine has been outfitted with equipment that increased the baseline wheel horsepower and torque levels from 236.7 hp and 289.4 lb-ft to 291.6 hp and 304.7 lb-ft. A sizable increase, it's hardly King Kong on the street-but the program Vinci outlined has jelled right, and each stage has provided the foundation for the succeeding modification.
This time he'll lasso some torque with Airflow Research's small, high-velocity ports and let it roll past larger valves and out freer-flowing exhaust plumbing. Vinci's Greg Warner baselined the engine and installed the AFR Mongoose Street 205 cylinder heads. Vinci says, "We've found that headers on an LS truck make a much stronger statement when they are installed after the new cylinder heads."
Though the Bassani system is patterned after the factory installation, the headers won't bolt to the Magnaflow high-flow cats or the OE cats; both have three-bolt flanges, while Bassani does it with two bolts and a ball-and-socket fitment. Warner responded with a clean cut-and-close operation, adapting the Bassani exhaust header extensions to the Magnaflow cats using the retrofit pipes supplied in the header kit.
There is a slew of aftermarket castings available for the LS engine as of late, though AFR accepted the challenge more than two years ago and offers a couple of excellent concepts for street use; the 225cc variation is geared toward larger displacement (396ci and up) and higher specific output, but for our purpose the small, high-velocity ports of the 205cc head will become a double catalyst. This month, they supersede the stock cylinder heads to enhance the combination with slightly smaller high-velocity intake ports (210 versus 205cc) and larger, solid-stem stainless steel valves (2.02/1.60 versus 1.89/1.55). To retain the LM7's 9.5:1 compression ratio with the OE flat-top pistons, AFR milled the surface to whittle the as-sold combustion chamber volume from 66 to 61cc.
If we'd quit the tune-up here, we'd have planed a few more thousandths from the deck to bump the static compression ratio to at least 10.0:1. But since we're going to apply forced induction, the 9.5 would give us margin for detonation and still provide plenty of squeeze for good low-speed throttle response in this normally aspirated configuration. To sustain the supercharger's riot, AFR casts its heads with a 3/4-inch-thicker deck surface. All ports are CNC-milled, and the rocker stud bosses are reinforced and located 0.100-inch taller than the OE (requiring longer pushrods) and include thick-wall runners and ductile iron interlocking valve seats. According to AFR, at 0.600-inch lift, the 205 pushes 70 cfm more air (300 cfm total) than a stock LS1 casting. This cylinder head is also squeaky-clean smog-legal everywhere and carries California EO#D250-4.
Our controlled experiment kept every-thing else the way it had been before the cylinder head change, including the Crane 1.8:1 rocker arm kit, hot hydraulic-roller camshaft, springs (130 lb/in closed), locks, and retainers. So that Greg's down-time would be minimal, Crane furnished us new hardware for the naked (valves only) AFR heads.