The electronic fuel-injection systems that have been installed on Corvettes since the early '80s have improved dramatically over the years. So has the tuning software that provides the capability to enhance factory calibrations. The result of all this electronic wizardry is a series of engines that produces more power, delivers better fuel economy, and generates lower exhaust emissions than would otherwise be achievable. But beneath all the wiring harnesses, sensors, relays, and controllers are engines that still require air, fuel, and internal combustion to create torque, horsepower, and carbon footprints.
That being the case, the recipe for a hearty "horsepower cake" still requires the proper mechanical ingredients; electronic alterations are simply the icing on that cake. The performance packages developed by Chris Harwood and Wes Burch of Xtreme Motorsports in Blountstown, Florida, address that fact by using selected mechanical components along with ECM/PCM calibrations developed for each combination of mechanical components.
One of the latest Xtreme packages includes a selection of bolt-on components designed for '08 and '09 LS3-powered Corvettes. It consists of Melrose long-tube headers, Random Technology high-flow converters and cross-pipe exhaust system, Callaway Honker cold-air system, 10mm plug wires, and custom tuning. Noticeably missing from the package is an aftermarket exhaust system. According to Harwood, "We have a lot of customers who really hate the drone you get with a lot of aftermarket exhaust systems, so we put together a basic package that keeps the stock mufflers. If noise isn't an issue, or if a customer likes the tone of a certain system, we can always add it to our package. We've found most low-restriction systems are good for 8-10 hp at the wheels compared to the dual-mode stock mufflers."
Considering the 60 hp and 46 lb-ft of torque increases delivered by the Xtreme package, the stock mufflers obviously aren't all that bad. And with headers and high-flow converters in place of the stock components, the exhaust is relatively quiet, yet has a surprisingly authoritative tone even when the bypass valves are closed.
All LS-series engines respond well to exhaust-system components that provide improved airflow capacity, but the LS3 is particularly responsive, owing largely to its 6.2L displacement and large-diameter valves. Although the exhaust valves are only marginally larger than those used in LS1 and LS2 engines, the LS3's 2.16-inch intake valves and 4.060-inch bore diameter translate to increased exhaust volume at the end of the combustion cycle. Consequently, modifications that improve exhaust flow provide an extremely attractive repositioning of horsepower and torque curves.
Long-tube headers are particularly beneficial when installed on engines with above-average breathing capability. The ceramic-coated Melrose headers included in the Xtreme package feature 1 3/4-inch primary tubes and 3-inch-diameter collectors. They connect to a pair of Random Technology SuperStainless catalytic converters, which feed into a 3-inch Random Tech stainless steel cross-pipe exhaust system. As a means of optimizing exhaust flow, the cross-pipe retains a 3-inch diameter all the way back to the exit end, where the pipes are reduced to 2 1/2-inch diameter so they fit the stock pipes that lead to the mufflers.
Harwood and Burch have also developed an alternative package that incorporates 1 7/8-inch headers and a Random Tech 3-inch converter/cross-pipe exhaust-system assembly. In addition to fitting C6 Z06s, this package can be installed on LS2- and LS-3-powered C6s and connected to a pair of Z06 mufflers to create a full 3-inch exhaust system. The intent of this configuration is to provide maximum power with no drone. (That system wasn't installed on this particular vehicle because it had the optional dual-mode LS3 mufflers.)
According to Harwood and Burch, with either system, the headers and converter/cross-pipe assembly are relatively easy to install. Once the original exhaust manifolds have been removed, the headers slip into place with a minimum amount of pushing, shoving, cussing, and bleeding. The passenger-side header slips in from the top, while the driver-side is more easily moved into position from beneath the car.
It's actually easier to install headers on a C6 than on a C5. LS2 and LS3 engines aren't equipped with an external air pump, so the headers don't require an external plumbing or connections. Minor changes in the chassis and drivetrain also reduce installation obstacles. According to Harwood, "I really like the Melrose headers. They're easy to install, fit very well, and make great power."
Slip fits between the headers, converters, cross-pipe, and exhaust system continue the easy installation theme. Band clamps, which overlap the collector on one end and the converter on the other, assure leak-free connec-tions, as do the original clamps at the cross-pipe-to-exhaust system joints. Harwood typically welds the remaining connections, where the converter out-lets slip into the cross-pipe, but clamps are also an option.
As a complement to the exhaust enhancements, the Xtreme package includes a Callaway Honker to bring a higher volume of cold air to the throttle body. The Honker achieves its airflow advantage through a replacement filter element and housing in combination with a modification to an existing shroud. That modification allows cold air to flow directly to the filter element.
As is obvious from the dyno charts, the Xtreme LS3 package works "Xtremely" well, delivering 431 hp and 412 lb-ft of torque--increases of 60 hp and 46 lb-ft over the maximum readings recorded prior to modification. As impressive as those numbers are, they don't tell the whole story. Although large numbers at the top end of the horsepower and torque charts are important at bench-racing sessions, "my Corvette is faster than your Corvette" fests and other verbal contests, they don't have much influence on normal street and highway driving. On the other hand, midrange torque brings an ear-to-ear grin to a driver who slowly rolls into the throttle when pulling away from a traffic light or the idiot in the next lane.
In addition to the improvement in peak power levels, the Xtreme package also delivers torque increases of more than 15 lb-ft from 2,200 to 3,000 rpm, 20 lb-ft from 3,100 to 3,300 rpm, and 30 lb-ft from 3,400 to 3,800 rpm. At 3,900 rpm and above, torque increases are 40 lb-ft or more. Keep in mind that throughout all the testing, the '08 C6 was in emissions-compliant, street-legal form.
But that's just the tip of the iceberg. If an LS3 (that has literally never even had its valve covers removed) can put 431 hp to the ground through an automatic transmission, consider its horsepower potential after a set of ported heads and performance camshaft are installed. Do I hear 500 hp, anyone?