In 1985 GM released one of its last major updates to the first-generation small-block Chevy, the Tuned Port Injection (TPI) system. It was the General's first foray into individual-runner and port-fuel-injection technology and a major step in the gradual climb out of the horsepower Dark Ages for the Corvette.
The TPI intake lived in the hearts of C4 Corvettes for seven years before the "Gen II" LT1 small-block replaced it in 1992. During that span, the Corvette and its mountainous low-rpm torque were king. The L98's 18-inch intake runners were tuned for producing maximum power at lower rpm, yielding as much as 250 hp and a whopping 340 lb-ft in the '90 and '91 models. But while the tuned runners produced gobs of power down low, their length and somewhat narrow diameter ended up starving the engine of air at around 4,500 rpm.
Compared with the broad powerband in today's LS3-powered Corvettes, the narrow power range of even a highly modified TPI engine-and the frequent shifts that result-can greatly reduce your chances of winning a close race, despite the abundance of available torque.
Over the years, there have been numerous efforts to improve upon the stock TPI intake. These include larger intake bases and runners, short-runner intake manifolds (similar to the LT1 intake) and, of course, the venerable John Lingenfelter-designed ACCEL SuperRam (above right). Compared with the stock TPI intake, it features a larger plenum and throttle body along with slightly shorter (approximately 15-inch) runners. The result is an intake that is still tuned to make lots of torque down low but can also breathe and make power beyond 4,500 rpm.
But much to the dismay of many a torque-loving Corvette owner, the SuperRam is no longer in production, and good copies are becoming more expensive and difficult to find. Additionally, the SuperRam was developed in a time when free-breathing aftermarket cylinders were not widely available. Lingenfelter created an excellent manifold for use with stock ported heads (which flow around 280 cfm), but with the cylinder-head technology of today delivering an easy 300 cfm of flow, a need arose for a manifold designed with these capabilities in mind. Edlbrock's Pro-Flo XT looks to have the potential to fill this niche.
The Pro-Flo XT (above and top left) was designed from the ground up, utilizing computational fluid dynamics and CAD technology. It features a 90mm inlet for a front-mounted LSX-style throttle body on a 190ci central intake plenum. The plenum feeds eight individual 5.5-inch-long intake runners, which taper down approximately 30 percent to the intake port. Edelbrock says its tests have shown that the Pro-Flo XT beats the company's previous single-plane-style Pro-Flo 2 EFI manifold across the board, picking up as much as 15-20 hp on some combinations.
It's no coincidence that this tunnel-ram-style intake looks very similar to the custom sheetmetal manifolds popular with serious racers. The medium-length runners are tuned to make power across the board and into the 6,500-rpm range, and their tapered design is intended to ram air into the combustion chambers for maximum power. The Pro-Flo XT is 9.05 inches high at the tallest point-the throttle body flange-making it taller than the SuperRam. With the low hoods of Corvettes, and the C4 in particular, this could be a major roadblock. We set out to determine what it would take to make this intake fit a fourth-gen Vette, and how it would perform against the mighty SuperRam.