For years LT1 enthusiasts have been running well into the nines with factory LT1 and LT4 intake manifolds. With some porting the factory castings have proven capable competitors, even on big-cube and forced-induction motors. However, the brilliant minds at Edelbrock knew that using the knowledge and experience they've achieved through decades of intake manifold development could improve upon GM's 15-year-old design. As a result, Edelbrock created the RPM Air-Gap LT1 intake manifold (p/n 7107) to significantly improve output between 1,500-6,500 rpm, while also pulling the plenum away from the splashing oil in the lifter valley to decrease heat soak. The larger runner cross-section was designed to work with Edelbrock's Performer LT1 head (p/n 61909) or another port-matched LT1 head; however, for the first part of our test we wanted to see what would happen if we bolted Edelbrock's intake to the stock heads.
RaceKrafters of Lancaster, Pennsylvania, was not only happy to lend us some dyno time to complete the test, but the top-notch engine shop even whipped up a fresh 355-cid LT1 with refurbished stock heads (10.7:1 compression) and a new valvetrain. Since the cam in the core motor was shot, it was replaced with Comp Cams smallest grind measuring 210/220 duration at 0.050, 0.500-inch lift and 114LSA with 4 degrees of advance. When rigged to RaceKrafters' engine dyno with 1.75-inch long-tube headers, dual Flowmasters, K&N cone air filter, MSD coil, 6AL ignition, and FAST XFI computer, the LT1 was not far off from your typical bolt-on combo while still controlling as many variables as possible for the sake of the test. In addition, these key components will provide a firm foundation as we continue to build up the LT1's top end. Follow along for a soup-to-nuts comparison of Edelbrock's intake versus stock, and its subsequent test.
With the 52mm throttle body bolted up, the top end was definitely flowing more air as Bob added another 6 percent fuel.
Horsepower went back up to 350.3 at 5,500 rpm and torque climbed to 391.3 lb-ft at 4,000 rpm (at 13.7:1 AFR). Average torque was now up to 376.10 lb-ft and horsepower hit 302.41.
The factory LT1 intake manifold is clearly a well-designed piece, and hard to beat when paired with the stock heads. Despite being more efficient and higher flowing, as indicated by the leaning out of the air/fuel ratio with the addition of the intake (and 52mm throttle body) and its ability to make similar power with a much smaller plenum volume, the Edelbrock intake did not manage significant enough improvement over the factory manifold to warrant its $460 price tag if it is to be the only change. However, most likely this is only because with the larger runners the mismatch from the manifold to the heads becomes significantly increased, causing a restriction. Therefore, in our next installment we plan to test the two intake manifolds back-to-back again, only with the larger ported heads Edelbrock had originally intended for use with the manifold.