Part I: Baseline Testing Edelbrock's RPM Air-Gap LT1 Intake Manifold With Stock Heads
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.
When I arrived at RaceKrafters, our LT1 test motor was already bolted up to the dyno. For baseline testing, the stock LT1 intake and throttle body was left on. However, to ensure reliability a new set of 24-lb/hr injectors were installed. For better real world simulation, and to protect the motor, a K&N cone air filter was used-as opposed to running an open throttle-body. | 
As it would allow the most data collection and precise fueling control, a FAST XFI computer was used to map the fuel curves. The spark was controlled separately through an MSD 6AL ignition box, which was connected to an MSD high-output coil. | 
Through a set of 1.75-inch long-tube headers and Flowmaster dual exhaust, the mostly stock 355 LT1 made a healthy 350.4 SAE horsepower at 5,600 rpm and 386.7 lb-ft of torque at 4,100 rpm on 93-octane pump gas with 38 degrees of total ignition timing. If this seems a bit high, please take into account that the motor has a good-sealing set of brand-new rings and a Comp Cams 210/220-duration bumpstick. |
Edelbrock's RPM Air-Gap LT1 intake (p/n 7107) uses a relatively short-runner tunnel-ram design, which pulls the plenum away from the hot oil that splashes against the bottom of the intake to prevent heat soak. The optimized runner shape, size, and length were designed to take advantage of the Performer LT1 cylinder head's larger ports. Stock or other aftermarket castings could just as easily benefit from enlarged runners with port matching. However, for this portion of the test we intended to see what this intake could do on the bone-stock heads. | 
Though the runners and throttle body opening are already a sizable increase from stock,... | 
...there seems to be enough material for further porting. Like the factory intake, inside the runners and plenum the rough, cast finish is apparent, which had Bob Wise [of RaceKrafters] salivating to pull out the Dremel grinder. Not so fast! |
Off came the stock intake, and with a bead of fresh silicone the Edelbrock intake was set into place and bolted down. The Edelbrock unit is a direct factory replacement, and aside from cleaning out a few boltholes, it should bolt right up with the factory bolts. | 
With the stock throttle body still choking the Edelbrock intake, the motor initially went 0.2 rich, and Bob pulled fueling back 2 percent. He soon learned, though, that the Edelbrock intake actually made better power with a leaner mixture (and the same timing). With the final tune, peak power curiously dropped to 346.8 hp, while torque climbed only incrementally to 387.7 lb-ft (occurring at the same rpm as stock). But average power did increase slightly from 372.51 to 373.19 lb-ft and 299.81 to 299.99 hp. Perhaps the throttle body was holding it back? | 
To eliminate restriction with the stock 48mm throttle body, we had Edelbrock send out its 52mm Twin Throttle Body (p/n 3809). This 50-state street-legal piece has a sculpted smooth entry and a much larger opening with high-quality throttle blades. |