Edelbrock RPM vs. RPM II Intake Battle on the Dyno
Edelbrock muscles back to the front ranks of the street EFI intake-manifold market with its revised Performer RPM II intake
In our May '03 issue we introduced the Performer RPM II intake manifold from Edelbrock ("Biting Back," p. 52). A promising revision of the long-popular Performer RPM for fuel-injected 5.0 engines, the RPM II looked set to meet its goal of pro- ducing the same torque and mid- range horsepower as the RPM, while improving on the high-end horsepower.
Then, in our July '03 issue, we debuted our newest dyno test mule, a 347 short-block from Coast High Performance ("True Power," p. 51). In that article, it was wearing Ford Racing Performance Parts' new Z304 cylinder heads, a Demon carburetor, and a Performer RPM carbureted intake. The combination thumped out 427 hp and 435 lb-ft of torque.
This month we're combining all the players into a fuel-injected review. We had several goals. For starters, we wanted to independently verify Edelbrock's power claims for the RPM II (+9.6 hp comparing power peak to power peak on a 302). Next, we wanted to show what the RPM II would do on a 347 as opposed to a 302. Finally, and not incidentally, we wanted to get our dyno mule configured to run using EFI rather than a carburetor.
The New Intake
What puts the II into the RPM is mainly an enlarged crossover--the runner connecting the throttle body to the runners. The Performer RPM uses a tube-shaped crossover, which has proven restrictive at high rpm and lacking in good flow distribution to the lower intake runners. The RPM II's answer is a "reverse taper"--the triangle shape in the upper intake.
Horse Sense: Our test engine used an A9L computer,24-lb/hr injectors, and an 80mm mass air meter from Pro-M. Historically,this combination will return around 10 more horsepower than Ford RacingPerformance Parts' M-9000-C52 Cobra Kit, which also uses 24-lb/hrinjectors and computer. The difference is the Cobra Kit uses a 70mmaluminum throttle body with a central vane, while our plastic 80mm hasno central obstruction.
The triangle crossover was not enough high-rpm performance boost by itself, so Edelbrock also enlarged the upper runners. This was easily done in the upper, as the casting cores had to be reworked to accommodate the new crossover anyway. In the lower intake, the existing RPM casting is used but with additional machining steps. The first inch or so of the runners is CNC port matched to precisely mate the larger runners in the new upper with the smaller runners in the lower. Another change is milling the upper/lower mating flange of the RPM II lower by 0.375 inch. This allows packaging the taller RPM II upper.
These extra machining steps have led Edelbrock to sell the RPM II only as a stand-alone assembly of upper and lower intakes. Furthermore, RPM II uppers will not work on existing RPM lowers unless the lower is port matched and milled. Logically, if you want an RPM II intake, you should buy both the upper and lower, and as that's the only way Edelbrock sells them, that's the only option.
Curiously, the RPM II has a slightly different bolt pattern between the upper and lower castings than did our early Performer RPM intake (Edelbrock calls such pre-'98 units "eight-bolts" due to the number of upper to lower attachment bolts). This is immaterial unless you're Frankensteining a bunch of parts together, but then it would be good to know Edelbrock changed their bolt pattern five years ago.