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.
To put more numbers to the airflow changes, the upper intake runners went from 1.90-inch cross sections in the RPM to 2.58-inch cross sections in the RPM II. That's about a 30-percent gain. In area, there's approximately 8 square inches at the throttle-body exit in both the Performer RPM and the RPM II intakes, but the RPM II opens to 12 square inches at the plenum (between the end of the cross- over and where the runners begin). Edelbrock says this gives nearly a 20-percent airflow gain over the RPM, and that the extra power should show up around 4,750 rpm and continue on from there. As the company puts it, "This new manifold ... has been designed to run in the 1,500-6,500-rpm range. It has shown significant gains in horsepower (17 hp with our PN 3722 camshaft) over our existing (Performer RPM) and consistent gains (10 lb-ft at 4,000 rpm) over the Holley SysteMAX II in prototype testing." The Performer RPM II carries PN 7123.
Our Z304 Heads
Also new are FRPP's Z304 cylinder heads. Designed with the more advanced enthusiast in mind--the guy who's ahead of GT-40s but not wanting all-out canted-valve heads--the Z304 is sold bare so the owner can select his own valvetrain parts. It was also a clean-sheet design, so it offers the latest in FRPP's thinking on large valves, high-flowing ports, durability of the firedeck, and so on. While designed to bolt to a 5.0 block and to accept all production hardware (valve covers, manifolds, and so on), the Z304 is not a pure bolt-on cylinder head in the sense of the GT-40 line of heads. Instead, it's expected the Z304 owner will stop at the machine shop first to install the valves and springs, guideplates, and rocker studs.
On our carbureted 347 test, the Z304s were 24 hp ahead of the GT-40X heads we ran as controls, so they're a definite notch up the food chain.
Our Pet Mule
Then there's our 347 dyno mule. A Coast High Performance Street Fighter short-block, the engine is really a 342. That's a stock 5.0 bore (4.00 inches) with a 3.400-inch stroke crankshaft. We're sticking with the 347 name because that's what everyone knows these engines as--and they really measure that with a 0.030-inch overbore.
We've detailed our 10:1 compression dyno mule several times before, so we'll just reiterate that it has forged Probe pistons with multiple valve reliefs (accepts any aftermarket head without flycutting the piston) and a CHP hydraulic-roller camshaft measuring 0.500 inch of valve lift and 220 degrees duration at 0.050-inch lobe lift. It has a somewhat "cammy" idle, but it's nothing the mass air EEC IV can't handle. It would make a fine street cam in a project car.
There's nothing trick about the injection or engine management we ran for this test. It was plain, old EEC IV with an 80mm Pro-M mass air meter, 24-lb/hr injectors, an A9L computer, and a stock wiring harness that had been slightly modified to mate with the dyno. By modified, we mean the battery leads were optimized to hook to the dyno battery, ditto with the ground wire, and so on.
In fact, as we had many challenges digging all the EFI gear out of the recesses at Westech--it had been a while since we had run injected--our EFI system was downright bare bones. The fuel-pressure regulator was stock (nonadjustable), and we didn't hook up oxygen sensors. As EEC IV runs in open loop at wide-open throttle, the oxygen sensors are really just along for the ride in dyno testing such as this. We'll add O2 sensors in the not-too-distant future, as we want to work toward duplicating the in-car environment as closely as possible in our engine dyno testing. That they were missing in this test has no bearing on the power figures, however.
We Test, Finally
This time around, just getting our 347 injected and on the dyno with a box-stock Performer RPM intake was excru-ciatingly slow and stressful. In reality it was nothing, just that half the injection parts had been strewn across Southern California and no one seemed to know where any of them were, and when they were all found, they had to be gathered together. As always, we were hugely helped by friends. Mark Sanchez at Advanced Engineering West stepped up with injection parts at the last minute, and Jon Mihovetz of Accufab and Steve Brule at Westech put in time and parts, too. And we can't forget that our friends at All Mustang Performance came through with a stock 5.0 engine that we could scavenge for EFI hardware. We'd never have made it without these folks.
So, after all the running around we felt it was something of a miracle when the 347 barked to life. Naturally, as soon as we tried to pull some power the engine proved lazy, along with a few pops through the intake. We tried a power pull and got a definitely unhappy 350 hp or so.
At times like these--a sick-sounding engine after a week of thrashing just to put the silly thing together--you'd swear something terminally bad had infested the computer, the wiring, or who knows what. And in the end it's always something dumb. This time, hooking a battery charger onto the dyno's battery did wonders, not to mention correctly setting the ignition timing, which in a complete haze yours truly had blithely checked--at precisely TDC.
Knowing that magazine types should not be let loose with tools, Steve Brule yanked the SPOUT connector and rechecked the timing himself, putting it at a stock 10 degrees BTDC. Now the engine sounded as if it would run, and a quick pull netted almost 400 hp and 419 lb-ft of torque. Bumping the timing to 14 degrees initial returned 406 hp and 434 lb-ft of torque.
Steve immediately reset the timing to 18 degrees. We were on 91-octane pump gas and not too sure about that much lead, but Steve wasn't worried--it's not his engine!--and bingo, we saw 414 hp. Obviously, the engine had been ready for more ignition timing.
Still, 18 degrees is plenty of lead. We've seen many 5.0s in the old days--before the widespread use of computer chips--that used this much initial timing, so it's hardly unprecedented. It's possible the Z304 combustion chamber wants extra timing, though we wouldn't really think so as FRPP heads have historically had "fast" combustion chambers rather than "slow" ones. The other possibility is the timing pointer or damper on our engine is slightly askew. In any case, 18 degrees proved the happy spot for the ignition.
At this point we went off on a tangent, fiddling with the barometer and other factors due to the weather. It was unseasonably hot and humid--99 degrees and doggone near liquid air in the dyno cell--and we wanted to make sure the correction factor was correct. After a handful of pulls checking raw data and looking up old data for com-parison, all of us were happy with the correction factor.
Finally, we were ready to make a few pulls for repeatability, which worked out fine. The official power figures for the Performer RPM were then obtained at 414 hp at 5,500 rpm and 439 lb-ft of torque at 4,600 rpm. This is great power. Once again the 347 had made an easy 400 hp, something a 302 really doesn't do, especially not with a fuel-injected manifold. This was nearly the same power this long-block and cam had made with a carburetor, which was another great sign. Happy to know the injection was working correctly, we moved on to the RPM II intake.
Performer RPM II
With the Performer RPM run, we switched to the RPM II. It took about an hour to remove the first intake, lay on a new bead of RTV along the valley ends, set on the new intake, swap over the injectors, put on the upper intake, fit the throttle body, and so on.
With the timing dialed in and the fuel situation looking good on the data acquisition, there was not much more to do other than run the combination several times to even the temperatures and check repeatability of the data. This took five pulls, all of which were close to each other, and all of which were at least 10 hp ahead of the RPM. Our final, official figures of 428 hp at 5,400 rpm and 438 lb-ft of torque at 4,600 rpm showed the RPM II is substantially ahead of the RPM in horsepower and right there in torque for a winning combination.
Looking at our power curves, we had a minor but repeatable trade-off across the curves. In other words, the curves between the RPM and RPM II meander back and forth across each other three times before diverging in the higher rpm range. To get a better handle on which intake really had more area under the curve, we averaged the power and torque from 2,800 to 5,800 rpm with the following results.
RPM Average - RPM II Average
338 hp - 343 hp
414 lb-ft - 419 lb-ft
Change +5 hp +5 lb-ft
Clearly the RPM II is ahead of the RPM, with this set of averaged numbers showing the RPM II offers more torque than the RPM when measuring across the powerband. This is not the case with the peak numbers, which are as follows.
RPM Peak - RPM II Peak Change
414 hp - 428 hp = 14 hp
439 lb-ft - 438 lb-ft = -1 lb-ft
Summed up, trading a bare 1 lb-ft of torque right at the peak, but gaining 5 lb-ft of torque when averaging torque across the powerband, and netting 14 hp at the top end, is definitely a good thing.
About the only bad thing we can say about the RPM II is that if you already have a Performer RPM, you'll need the lower as well as the upper to play. On the other hand, the RPM II power gains are not so great that other breathing techniques could net the same results, so if you already have a Performer RPM, look at the exhaust or camming. And, finally, if you're in the market for a performance intake, the RPM II looks to be a serious contender for top EFI intake honors in the streetable realm, something the RPM wasn't quite up to.
What a neat street engine this would make. Go ahead and figure on giving up 20 or more horsepower by the time it gets in the car (belt-driven accessories, intake air tube, exhaust) and still you have a 400hp thumper with plenty of torque. It would be tons of fun and hardly so high-strung it would tire you out on cruise night.