The Street Game

A closer look at Edelbrock's Air Gap Performer intake

Although good for its day, the original Edelbrock Performer intake still appeared to be based on design philosophies of the mid 1950s. The thinking of the day was that long, gently curving runners were good for flow but negatively impacted fuel atomization and distribution. A GM-authored SAE paper on the subject justified sharp edges at the plenum/port runner junctions not only on the grounds of but also power output. A probable consequece of this paper was that most aftermarket manifolds of the '60s to the late '70s exhibited somewhat angular port designs. Such ports were easier to make and supposedly aided fuel breakup but, with advancing carburetor design, this may have been questionable. The additional flow produced came about mostly from larger port cross sections. This approach certainly produced more top end power, but reduced port velocity at lower speeds inevitably brought about reduced low-speed torque.

With the introduction of Edelbrock's new generation of small-block Chevy Performer manifolds in the late 1990s, it was apparent that the earlier accepted design practice was being seriously questioned. Past ideas that did not hold up appear to have been totally abandoned. The question we are asking here is, is there a real benefit to the new thinking and if so where, and by how much?

Stock Manifold Capability
In terms of low-speed output, the stock GM Q-Jet manifold does well. With few exceptions, the aftermarket intakes predating the new Performer series produced less torque below about 2750 rpm. This tells us that either the stock pieces are not all bad or that the aftermarket offerings of the day were not all good.

Analyzing a stock GM intake (Fig. 1) demonstrates mediocre overall flow and flow distribution. Flow tests using a stock head with 180 cfm at 400 thousandths intake lift reveal an average flow drop of almost 20 percent and a 27 percent variation in cylinder-to-cylinder flow. Adding a stock Q-Jet carb to the system reduces flow by a further 7 percent. The old Performer was significantly better than stock and reduced cylinder head flow by only 9.8 percent when tested with the manifold only. Cylinder-to-cylinder flow variation was also less at 17.5 percent. Because of its lower resistance to flow, the Performer put greater demand on the carb so when installed the carb droppped flow by some 7.7 percent.

Running the same tests on the current stock-height non-Air Gap Performer paints a far brighter picture. Installing this on the head drops flow on average by only 6 percent while the flow distribution varies by a little over 9 percent. Again, because the manifold is now allowing the head to communicate greater demands to the carb, installing the carb causes a proportionately greater drop in airflow. The point that is beginning to emerge here is that the better the manifold, the greater the carb airflow required to effectively service the system.

Manifold Runner Shapes

It's apparent the current Performer range is far better than than those they replaced and it's easy to see why when runner forms are compared. Although the large radius turns of the latest Performer can be seen externally, the full extent of the differences are really apparent when runner casting core shapes are compared. Fig. 2 shows the conceptual difference in port form of a stock intake, the first version of the Performer, and the current design. The new range of Performer intakes considerably out flows its predecessors and does so with port cross sections smaller than ealier designs, though it is still larger than the stock manifold.

For the small-block Chevy, Performer manifolds are available in three main forms (Performer, Performer RPM and Performer RPM Air Gap), each of which has its own sub-groups to utilize a variety of carbs including Edelbrock's Q-Jet and Performer series, Holley and Demon carbs. To determine which might suit your requirements, get an Edlebrock catalog or hit their web site. Our first tests here were done on a regular Performer which has a carb pad near stock height. The Performer RPM and RPM Air Gap are about one inch taller and do flow better.

Basic Motor Dyno Tests
Initial tests with a regular Performer on a basic low-compression 350 with stock exhaust clearly shows the big improvement achieved over earlier Performer designs. As Fig. 3 shows, the Performer, unlike its predecessor and most other manifolds, actually picked up a little low-speed torque over the stock manifold.

High Output Potential
To establish the top-end capability, a freshly-overhauled 383 small-block Chevy mule with a 7,700-rpm redline was used. Equipped with a port-matched, but otherwise stock single four-barrel Holley Strip Dominator of proven function, this motor cranked out 560hp. The intent here is to test manifold capability, not carb and manifold combinations. Achieving this meant each manifold must be paired with the optimal carb for that intake manifold, leaving only the manifold as a variable. This was done by testing with a variety of carb sizes and booster designs and for help in this area thanks must go to long-time Holley carb guru Norm Scenck.

Flow & Dyno Tests
Having baslined and optimized the 383 on the Holley intake equipped with a high-flow Holley carb, the next move was a precision port match on the Performer to the 383's heads. With this job done, a head from the 383 was used to make a flow comparison between the Holley race manifold and the Performer RPM. Our intent here is to demonstrate the importance of proper carb sizing because with a dual-plane 180-degree intake design, it's likely to be higher than you may expect. The results, Fig. 4, highlight the need for optimal carb selection for a given manifold. Because a 180 design divides the carb capacity in two (each cylinder seeing only half the carb's flow capability) the Performer range will be more sensitive to carb cfm than a single-plane race manifold (such as this Holley Strip Dominator). Installing a 914-cfm modified Holley carb dropped the race manifold's flow from an average of 245 cfm (column 2) to 241 cfm (column 3). The total flow reduction with the carb and manifold in place was 1.6 percent. Substituting the 914-cfm carb for a 1020-cfm carb reduced the flow loss to .82 percent (column 4).

The same tests on the bare Performer RPM showed its less-direct port routing caused a greater flow reduction at 4.9 percent (column 5, compared with column 2). Adding the 914-cfm carb to the race manifold produced an additional loss of only 1.6 percent but this same carb on the Performer RPM caused the flow loss to increase an additional 3.7 percent with it going up from 4.9 percent to 8.6 percent (column 6).

Substituting the 914-cfm carb for the 1020-cfm carb on the race manifold reduced the flow loss due to the presence of the carb to 0.82 percent of that shown by the bare manifold. This same carb on the Performer, where its flow, due to the 180 degree design, is cut in two, showed a 1.2 percent loss over the bare manifold. These figures serve to demonstrate how much more sensitive a two-plane manifold is to carb flow compared to a single-plane manifold. On the dyno, the Performer-equipped 383 produced 534 hp and outpaced the race manifold up to 4,900 rpm by a healthy margin. As far as top end is concerned, the 1020-cfm carb and Performer RPM hung on right up to the 7,400-rpm limit of the test, thus proving it has way more top-end cabability than can normally be expected of a two-plane street manifold.

Real Street Test
So far we have looked at the Performer's capabilities at the extreme ends of the engine spectrum. Now it's time to get real and bolt one, this time an RPM Air Gap model, to an engine that broadly represents the majority of applications it is likley to go on. For this the dyno mule was reconfigured. The fully-ported Sportsman II heads were replaced with a set of un-ported 195 Canfield heads which produced a 10:1 compression ratio with flattop pistons. Ninety-two octane unleaded pump fuel stoked the fire and exhaust dumped through 1.75-inch Hooker headers. Ignition was by a Performance Distributors HEI with a 9000 rpm capability. The Comp Cams valve train was run with both a 270 and a 280 degree hydraulic cam on 107 degree LCA, in at a 103 degree intake centerline. Before getting into test results, some serious credit needs to be issued to people who helped make this test happen. Speed shop boss Tony Brown of Atlantic Racing in Charlotte, North Carolina, loaned us an intake out of stock so we had a manifold on the spot. Crew chief Mervyn Bonnett, Bob Mc Donald and his cousin Derrick McDonald rushed in to provide much-needed manpower to overcome a lot of weather-related obsticles.

Every street guy wants the most power possible, so the Performer RPM Air Gap was pitted against the best in the Vizard arsenal: a Keith Wilson-modified Victor Jr. This, in ten years, has never been beaten for max output. For carburetion, both a 750 Road Demon and an 850 Race Demon were tested. Plug coloration indicated the Performer port forms favored certain cylinders for fuel. To compensate and maximize power with whichever carb was used, some stagger jetting as per Fig. 6 was employed. Fig. 7 shows the results with the 270 degree cam and the 750 Road Demon. As shown, the Air Gap Performer out-performed the Keith Wilson-modified Victor Jr. all the way up to 4,750 rpm.

On the low end, the Victor-equipped 383 could be pulled down to 1,800 rpm but with the Performer's greater torque the lower limit was 1,900 rpm where some 25 additional lbs.-ft. were seen. At the top end the Performer only gave away a little over 13 hp to the tricked-out Victor Jr. A test with the 850 Race Demon showed the carb/manifold combination to be credibly capable of just as much torque with either manifold down at 1900 rpm but only a scant more power at the top end. What this told us is that the head/cam combination's air demand was being met with the 750 Road Demon (which, when tested, flowed near 800 cfm).

When the 280 cam was installed, the 383's hp, when Air Gap Perfomer-equipped, climbed from a peak of 443 to 457. Best power this time by a 7 hp advantage went to the bigger 850 Demon. By contrast, the power with the tricked-out Victor went from 455 to 488, again with the bigger carb. At this point the advantage of the Keith Wilson manifold was starting to tell over the otherwise out-of-the-box Air Gap Performer. A look at the torque tells another story and one more relevant to street performance: At 1,900 rpm, the Air Gap Performer was down on torque by a barely-measurable amount, but our 383, when Victor equipped, was down by some 25 lbs.-ft. Translation--the Performer RPM Air Gap allowed the use of the larger 280 cam and 850 carb with almost zero penalty at low rpm.

The Air Gap Difference
Exactly what is the Air Gap's air gap worth? Temperature testing with an infrared heat gun revealed much. First, the heat soak from a hot engine will, in about 10 minutes, bring the runners of the Air Gap manifold up to that of a non-air gap one. Under full-power conditions, the runners of either type of manifold drop and stabilize after about 15-20 seconds at full throttle, but the Air Gap manifold drops (depending on ambient conditions) about 20 degrees more at 2,500 rpm and about 15 more at 5,500 rpm. The net worth of this in output is about 6 lbs.-ft. at 2,500 rpm and about 4 lbs.-ft. at 5,500. Nevertheless, there is an overriding concern here. If the carb being used has inadequate fuel atomization, the cooler runners can actually detract from output, so be sure to use a carb that does a good job on mixture preparation.

Conclusions
The bottom line here is that Edelbrock's Chevy Performer range of manifolds delivers everthing they claim and then some! The difference in top-end power is not an issue for a genuine fully streetable performance machine. Using a drag strip simulation program revealed that a typical 3,200-lb. automatic trans car with a sane street converter was faster down the strip than a Victor-equipped variant. It made enough extra ground at the start that it was uncatchable until way past the end of the strip and by then the race is over! If your requirement is for a real street performer, then a Performer is what you had best check out.