The time has come to buy a new intake manifold for your hot rod. There are plenty of choices on the market: single-plane, dual-plane, dual four-barrel, three two-barrel, tunnel-ram, and so on. You've narrowed it down to a single carb intake. Single four-barrel manifolds come in two basic configurations: dual-plane and single-plane. Both designs have been around for years, and each has its own benefits. But which is better for your motor's combination? The answer is it depends. But let's back up a bit and see how the two manifold types differ.
Different StrokesFirst, let's look at the most common race manifold (which, obviously, is the single-plane). Generally speaking, a single-plane intake includes a large, centrally located plenum which has reasonably straight runners leading from the plenum to the port entries in the cylinder head. In a single-plane configuration, there is a large common plenum under the carburetor. According to the experts, this "common" plenum allows each runner and cylinder intake port combination to draw from all four of the carburetor venturis at wide-open throttle.
As the partially vaporized air-fuel mixture leaves the base of the carburetor venturis, it forms as four individual mixture streams. When each of the cylinders places a demand on the plenum chamber, these mixture streams (or in some cases, portions of the streams) physically bend in the direction of demanding runner-port entry. The mixture "streams" combine to form a single "mixture river" which flows into the runner, eventually feeding the cylinder which is making the demand.
The beauty of a single-plane manifold configuration is that it allows each runner to withdraw a larger volume of air-fuel mixture during the available induction time span. Unfortunately, life isn't always simple-and neither are intake manifolds. As each cylinder withdraws a charge from the plenum, the "mixture streams" are forced to change direction constantly. Creating more havoc inside the manifold are pressure pulses which travel backward from the cylinder into the manifold runner and eventually into the plenum. And some engine combinations have more of this reverse pressure pulsation than others do. These constant directional changes in the plenum, along with pressure pulses, can create a healthy amount of turbulence inside the plenum.
Dual-plane manifolds, on the other hand, were used in great numbers on carbureted production engines. In these cases, low-rpm performance of the car was the primary concern. To enhance the low-rpm ability of the respective street car engines, a dual-plane manifold was almost always used. In essence this manifold is a two-in-one arrangement. Each half (or "plane") of the intake routes the air-fuel mixture from a separate plenum area to an individual group of cylinders (obviously, four cylinders in a V-8).
With each side of the intake separated from the other, individual intake runners are grouped so that 180 degrees of crank rotation separates the intake cycles of the cylinders fed by the same half of the manifold. Given the layout of the manifold, the runners can be (and, in practice, always are) long. Coupled with 180-degree separation of the cylinders, these manifolds, at least in theory, are best suited to engines which operate up to approximately 5,500 rpm. Of course, this also means that, in theory, this manifold configuration should be best used in a low-rpm, high-torque application.
Much For TheoryFrom our perspective, at least, we thought it would be interesting to compare two very good manifolds to see what the differences in dual- and single-plane manifolds really were. So we picked up the phone and called John Heida at Speedway Testing and posed the intake manifold question. Now, John flushes quite a few engines (drag race, street/strip, and street) through his dyno facility, and we thought he might have a perfect candidate for the test. He did. The small-block he had available was very typical of a solid, basic bracket engine. Specs for the engine are as follows:
355-cid small-block Chevrolet11:1 compression ratioAngle plug cylinder heads; 2.02-inch, 1.60-inch valvesMild porting (not by Speedway Testing)Chevy II oil pan1 3/4-inch fenderwell headersCompetition Cams bumpstick with the following specs:.530-inch intake lift.550-inch exhaust lift250-degrees intake duration @ 0.050 inch259-degrees exhaust duration @ 0.050 inchLobe separation of 106 degrees (installed at 102 degrees)1.5:1-ratio roller rockers780-cfm Holley vacuum secondary carb (brown secondary spring)
The tests were all accomplished with the following parameters: *Chevron 94-octane premium unleaded gasoline (see note) *Water pump driven off crankshaft *No mufflers *PCV system in place
(Note: In John's testing, he has found that Chevron fuel seems to make the best power with an air fuel ratio that appears "rich," which is in contrast to race fuels which appear to make more power with "lean" air fuel ratio numbers.)
Which Manifolds?So we had an engine. Which manifolds do we use? That was simple, too. We called High Velocity Heads, and they supplied a pair of intakes for the test: an HVH-Brodix dual-plane along with an HVH-Brodix SP1 single-plane intake. The dual-plane is a high-velocity piece designed primarily for wide-power-band street applications. It has a manifold height of 4.550 inches. In the opposite corner is the SP1. Obviously a single-plane manifold, the SP1 is a dedicated race piece with a manifold height of 5.875 inches. Meanwhile, Heida had, in his arsenal, an Edelbrock Performer RPM along with a Victor Jr. intake (both box stock). These proven manifolds would give us a good baseline. The test was set.
Testing, TestingSo far so good. What about the tests? Speedway Testing ran a number of different tests (something in the order of 15-plus pulls). Heida baselined the engine with an Edelbrock Performer RPM manifold, then swapped to the HVH-Brodix dual-plane, then swapped to the Victor Jr., and finally to the HVH-Brodix SP1. The first test, with the Performer dual-plane provided the following results:
Peak in baseline configuration was 412.9 hp at 6,200 rpm. Maximum torque was 400.6 lb-ft at 4,400 rpm (so much for the 5,500-rpm max peak power theory).
The second test duplicated the first, with the only addition being the addition of an HVH "Street Sweeper" carb spacer (this is a new configuration spacer that is designed specifically for street-style, dual-plane manifolds-more in the accompanying photos). The dyno results might surprise you.
The torque peak was 408.6 lb-ft at 4,600 rpm. Meanwhile, the horsepower jumped (over the baseline) to 419.7 at 6,200 rpm. The addition of the HVH "Street Sweeper" spacer added 7 hp to the Performer RPM intake, as well as 8 lb-ft of torque. You can see by examining the charts that the spacer allowed John to take the engine higher in the power band. It continued to pull at 6,400 rpm. Just as interesting was the brake specific fuel consumption. It dropped by a considerable margin. In simple terms, the addition of the spacer made the baseline combination more efficient.
In the next test, John removed the Edelbrock Performer RPM and installed an HVH dual-plane intake manifold. In this test, the dual-plane was run without a spacer.
As you can see, the small-block peaked at 423.6 hp at 6,200 rpm, while the maximum torque of 406 lb-ft occurred at 4,600 rpm. In comparison, the HVH dual-plane was slightly more efficient and made more power (actually quite a bit) than the baseline intake. It also made more power and a wee bit less torque than the Edelbrock Performer RPM with the HVH "Street Sweeper" carb spacer installed. You'll also note that Heida took the engine up slightly in rpm, simply because it seemed to like it (note that it was still climbing in peak power at 6,200 rpm).
At this point, John pulled off the carburetor and added a HVH "Street Sweeper" carb spacer. Results are as follows:
With the addition of the "Street Sweeper" carb spacer, the HVH dual-plane intake continued to make impressive power. The peak was 429.6 hp at 6,400 rpm while peak torque was 415.6 lb-ft at 4,700 rpm. As you can see, the spacer moved the power peak upstairs, with a gain of exactly 6 hp. Meanwhile, the torque peak went up by an impressive 9 lb-ft. Without getting into conclusions at this point, it's starting to look like this engine combination really "likes" the HVH dual-plane/"Street Sweeper" combination.
But we weren't quite done. For the next test, John replaced the HVH dual-plane with a tried and true Edelbrock Victor Jr. intake. Victor Jr.s are a stalwart in many forms of racing, and you see plenty of them on the street. How did the race-inspired single-plane fare against the dual-planes? Check out the chart.
It's easy to see that the race-inspired intake was much happier when operated in the higher-rpm ranges. It produced peak hp (421.4) at 6,200 rpm. Meanwhile, the peak torque was 399.2 lb-ft at 5,100 rpm. Compare these numbers to either of the dual-planes and it's becoming clear that a street small-block might really prefer a dual-plane intake arrangement.
But is this completely true? John bolted an HVH "Super Sucker" carb spacer (designed primarily for single-plane intake manifolds) to the Victor Jr. and came up with the following results.
John found a couple of horsepower with the spacer installed on the Victor Jr.
Peak was now 425.6 hp at 6,100 rpm (an increase of roughly 4 hp). Meanwhile, the torque peak with the spacer was 401.9 lb-ft at 4,900 rpm (a couple of lb-ft improvement, but at a slightly lower engine speed). We were close to the power produced by the HVH-Brodix dual-plane, but the torque was certainly down.
At this juncture, John decided to bolt on an HVH SP1 intake. The results of this test were remarkable:
Recognizing that the single-plane intake liked more rpm, Heida moved the rpm upward. He was rewarded with plenty of horsepower: The small-block produced a peak of 428.6 hp at 6,800 rpm coupled with 402.2 lb-ft of torque at 5,000 rpm. It was still short (although very slightly) of the best pull with the dual-plane intake, even when the engine speed was moved upward. To be quite honest, John tried a spacer on this combination, and it didn't really like it (no real power gain). Why not? Heida has a theory (and it's a good one): "The SP1 is a very well-developed intake manifold combination. It might work better with a spacer if the camshaft was different, but in these tests, the manifold worked perfectly right out of the box."
They Do Work As AdvertisedAs you can extrapolate from the above tests, it's easy to see that the manifolds perform pretty much as predicted by theory. A dual-plane simply makes more torque and power down low, but when the throttle is opened, the single-plane can eventually make comparable power (but it might take some work). Single-plane intakes can likely prove more powerful if the camshaft is revised to suit the intake.
So what's the point? In the end, a street-driven or street/strip machine will no doubt be a bunch happier with a dual-plane instead of a single-plane. On the other hand, if you're prepared to gear your car accordingly, fiddle with the camshaft and perhaps use a bit more converter stall speed, then the single-plane intake may eventually be superior. Name your poison. They both work. It's just that some intakes work better than others do.