The ongoing debate over selecting intake manifolds generally divides enthusiasts into two camps of conventional wisdom: single plane is for racing, and dual plane is for the street.
These well-defined lines of thought have become entrenched in performance engine philosophy, but do you know why? What is the technical rationalization behind this logic? How do engine dynamics such as firing order, back pressure, fuel metering, induction pulses and more come into play when designing intake manifolds? And more important, should you consider these dynamics when selecting an intake?
“Each manifold type has a purpose in the performance world,” explains Smitty Smith, technical sales coordinator at Edelbrock Performance. “It’s not just single-plane manifolds are for racing and dual-planes are for street. We have many customers racing with dual-plane manifolds. In some circle track classes, dual-plane manifolds are the spec manifold.”
For street use, a single-plane manifold works great on stroker motors.
— Smitty Smith
“Usually the camshafts and lifters are the limiting factors in developing dual-plane manifolds for the upper rpm ranges,” says Smith, who notes that Edelbrock started manufacturing intake manifolds in 1938. “We are conservative with our numbers, but there aren’t many hydraulic flat tappet lifters that can run 7,000 rpm.”
Turbulence can be a friend
On a V8 engine, the induction cycle of any given cylinder is long enough that it will overlap the next cylinder in the firing order. In the single-plane design, each cylinder is allowed to draw fuel-charged air from all four carburetor venturis. The air-fuel stream is constantly changing direction as the demand from the next cylinder in the firing order changes with engine speed. Back-pressure pulses may invade the plenum and generate turbulence. This reversion in the plenum may affect the carburetor’s fuel metering.
In many engine combinations, this turbulence helps keep fuel suspended and the air tumbling as it enters the combustion chamber. However, the overlap can be a disadvantage at part throttle and low speeds, especially with big camshafts when another cylinder in the overlap phase also opens to the common plenum. This can draw exhaust gasses into the cylinder during the overlap phase, which results in rough running at low rpm and less torque production.
“Then, when the rpm increases, the effect of valve overlap and velocity of the airflow do not matter as much as meeting the demands of the engine’s airflow requirements,” says Smith.
The dual-plane design separates the manifold into two plenum sections. Each plenum and set of runners connect to every other cylinder in the firing order; therefore, each side of the manifold is subjected to pulses from every other cylinder in the firing order. Unlike the single-plane manifold that has overlapping pulses every 90 degrees of crankshaft rotation, the dual-plane manifold only sees an induction pulse every 180 degrees of crank rotation.
“Dual-plane manifolds are often called 180-degree manifolds because of this,” says Smith. “Dual-plane manifolds do a better job at balancing the air from cylinder to cylinder to keep them more balanced throughout the rpm range because of the 180-degree design.”
Understanding induction pulses
More specifically, the induction pulses are transferred and enhanced to half of the carburetor in a manifold with divided plenums, especially at low air velocity where the carburetor’s booster functions can be utilized more efficiently. Remember, it’s all about the air signal back to the carburetor, and thus dual-plane manifolds earn a reputation for better low-end performance, smoother drivability and sometimes better fuel economy. For the street, however, single-plane manifolds are not as efficient.
“When you crack the throttle, the atomized fuel will make it to the cylinders where the valves are closed. A little of the fuel will gather there. When the intake valves open and the vacuum draws in that extra fuel with the incoming atomized fuel air mixture, it makes it a little sluggish,” says Smith.
A long-stroke engine, however, may see advantages cruising the boulevard with a single-plane manifold.
“For street use, a single plane manifold works great on stroker motors,” says Smith, adding that the larger cylinder volume helps dilute the fuel-to-air ratio into a more optimized combination at lower rpm. “That larger engine is a little more forgiving. It won’t be as soft driving away from a stop sign.”
Given that single plane manifolds tend to be a little unruly at idle and low RPMs for street use, there are some upgrades to consider.
“It’s gotta be something more than stock,” urges Smith. “Upgraded cams, upgraded exhaust and multiple spark discharge ignition systems like MSD can help, and probably the last thing is to put in a lower gear ratio to make it work better. An intake and carburetor are one thing; but when you start thinking about camshafts, you’ve got to think about gear ratio.”
Knowing where the air goes
Distribution is also an important consideration in manifold design. In a typical V8 engine layout, the cylinders on the far corners are further away from the plenum than the cylinders in the middle of the engine. This is an automatic challenge for single-plane manifold manufacturers because the flow paths to each of the cylinders are different. In most dual-plane manifolds, the two separate plenums lead to individual intake runners that are longer and closer to the same size. Longer intake runners take advantage of the natural pressure-wave pulses to provide a greater atomized air-fuel charge to the cylinder, which boosts low-end torque, improves idle quality and also part-throttle response.
While longer runners tend to work well at the lower rpm range, shorter runners favor the upper range of the rpm band. Plenum size also has a dramatic effect on power production. Smaller plenums, like those in a dual-plane manifold, do well in the lower rpm range while larger plenums, like the single plane manifolds, tend to boost the top end of the rpm range.
Smith cautions not to rely solely on intake runner length and plenum size for rpm range. “Runner cross-sectional area also plays a big role in rpm range,” he says. “An intake runner with a smaller cross sectional area produces a higher air stream velocity, which is great for lower rpm. Some of our dual-plane manifolds use a larger cross-sectional runner that runs as well as the single-plane manifolds into the 5,500 to 6,500 range.”
One of the first early design tricks from Edelbrock was slightly morphing a dual-plane into a single-plane intake.
“On some of our dual-plane manifolds, we will cut down the center divider; it varies from one manifold design to another,” explains Smith. “The amount that the divider is cut down isn’t determined by numbers on a dart board. This is all based on dyno testing and flow testing by our engineers. What this does is help balance the air signal to the carburetor for the air/fuel mixture at higher rpm. This technology goes back as far as Vic Edelbrock, Sr, and the old slingshot manifolds on flatheads.”
Selecting a manifold
“It’s not enough to bolt on an intake without considering the other components,” says Smith. “The rpm range for a specific intake should match the rpm range of the camshaft and the valvetrain. It should closely match the flow numbers for the cylinder head and even the carburetor’s airflow should be compatible to the intake. We’ve become big advocates in testing all the components with each other. Not just on the dyno but we test them in all types of street platforms.”
Suppose your goal is to optimize an existing engine combination with a new manifold.
“All intakes have a published rpm range that identifies where they are most efficient. This range is based on information collected from testing,” Smith says. “The published RPM range must be considered and is one of the easiest guides in selecting the right manifold.
“You have to take into consideration the whole picture,” continues Smith, “Without being specific, for street cars, a dual-plane manifold would probably be the best choice. Single-plane manifolds would work best in applications where the camshaft, rearend gears and exhaust have been upgraded and you seldom get below 2,500 rpm.
Cam manufacturers also advertise rpm ranges, and these numbers should be considered when selecting an intake.
“Keeping the intake manifold advertised range as close to the camshaft advertised range will help get the best performance for your engine,” advises Smith. “Keeping within 500 to 1,000 rpm between the intake manifold optimal performance range and the camshaft’s performance range is good. Exceeding that may cause problems. An intake range that is over 1,000 rpm higher than the camshaft may cause instability or off throttle hesitation problems.”
Smith also warns to be aware of fitment issues with regards to water-coolant ports, vacuum ports, carburetor flanges and hood clearance.
Manifold design has come a long way to help equalize air/fuel distribution to all cylinders. Utilizing engine dynos to analyze horsepower and exhaust gas temperatures for are time-honored testing procedures. However, before a new manifold is even cast and tested on the dyno, Edelbrock tests design properties using CFD calculations that simulate interactions of airflow across surfaces defined by boundary conditions. Computational Fluid Dynamics software has been used in the space and aviation industries for many years and now is popular in race-engine development.
“What has made Edelbrock intake manifolds research and development so advanced from the beginning is that Vic Edelbrock Sr. understood the need to rigorously test each prototype and design. As far as I know, Edelbrock was one of the first aftermarket manufacturers to have an in-house engine dyno for R&D. Starting with a Clayton 200 engine dyno in 1949, to our current R&D lab with three SuperFlow engine dynos,” sums up Smith. “Both Vic Edelbrock Sr and Vic Edelbrock Jr have insisted on hardcore data on manifolds that is supported by testing on the dyno and in the real world.”