How Do I Choose an Intake Manifold?
Which Makes For A Better Manifold: A Dual-Plane Intake Or A Single-Plane Intake? We Explain The Differences And Offer Guidelines On When To Pick One Over The Other..
“In most tests on engines up to 6,500 rpm, a modern high-rise dual-plane intake like this Chevrolet Performance example on a Chevy 350 generates the best overall average torque and power numbers throughout the curve. For max top-end power, choose a single-plane intake.”
The traditional design choice for most stock intake manifolds, a dual-plane intake manifold is essentially two separate intake manifolds separated by a divided and unequal-height plenum. Each half or "plane" routes air and fuel from that group's separate plenum area to half the engine's cylinders. The runners are grouped so that 180 degrees of crankshaft rotation separates the intake cycles of cylinders fed by the same half of the intake plenum. This separation, two slightly different plenum volumes, and the relatively long, curvy runner lengths necessary to clear the other group's runners produces great low-end torque—but on a low-rise dual-plane intake, it also creates a restriction above 5,500 rpm. Using a performance high-rise dual-plane intake, whose added height offers more room for somewhat straighter runners, can extend the usable operating range into the 6,000- to 6,500-rpm range.
On a single-plane intake manifold, all the engine's runners connect directly to a single common plenum with minimum bends, minimizing airflow restriction. With a single-plane intake, an engine's cylinders draw air from a larger-volume common plenum, which makes a smaller carburetor or air meter act larger. That's great on the top end, but at lower rpm the air velocity is lower, providing less signal to the carburetor—and that results in the low-rpm cylinder air/fuel charge-fill density being lower than it is with a dual-plane intake. Collectively, all this tends to reduce low-speed torque.
Another single-plane intake issue is that with a common plenum, adjacent cylinders in the firing order can rob some of the air/fuel charge. For example, on a Chevy V-8 with the traditional 1-8-4-3-6-5-7-2 firing order, cylinder 7 fires right after cylinder 5 on the left bank, and cylinder 4 fires right after cylinder 8 on the right bank. At low engine speeds the first cylinder in each pair receives more of the intake charge at the expense of the second cylinder in the pair. This can make it hard to tune in off-idle transition performance, particularly in heavy vehicles with mild "highway" final drive ratios. At high rpm, this tendency pretty much goes away due to higher flow velocity. EFI has less of an issue with this problem because fuel injection doesn't depend on a vacuum signal to get the system working and is reduced even further on those higher-end port-fuel injection systems where injector bandwidth can be individually tailored to each cylinder.
Those old Ford FE big-blocks can really benefit from a high-rise dual-plane aftermarket aluminum intake. Some of the old iron factory FE intakes aren't too bad in the flow department, but they sure are heavy. This is Edelbrock's Performer RPM FE dual-plane intake manifold (PN 7101), a leg up in both power and torque for those ancient Fords. Photo: Edelbrock
Choose a high-rise dual-plane intake if you need a broad torque curve to enhance low- and midrange power at speeds less than 6,000 to 6,500 rpm.
Choose a dual-plane intake to tame a big carburetor.
Choose a dual-plane intake for better idle quality, off-idle throttle tip-in response, and better air/fuel charge distribution to the cylinders at low rpm.
Choose a dual-plane intake if you want longer runners to move the rpm band down.
Choose a dual-plane intake if you want a less peaky powerband with more rpm separation between the torque and horsepower peaks (more area under the curve).
Choose a dual-plane intake for more average power and torque.
Choose a dual-plane intake if you have mild gears in the rearend.
Choose a dual-plane intake if you just want to run an out-of-the-box carburetor without spending a lot of time dialing in the carburetor for best driveability.
Choose a dual-plane intake if you need an exhaust heat crossover passage for emission compliance and/or decent cold-start performance with a carburetor. (Exception: Edelbrock's RPM Air-Gap series or other manufacturers' equivalents don't have a crossover passage. )
One of the best all-around dual-plane intake manifolds is this Edelbrock RPM Air-Gap (PN 7561) that fits big-block Chevy oval-port heads. If emission compliance isn't a factor but a dual-plane intake's broad powerband is still desirable, consider the Air-Gap series for any street engine. Unlike big E's Performer RPM series, as its name implies the RPM Air-Gap intakes keep the runners and plenum isolated from the valley and allow cooler air circulation under the plenum for greater air/fuel charge density. Also note the partially cut-down plenum divider that helps enhance top-end flow on a hungry big-block Chevy. Photo: Edelbrock
Stay Single Or Hook-Up With A Dual?
Although we've all seen and proven exceptions to these rules of thumb, in general when it comes to deciding between a single-plane vs. a dual-plane, mild carbureted street engines should use a modern high-rise dual-plane intake configuration, such as Edelbrock's Performer RPM or RPM Air-Gap (or other manufacturers' equivalents). For competition use with a matched camshaft, carburetor, torque converter, and steep rear gears, go with a single-plane intake manifold.