Intake manifold selection depends on how you intend to build and use your Cleveland.
Street engines operating between 2,000 and 5,500 rpm generally call for a dual-plane intake manifold, which offers longer intake runners that enhance lowto mid-range torque. Classic dual plane manifolds offer a nice combination of good, low- to –mid-range torque while coming on strong at high RPM. Vintage Cobra high-risers, Offenhausers, Edelbrocks, Weiands, and Shelby manifolds are good examples. Edelbrock’s Performer RPM and Weiand’s Action Plus manifolds have taken this old-school, high-ceiling runner approach and refined it for even better performance across RPM ranges. Plenums and runners are engineered to create less turbulence, which improves velocity and fuel atomization.
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When most people think “street” they tend to think dual-plane. However, if you’re adding displacement via stroke, you have to start thinking of your engine as you would a big-block. You no longer have a 351, but instead a 383, 408, or 430. With stroke comes more torque as well as horsepower because you’re moving more air/fuel through those ports. This is why a single-plane, straight-shot style may work better than a long-runner, dual-plane. Often it is a matter of trial and error. You may have to try both to determine which works best for your Cleveland.
Dual- and single-plane manifolds offer different power curves. The dualplane offers a broader torque curve, from 2,000 to 5,000 rpm. The single-plane moves your torque curve higher and increases horsepower, especially with a stroker. Dual-plane manifolds work best for straight street use. Single-planes are more for high RPM and high displacement, where you’re seeking more horsepower as well as torque.
Dual-plane high-rise manifolds don’t always have to be new. Vintage aftermarket dual-plane, high-rise manifolds yield the benefits of low-end torque and high-RPM breathability and can be found at swap meets all over the place. They do well on the street in stop-and-go driving, and they yield plenty of power when it’s time to rock.
Long intake runners and a dualplane design are two reasons you can achieve good low- and mid-range torque from a carbureted engine. You also want cool air, both ahead of the carburetor and beneath it. To get cool air before the carburetor, you need to source cool air from outside. Underhood air is much hotter than the ambient air outside. If you can drop the intake air temperature by 50 to 80 degrees F, it makes a considerable difference in thermal expansion inside the combustion chamber. You can net nearly 10 percent more power this way.
You get cooler air with a hood scoop or a ram-air scoop at the leading edge of the vehicle. Ram air can be sourced through the radiator support or beneath the front bumper. Ram-air kits can be sourced from Summit Racing Equipment or your favorite speed shop.
Getting cool induction air after the carburetor takes closing off the manifold heat passages from the exhaust side of the cylinder head. You do this by installing the manifold heat block-off plates included in most intake manifold gasket kits. Manifold heat is needed only when the outside air is really cold. A cold intake manifold does not allow the fuel to atomize as well as it does in a hot manifold, which causes hesitation and stumbling.
A low-restriction air cleaner that effectively filters out dirt while allowing healthy breathing at the same time is best. Reusable, washable air filters from K&N, AEM, Spectre, aFe, Injen, and others meet the challenge effectively, but they don’t come cheap. They can be washed and reused, which actually saves you money in the long term. Some don’t even have to be oiled, which means all you have to do is wash, dry, and reinstall. A reusable fabric/fiber air filter is money well spent in terms of performance and longevity.
Written by George Reid and Republished with Permission of CarTech Inc