351 Cleveland Engine Induction Guide: Intake Manifold

Intake manifold selection depends on how you intend to build and use your Cleveland.

 

Dual-Plane

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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It is one thing to choose the right carburetor and another thing to pick the right carb and manifold combination. Use dual-planes for driving and low-end torque, and single-planes for high-RPM use. This is a single-plane Edelbrock Torker 351, which is not a good street manifold, but good for drag racing and high RPM. This manifold lives at 3,000 to 7,000 rpm. It does not fit the 351C with 2-barrel heads.

This is the Trick Flow Track Heat (PN TFS-51600111) single-plane intake manifold for the 351C (9.200-inch deck height block) with 2.100×1.500-inch intake ports. This is a street/strip manifold designed for high- RPM use at 3,000 to 8,000 rpm, which is where it lives. What you get from this manifold is peak torque in the 4,500- to 5,500-rpm range and peak horsepower from 6,000 rpm up. Best carburetor for this manifold is the Holley 4150 double-pumper.

Edelbrock has answered the call of Cleveland quite well in recent years. The Performer LB 351C-4V manifold is designed for 351C engines with the factory 4-barrel heads. What you get from this dual-plane manifold is the Holley 4150/4160 baseplate flange pattern and long, high-ceiling intake runners that improve low- to mid-range torque. Valley width is 6-31/32 inches for the 1970–1974 351C only. There’s also the Performer 400 non-EGR and EGR manifolds for the 351M and 400 engines with long runners for good low- to mid-range torque, which are compatible with 2-barrel heads. Valley width is 8 17/32 inches for the tall-deck 351M and 400.

Single-Plane

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.

 

Power Curve

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.

 

Cool Air

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.

 

Edelbrock took 351C induction a step further with its Performer RPM Air Gap 351C dual-plane manifold (PN 7564). The Air Gap isolates runners from engine heat, which helps control induction air temperature. Runners are generous in size and offer length, which means great low- to mid-range torque. The Air Gap is designed for Edelbrock’s own Performer Cleveland heads plus factory 2- and 4-barrel heads.

Magazine tech writer Richard Holdener was curious about what could be done with a 400, so he put one to the test at Westech Performance Group using a variety of approaches. Richard quickly put to rest the idea you can’t do anything with the tall-deck 400 Cleveland. He fitted this one with Pro Comp heads, Edelbrock Air Gap induction, and a Holley HP 750-cfm carburetor (to name the things you can see) and made 489 hp and 502 ft-lbs of torque. Richard stresses the 400 is the legendary 351C with a raised deck and 4.000 inches of stroke; so why the bad rap? This engine can make real power given cylinder heads, induction, and a hot cam.

 

 

When Richard Holdener installed this single-plane CHI manifold, an 1,150-cfm Holley Dominator, and a custom-ground cam from Cam Research, Westech’s dyno measured 568 hp and 542 ft-lbs of torque.

Australian Scott Cook is reproducing 351C-4V iron manifolds identical to original castings: D0AE-9425-L complete with casting date codes and other markings including Cleveland foundry.

Port matching is an important key to power. Port mismatches lose power due to turbulence at the manifold and head. Even if port mismatch is slight, you can take an intake manifold gasket and blend the ports. The gain is significant.

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.

 

Another mistake I see from time to time is gasket, manifold, and head mismatch so that oil is drawn into the intake port from the valley. A very thin film of Permatex The Right Stuff along the bottom like this eliminates risk of port contamination.

There are many theories on whether or not carburetor spacers help. I know from years of watching dyno testing that spacers do make a difference in power; not so much at the high end, but at the low to mid range where they enhance torque. If you have hood clearance, go with 1/2 to 1 inch and watch the difference in torque. This is the HVH Super Sucker from MPG Heads/Cam Research, which is sculptured internally to improve fuel distribution into the plenum. The manufacturer claims up to 47 hp possible.

MSD’s Atomic EFI throttle body injection system is an efficient power-adder drop-in replacement for your carburetor. It fits onto a standard Holley 4150/4160 flange and can be installed in a day with fewer connections and hand-held tuning you can do yourself. No laptop required.

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