Looking straight down into a dual-plane intake manifold like this Weiand Stealth Air Strik
Playing With Plenums
Otis Linton, Santa Maria, CA: Stories have been told about cutting out the divider that splits the plenum in the intake manifold on a small-block Chevy. Have you ever tested this? I've always wanted to try it.
Jeff Smith: Don't you just love a guy who comes right to the point? Let's see if I can be as parsimonious with an answer. The divider is the vertical wall that separates the two small plenums on a dual-plane intake manifold. This creates two separate intake tracks that split the engine into a pair of four-cylinder engines. On a small-block Chevy the cylinders are separated 90 degrees apart by the firing order. With a firing order of 1-8-4-3-6-5-7-2, this combines cylinders 1 - 4 - 6 - 7 together in one plenum and 8 - 3 - 5 - 2 on the other. Looking down through the top of the manifold with the carburetor removed, the first thing that becomes apparent is that each plenum is rather small and that each half is fed by one primary and one secondary barrel on a typical four-barrel carburetor.
The advantage of a dual-plane manifold is that the intake runners are significantly longer than a single-plane. Longer runners boost the midrange power-roughly between 3,500 and 5,000 rpm. A single-plane intake employs much shorter runners, which emphasizes top-end power at the expense of midrange torque. These are general characteristics that can and do overlap depending on individual intake design. One disadvantage of the dual-plane is that one plenum floor tends to be higher than the other, creating a smaller common (plenum) area. At greater engine rpm, the high-speed air and fuel column through the carburetor tends to crash into this shallow floor, disrupting flow and causing air/fuel separation. This then turns some fuel back into a liquid, which is much harder to burn. This same situation can occur with a single-plane intake manifold, which is why open-plenum carb spacers tend to make more peak horsepower on some manifolds. Adding height between the bottom of the carburetor base plate and the floor of the manifold gives the air and fuel a much gentler radius to make the 90-degree turn into the intake ports. But we digress.
Removing the divider wall between the two small plenums in a dual-plane intake manifold first allows the two plenums to communicate with a larger common area from which to pull air and fuel, which improves high-speed power. Often removing this wall will help power everywhere in the entire engine rpm operating range. Other times, cutting this wall will help the top-end power but hurt the low-speed torque. One way you could test this before modifying the intake manifold is to add a 1/2- or 1-inch-tall open-plenum spacer to the intake manifold (assuming there is sufficient vertical room to the hood when the spacer is added). Adding an open plenum spacer will also affect the fuel delivery curve, so that's one thing to watch carefully. Adding a spacer tends to lean the fuel curve because the signal to the carburetor has been reduced slightly. If the carb is already on the rich side, an increase in power may be due to leaning out the air/fuel ratio slightly-or some combination of manifold design change and air/fuel ratio change. You can see why testing all this stuff can get complicated.
Cammers Are Cool
We can never get enough of the Ford Cammers. This one was at the 25th anniversary Knott's Berry Farm Show. Stuffed into a '64 Fairlane T-Bolt tribute car, the motor displaces 440 ci with a Velasco steel crank, 14:1-compression Arias pistons, and that iconic Hilborn injector stack setup. The car is a tribute to Emmett "Rattlesnake" Austin.