Manifold Destiny - Part 2

National Dragster, May 7, 2004 by Smith, Evan J

More on selecting the best intake manifold for your application

The intake manifold is an integral part of any engine's induction system. Part 1 (April 23) covered the basics of airflow, ram tuning and manifold types; Part 2 covers the variety of manifold designs available and how each variant affects performance on the street and the strip.

When it comes time to bolt up a new intake, you'll find dozens from which to choose, and picking the best can be a challenge. The good news is that with just a little information, you'll know what to expect in terms of performance simply by looking at the design. Carburetors and cylinder heads are normally rated by the amount of air they can flow (measured in cubic feet per minute); intake manifolds usually are not. For most, manifold selection is based on design rather than on flow, so it's important to know what you're looking at. Typical designs are of the single-plane, dual-plane, tunnel ram, and individual runner variety. Additionally, there are manifolds for big-blocks and ones for small-blocks, and there are intakes for street, street/strip and track-only use. Before selecting, consider your budget, rules that apply to your car's specific class, engine displacement, rpm range, desired, use (street, strip, etc.), vehicle weight, gearing, and hood clearance.

The-simplest manifold design is the single-plane. A single-plane manifold has one common plenum that feeds each of the intake runners. The runners often have one turn and are somewhat short when compared to dual-plane manifolds. More often than not, single-plane manifolds don't work well on mild street engines because the design favors high-rpm operation, and most street engines need lower-rpm torque, which provides good "tip-in" throttle response. Another type of intake used on the street and the strip is the tuned runner, or tuned-port intake, for EFI engines. By using long runners, manufacturers can optimize low-rpm torque, which can make a small engine act like a larger one.

A good choice for carbureted street/strip applications is the dual-plane. A dual-plane manifold acts like two small single-plane manifolds in that one side (of the carburetor and manifold) feeds four cylinders and the other side feeds the other four. According to Keith Wilson of Wilson Manifolds, "At low rpm, most engines don't have a strong signal, so a dual-plane manifold will divide the banks so that each pulse draws from one side of the carburetor. With this arrangement, airflow is increased, and this will help fill the cylinders during midrange operation and during part-throttle." The result is good throttle response and midrange power. Long-runner EFI intakes accomplish the same thing because each cylinder pulls from just one runner.

The best choice for racing applications is a modified (read: ported) aftermarket or sheetmetal intake. Most high-performance manifolds are built from either composites, cast aluminum, or sheet metal. Specialty manufacturers like Hogan's Racing Manifolds and Wilson Manifolds will build a custom intake for a specific engine. The benefit is that the plenum and runners will be custom engineered to your engine in order to help develop maximum power where you need it.

Wilson explained, "The best on-track performance doesn't always mean making the most peak power. For instance, in circle-track or road racing, you may need a lot of power to get out of the corners, so you may use an intake that favors midrange torque. The engine may only be at peak power or peak rpm for a very short time at the end of the straight, but it may be in the midrange many times during the race, so it's more important to make greater power in that rpm range. That's what will improve lap times. And it's not that different in drag racing: The dyno may show more power one way, but the car will be faster on the track with more average power and less peak. It's important that the racer know what he or she wants to achieve."

Choose your weapon

Racers should remember that every car is different, and the intake that worked on your buddy's car might not make your car fly. I recommend that you pick your parts according to how you expect to use your engine.

Recently, I watched a guy test his '66 Chevelle on the chassis dyno. It had a mild 454 with a hydraulic cam and an 850 Holley on a single-plane intake, and it was backed by a Turbo 400 transmission, a tight street converter, and 3.73 gears. It made 320 rear-wheel horsepower and 335 pounds-feet of torque. By switching to a dual-plane intake, the engine picked up 25 horsepower and 30 pounds-feet of torque. Though the performance numbers looked great, the car performed much better in real-world driving conditions.

As a basic rule, engines of smaller displacement (or engines that rev low) draw on the intake system less than larger engines, so smaller ports are required to maintain velocity and efficiency. Nevertheless, as rpm and/or displacement are increased, more air is required, and this requires a larger intake tract. Because the 454 in the Chevelle had a mild cam, tight converter, and street gears, it needed to be efficient at low rpm. With the single-plane, it was anything but. Had the owner swapped cams and added 4.56 gears and a looser converter, the single-plane would have been a better choice.