Is Bigger Better?

The plenum in a stock manifold is typically smaller to keep air velocity high. Likewise, the cross-section of the runners is also small to keep the air moving at maximum speed into the cylinder ports. This provides good idle quality and throttle response, but also limits how much air the manifold can flow at higher engine speeds. Eventually the speed is reached at which the engine will try to pull in more air than the stock intake manifold can flow. That’s when the stock intake manifold needs to be upgraded to a performance manifold with a larger plenum and larger runners.

One manifold manufacturer said for maximum high rpm performance, the plenum volume should be equal to or greater than the engine’s displacement in cubic inches – especially on a stroker motor.
In recent years, the popularity of stroker kits combined with the availability of larger and larger displacement aftermarket engine blocks has created a whole new generation of monster-sized motors. We’re now seeing smallblock Chevys with displacements over 427 cubic inches, and big block Chevys with displacements well over 600 cubic inches.

Consequently, performance intake manifolds that were designed for small block and big block motors of a decade ago may not have enough plenum area and runner volume to keep up with today’s extreme engines. So intake manifold manufacturers have been redesigning their products to better accommodate the increased breathing requirements of today’s larger displacement stroker motors. The latest “improved” manifolds can often generate an additional 20, 30 or even 50 or more horsepower on many of these larger displacement engines compared to what was possible using an older manifold design.

Jim Davis of Professional Products said his company’s engineers redesigned their original “Hurricane” manifold for smallblock Chevys to handle today’s larger displacements. The new “Super Hurricane” manifold has a larger plenum and runners for engines over 400 cubic inches. Davis also said the company has come out with a new “Hurricane Plus” manifold for smallblock Chevy engines under 400 cubic inches that has better flow characteristics and delivers 10 to 15 more horsepower over the older design.

Runner Length

Runner length also affects the rpm range where an engine makes the most power. Longer runner lengths have a “ram” effect that helps keep the air moving forward as the intake valves open and shut. When an intake valve opens, there is a short lag before the cylinder starts to pull air through the runner into the combustion chamber. A longer runner helps maintain the inertia of the air column so it will fill the cylinder faster.

When the intake valve slams shuts, the momentum of the incoming air hits a roadblock, and a pressure wave rushes backwards through the intake port and runner. A longer intake runner tends to keep the air moving in the right direction in spite of the reversionary pressure pulse that is trying to push it backwards.

Shorter runners, on the other hand, usually flow better at higher engine rpm. Reducing the length of the runners may allow the engine to make more power at the top end, but the trade-off may be a loss of power and torque at lower speeds. When choosing an intake manifold, therefore, the runner length should match the engine rpm range where the engine is built to make the most power. If you are building a low rpm torque motor, you want an intake manifold with longer high velocity runners. On the other hand, if you are building a high revving motor, you will probably want a manifold with shorter runners or runners with a larger cross-sectional area to flow more air.

Runner Curves and Manifold Heat

Another important factor that influences airflow and how much torque and power an engine can make is the angle and curvature of the intake runners. In an engine with a carburetor, the intake manifold should not have any sharp turns because it can cause the heavier droplets of fuel to separate from the air/fuel mixture. This is not as critical in fuel injected engines because only air flows through the manifold. The fuel is sprayed directly into the intake ports by the injectors that are mounted in the intake manifold just above the ports.

On carbureted engines, the intake manifold is “wet” and contains fuel droplets. When a cold engine is first started, additional heat is needed to help vaporize the fuel. An exhaust crossover passage is often incorporated into the stock manifold to redirect exhaust under the plenum so the manifold will warm up quickly. On a performance engine, you don’t want heat in the intake manifold because heat decreases air density and power. So many aftermarket performance manifolds eliminate the heat crossover passage. Some manifolds raise the intake plenum and runners away from the engine so air can flow under the manifold to help keep it cool.

A fuel injected engine also does not require any heat in the intake manifold to aid fuel vaporization because the manifold is “dry” (no fuel vapor). This means the incoming air can be cooler and denser to produce more power (which is another advantage of fuel injection in addition to better cold starting). This also means the intake manifold can be made of lightweight plastic since the manifold does not have to withstand heat like a metal intake manifold on a carbureted engine.

Plastic intake manifolds are common on many late model engines, and one of the advantages of using plastic (besides saving weight) is that it can be cast to optimize airflow. Terry Wilson of Wilson Manifolds unveiled a new plastic intake manifold for the smallblock Chevy at the recent Performance Racing Industry trade show. He said the manifold weighs only 7 lbs, runs much cooler than an aluminum manifold, and is cast to flow as well as CNC ported aluminum manifold. The manifold will be in production this spring, and will retail for around $800, which Wilson says is about half the cost of what a CNC ported race-ready aluminum intake manifold would cost.

One of the disadvantages of a plastic manifold, however, is the risk of breakage if the engine backfires - which can be a danger when an engine is fitted with a nitrous oxide system. Plastic manifolds also require an entirely different casting process, so that’s why most aftermarket intake manifolds continue to make their products out of cast aluminum, including manifolds for late model fuel injected engines that have stock manifolds made of plastic. They say an aluminum casting provides strength, can be polished, plated or coated, and can be easily ported and modified.

Runner Design

The angle at which the runners in the intake manifold line up with the ports in the cylinder head should be as straight and smooth as possible to optimize airflow. That’s why “high-rise” intake manifolds produce more power than “low-rise” manifolds. The runners in a high-rise manifold have a straighter shot at the ports in the cylinder head. The incoming air does not have to change direction as much, so it flows more easily into the intake port and combustion chamber. And if the ports in the intake manifold are carefully matched to those in the cylinder heads, there will be no sharp edges or misalignment to disrupt airflow.