An intake manifold is more than the plumbing that connects the carburetor or throttle body to the ports in the cylinder head. It is an integral part of the induction system that has to match the airflow characteristics of the cylinder head and camshaft, as well as the displacement and rpm range of the engine.
A well-designed manifold that is properly matched to the engine’s requirements will make more torque and horsepower than a manifold which is mismatched to the engine.
Stock intake manifolds are often a compilation of compromises. Stock manifolds are typically designed to minimize manufacturing cost, to accommodate emissions fittings, to fit a tight engine compartment with limited hood clearance, and to provide good low- to mid-range performance, fuel economy and emissions. Most stock engines spend 95% of their running time between idle and 3,000 rpm, with rare bursts above 5,000 rpm.
Consequently, if the engine is modified with a hotter camshaft, larger carburetor or throttle body, and/or bigger heads, the stock manifold will usually run out of air above its original design speed and hinder power rather than build power.
As an example, the stock intake manifold on a Chevy 5.7L with tuned port injection, or the one on a stock Ford 4.6L V8 are both well designed for low to mid-range torque and power. In fact, each will usually out-perform most aftermarket manifolds at lower engine speeds. But if the engine is being modified to make more power, the stock manifold usually runs out of air above 5,000 rpm and becomes a restriction.
Dual- and Single-Plane Manifolds
On a V8 engine with a two-barrel or four-barrel carburetor, most stock manifolds have a split-plenum, dual-plane or “180 degree” configuration. Four of the cylinders (two on each side) draw from one of the primary barrels in the carburetor, and the remaining four cylinders draw from the other primary barrel. The intake manifold essentially splits the V8 engine into two V4s. The reason for doing this is to keep intake runner velocity high so the cylinders will fill quickly and produce maximum power and torque at low- to mid-range rpm.
This design, though wearing the “stock” label, isn’t necessarily bad. A performance manifold that is designed for a street application will often retain the split-plenum or dual-plane design for this same reason. These types of manifolds can be a good choice for heavier vehicles with automatic transmissions, stock gearing and engines that won’t rev much beyond 5,500 to 6,500 rpm.
But if an engine has a longer duration camshaft, stiffer valve springs, bigger cylinder heads and gearing for revving to 7,500 to 8,500 rpm or higher, an intake manifold with an open plenum, single-plane or “360 degree” configuration is usually the best choice. Opening up the plenum allows all of the cylinders to pull from all the barrels in the carburetor or throttle body for more airflow at higher engine speeds. This makes a significant improvement in mid-range and high rpm power, but may sacrifice some low-end throttle response and torque. Even so, a well-designed single-plane manifold will almost always outperform a split-plenum dual-plane manifold from 2,500 rpm and up.
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 small block 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.
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 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.
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.
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.
Variable Displacement Intake Manifolds
To increase engine torque at low rpm and power at high rpm, some late-model engines have variable displacement intake systems. The intake manifold has two sets of runners long ones that are used at low engine speed to keep air velocity high, and shorter runners that open up at high rpm to increase total airflow. This is accomplished by using a “tuning valve” in the intake manifold plenum.
At low speed, the tuning valve is closed and forces air to flow through a longer set of intake runners. Above a certain engine speed, or as engine speed and load increase, the PCM changes the position of the tuning valve and opens up additional runners for increased airflow. The valve is usually controlled by a solenoid or stepper motor mounted on the intake manifold. On a V6 or V8 engine, there are often two control valves.
One of the first engines to use this setup was the Ford Taurus with the Yahama-designed SHO engine. Since then, it has become common on many import engines as well as many Ford and other domestic engines. Some aftermarket intake manifold manufacturers have used a similar approach by casting dividers into the intake runners on a dual-plane manifold that mounts a four barrel carburetor. At low speed, the primary carburetor barrels flow air into the smaller, high velocity runners. When the secondary barrels open up, they feed into the larger secondary runners to increase overall airflow.
Cast or Custom?
The most affordable aftermarket manifolds are those which are cast and mass produced to fit the most popular engines. Four barrel manifolds for SB/BB Chevy and Ford engines account for the vast majority of aftermarket manifold sales, but Chrysler, Pontiac, Oldsmobile and sport compacts such as Honda also represent significant market segments.
Custom manifolds CNC-machined from billet aluminum or hand-built and TIG welded from aluminum sheet stock and tubing are high-end products for racers with deep pockets. A custom manifold can be optimized for virtually any engine application, but it takes time and money to create one of these masterpieces in metal. A custom-built manifold can literally costs thousands of dollars. But if the manifold delivers a few extra horsepower over anything else that is currently available in an off-the-shelf product, it’s probably worth it for the serious racer.
There’s a growing market for multi-carburetor manifolds to fit certain engines and many of these original manifold designs are still being cast and sold today for retro rod builders who want that classic look. What’s more, other manifold makers and suppliers have come out with “classic” manifold designs of their own for certain engines. These include three-duce manifolds for Pontiacs and Chevys, as well as dual quad manifolds for Chevys and Fords.
EFI or Carburetors?
Though carburetors disappeared from engines back in the late 1980s, they have remained in use for most forms of racing. In some cases (NASCAR, for example), rules prohibit the use of anything but a traditional carburetor. Because carburetors are mechanical, some say they are easier to modify and tune. But the proponents of fuel injection say fuel injection provides the best overall performance and tunability because the air/fuel ratio can be changed almost instantly to match changing driving, track or weather conditions.
Cold starting is much better with fuel injection because no choke is needed. The downside of fuel injection is that it requires more hardware (sensors, a pressure regulator, high pressure electric pump and control module) and it is electronic (which still intimidates many old timers but not the younger generation of engine specialists).
Many aftermarket manifolds that were originally designed for carburetors can be easily modified to work with throttle body injection. By adding plugs atop the intake runners that can be drilled out to accept fuel injectors, manifold suppliers have broadened the market potential of many existing manifold designs.
There is also an emerging market to “retrofit” newer fuel injected engines with a carburetor so the engine can be used in an older vehicle, a street rod or even a racecar.