Sumflow guide to Intake manifold design

The early fuel injected Corvettes had strange looking intake manifolds. By the time that you finish this you will know why. You will be able just glance at an intake manifold and know what it is all about. Intake manifolds are designed around three things. Width for velocities, length for sound resonance, and plenum volume.

1957 Corvette Intake manifold

The almighty cathedral of Corvette power was limited by being squished between the heads of the V8 and the downward sloping front hood. If the ram tubes could have been made longer, it would help low end torque, because the velocity of air coming down a long tube would just pound into the intake hole. But if it was shorter it would not run out of breath as soon and run higher RPMs. There were two approaches to Fuel injection at that time, timed injection and continuous spray injection. Both now obsolete because of direct injection and the comeback of super and turbo charging.

Cutaway Vette fuel injection manifold

You can see a lot in the above picture, how the air was rammed down the interior velocity stacks to the inlet valves. To save money Corvette went with continuous spray injection. It really cost a lot because it was a low volume sports car.

Horizontal intake manifold

Here is pretty much the same design but with the air chambers laying horizontal instead of standing vertical. In this design they are not limited by the bonnet slope. You can see how the air box is extend wider than the ram tubes. That is because air flows the slowest where it touches the insides for the chamber. The end of the stacks must keep away from the air box, feeding from the faster moving air in the middle. The air rubbing against the pipe wall has fairly a lot of friction working against it, so it moves at a slower velocity. The air running in the center of the pipe , is rubbing against other air molecules, so it has little friction, and is able to move at a higher velocity. This is why the trumpets run clear of walls and floor inside the chamber. But also the stacks closest to the intake will suck in some air, the plenum chamber must be squished to keep the pressure on, as it moves away from the opening to compensate for the decreasing amount of air reaching the outer stacks.

Cutaway

So, the collector/resonance chamber uses air pressure from the cars motion to increase the pressure inside of the manifold. When the air from the intake tube enters the wider air intake chamber the pressure increases because the air slows down. This increased air pressure in the plenum chamber has a positive effect on engine power, both because of the pressure itself and the increased air density this higher pressure gives. Small plenum volumes lower the rpm of peak torque. Larger plenum volumes increased the rpm of peak torque. A plenum that is too small for the motor will starve the engine of air, a plenum to large will bog it.

Sound waves cause pressure waves that are directly proportional in strength to the amplitude of the sound wave. These can be used to create positive manifold pressures. The intake pulses in the intake system, which originate from the pressure waves created by the opening and closing of the intake valves are optimized to create a supercharging effect to boost air intake at certain RPMs. Acoustic resonance effects are more important to the rather weak wave action that occurs at low engine speeds.

Cold Air Intake

The air flows in from the outside, bringing in unheated air by the throttle. Any air cleaner today will flow what cars need, they are mainly designed to be quite. The restriction to how much air can get in is caused by the size of the throttle valve, not the filter or intake tubes.

Plenum resonance chambers

The cold air flows through the throttle into the plenum chambers. You can see how the nylon intake manifolds are tapered away from the inlet, the ends of the chambers are rounded, and overhang the stacks. This is another antiquated design because the plenum is of fixed length. This car uses a timed injection, but newer cars have direct injection, and variable length-resonance systems, which boosts torque by allowing the engine to draw from different velocity airflows.

Variable length-resonance system

Here you can see the second crossover pipe. When it opens, each cylinder bank can draw from airflow “excited” by the resonance created by alternating induction between all six cylinders. In essence, “dual resonance” creates two induction paths for each cylinder. Starts with a “short” path, but when the resonance flap opens the cylinders draw from longer passages.

http://horsepowercalculators.net/intake-manifold-design/intake-manifold-design

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Air meter, fuel meter and intake manifold

http://www.illustratedcorvetteseries.com/images/ICS_164_1957_First-Fuelie.jpg

Velocity stacks

Air moving rapidly over a Velocity stack, creates a vacuum  inside the opening when the air is at a perpendicular angle to the length of the tube. 

Turbulence

At high speed, the rushing air tends to create a partial vacuum inside the tube.  The condition is counterproductive to air flow.  The faster you go, the worst it is.  Vacuum created by the engine is trying to coax air into the cylinders and the high-speed air flowing over the open end of the stack is pulling it out.

Reversion creates other problems.  Reversion refers to reversed airflow, or in simpler terms, it’s when air in the intake runner reverses direction for a split second.  The condition is caused when a burst of pressure escapes into the intake runner from the cylinder during valve overlap.

Reversion creates resonance shock waves inside the tubes, which exit the open end of the tube at various rates depending on engine speed.  It has also been proven that these shock waves interfere with each other when the stacks are in close proximity.

By putting a plenum over the velocity stacks, air entering the "filled space," is slowed, smoothed, and straightened.  The plenum then becomes an endless source of calm, clean air.  Shock waves dissipate within the confines of the plenum without interfering with the shock waves emitted from an adjacent stack.