Velocity stacks may not be the absolute coolest thing you can put on your car, but... actually, you know what? They are the coolest thing you can put on your car. Here’s what they do, how they work and why everyone goes nuts over them.
Velocity stacks are also called intake trumpets, which gives you a bit better idea what they are. They’re little trumpet-shaped air intakes for your engine. The trumpet shape smooths the flow of air into the intake, allowing the most air possible to flow through the given volume of the intake. Put simply, more air means more power for an engine. Put more complicated-ly, there are three clear reasons for why people go nuts for little trumpets on their engines. These get get only slightly more complicated-ified as they also involve two other bits of tech, namely carburetors and individual throttle bodies.
In case you somehow don’t know, a carburetor is an ancient wizard’s tool that mixes air and fuel together before it’s sucked into your engine. In the early days of the automobile (that is, everything before I was born), carburetors were the most common and basic way that engines got fuel.
Only after the dawn of reliable electronic fuel injection did we start to see carbs disappear from normal family cars. If you want to learn more about how carburetors work, go to the weird part of town and shout at the top of your lungs “BOY I SURE MISS OLDSMOBILE” and some old man will come out of the shadows to tell you about the Holley he had on his Cutlass 442.
Individual throttle bodies are a bit easier to understand. Most cars just have what you would call a single throttle body. There’s one point where air gets pulled in from the outside, and it all rushes into an intake manifold responsible for feeding each of your engine’s cylinders. Individual throttle bodies have one point per cylinder for air to rush in, so there’s much less delay and disturbance for outside air getting into your engine. This quickens the response of your engine and gives you a chance to tune your engine right.
If you want to learn more about why individual throttle bodies (often abbreviated as ITBs) are hella sick, bro, just walk over to the far corner of your local community college parking lot and yell “HONDAS ARE COOL AND ALL BUT BMWS ARE BETTER” and four guys will run out of the shadows to argue with each other about their intake setups in front of you.
Alright, back to the good stuff. You see velocity stacks on cars for three basic technical reasons, each of which focus on getting more air into an engine: first, they’re an easy way to change the length of your intake, which is a good way to help tune your engine.
Second, they smooth out how air rushes into your intake. Smoother air flow means more air flow, and more air means more ability to burn fuel, and burning more fuel means more power. Third, this cleaned up and controlled air prevents certain kinds of carburetors (that is, non-plenum carbs) from barfing fuel and catching on fire.
Changing The Length Of Your Intake: One of the reasons people put velocity stacks on their cars is they’re an easy way to change the length of your intake. You can have a long velocity stack, or a short velocity stack. Different engines perform better with different lengths, as this excellent thread “The Misunderstood Velocity Stack” on a vintage Volkswagen forum explains:
Race tuners know there is a specific length from the carb mouth to the intake valve that produces the best power at a specific RPM. For racing, you can change this by just changing the velocity stack!
For the street, you have to compromise. The short Weber manifolds most kits have work just fine with the approximately 3" high velocity stacks in the same kit. For variable RPM road racing, a 1.6 engine redlined at 6000 needs about a 16" runner length, meaning the total measurement from carb mouth to intake valve.
Shorter runners lengths give more power at high RPMs, longer lengths give more power at lower RPMs.
What the length of the intake runner is doing is tuning the pulses of air rushing through the intake. Get the length right, and you’ll have pulses of air rushing towards the intake valve on the cylinder for a kind of supercharging effect.
This is called ‘resonance supercharging,’ as the great Ate Up With Motor explained in an old article on the cross-ram intakes you found on early ‘60s Dodges:
When an engine is running, the downward motion of the piston during the intake stroke creates engine vacuum that draws the air-fuel mixture into the cylinder. However, as we’ve just noted, the intake valve is closed more often than it’s open. If the incoming mixture reaches the valve at a point when the valve is closed, the leading edge of the onrushing mixture will stop abruptly as it hits the closed valve, building up pressure that eventually forces the mixture back up the intake runner. When this pressurized air hits the opposite end of the runner, it reverses direction and heads back toward the valve. This sets up an oscillating pressure wave moving within the intake runner at the speed of sound (which in air varies considerably depending on atmospheric pressure, humidity, and ambient temperature, but is approximately 1,115–1,132 feet per second or 340–345 meters per second at sea level on a balmy spring day.)
If the valve is open when this newly pressurized intake air reaches that end of the runner, the mixture will be forced into the cylinder at higher-than-atmospheric pressure, just as if the engine were using a mechanical supercharger. As with a turbocharger or a supercharger, this supercharging effect — called resonance supercharging — allows more air and fuel to be packed into the cylinder, producing more power.
Naturally, resonance supercharging only provides a benefit at the points where the compressed mixture reaches the intake valve at a point where the valve is open for business. [...] Therefore, the supercharging only occurs part of the time, at certain engine speeds.
What engine speeds, you say? That depends on the frequency of the pressure wave. If you stayed awake in high school physics, you may dimly recall that the frequency of a wave is inversely proportional to its wavelength (that is, a short wavelength means a high frequency and vice versa). In this case, the wavelength is determined by the length of the space in which the pressure wave can move, i.e., the length of the intake runner. All else being equal, the longer the runner, the lower the engine speed at which resonance supercharging occurs and vice versa.
You can tell that this isn’t an innate ability of velocity stacks, but velocity stacks allow you to get your engine tuned right. I was recently up at Team O’Neil Rally School and saw one of their Fiestas getting put together in their shop garage.
The 1.6-liter naturally aspirated engine had just gotten a new cam, and to go along with it they’d also put in individual throttle bodies to get the right runner length. You can see how each cylinder gets its own individual throttle body, unlike a stock engine that would have one tube for each cylinder pulling air from an air box with a single throttle body.
Team O’Neil also straight up put larger throttle bodies on the engine. A bigger cam meant more appetite for fuel and air, hence the bigger valves to gulp air in. Conceivably you could put a single larger throttle body on a normal air intake system, but velocity stacks and individual throttle bodies most easily achieved the desired result.
Smoothing Out How Air Gets Into Your Intake: This one is pretty simple. These intakes have little trumpet-shaped curves to their tops, where outside air first rushes in. If you only had a straight pipe, the air would bunch up at its top and struggle to get in, whereas a curve (one that varies based on the whole shape of the velocity stack as a whole) lets the air enter more smoothly. Here’s a handy diagram I found on Wikipedia that shows what I’m talking about:
This trumpet shape also makes these things hella hella loud. Again, this trumpet shape works just as well at making your younger brother loudly mouth-fart his way through high school band as it does make a BMW scream at hard throttle.
This noise alone is enough to encourage most people to go ahead and get velocity stacks/individual throttle bodies for their engine. More power is good; more power with the howling bark of intake noise is better.