Ask RideApart: What Are Velocity Stacks and Throttle Bodies?

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You ask, the community answers. It’s Ask RideApart. This week: What are velocity stacks and throttle bodies and why are their dimensions important?

This question comes from reader Holden Lewis, who asks: “I often see motorcycle articles that refer to different lengths of velocity stacks, and different diameters of throttle bodies. What are velocity stacks and throttle bodies, and why are their dimensions important?”

Who knows the answer and, more importantly, who can explain it in the kind of simple, clear language that even the RideApart staff can understand?

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  • Robert Phair

    Well, any air delivery system, especially in modern engines, is a very complex topic, but the short answer is as follows:

    Velocity stacks are cylindrical tubes that terminate in a sort of trumpet-bell shape. They’re placed at the point of air intake on an engine, and serve to assist that engine in taking in as much air as possible. The shape of the stack, with the flared end, is used in order to smooth airflow as it is drawn into the engine. The bell-shape at the end provides a tapering diameter that allows maximum airflow over a given volume. By contrast, sharp edges, like a simple hollow tube, create high-pressure zones around the point of intake, which decreases the overall flow of air. When designing any engine, one of the goals should be to provide as smooth and unrestricted a path for the airflow as possible, and velocity stacks can help to achieve this.

    Stack dimensions are important because, with properly tuned length and overall diameter, along with a good taper, it’s possible to make use of resonant frequencies in the stack to actually slightly increase the intake pressure over the ambient atmospheric pressure. This provides an engine with a slightly denser charge of air as the intake valve opens. More air means more fuel to burn, which equates to more power over a specific RPM range. Depending on the engine and the geometry of the stack, this can occur at different places in the engine’s operational rev range, although the net increase is on the order of two to four percent.

    The throttle body is another important piece of the air-delivery puzzle. Throttle bodies are used to regulate the amount of air allowed into the engine, which in turn determines how much fuel an engine can burn at any one instant. Generally, this is done with sliding or rotating plates that move in response to the handgrip or accelerator pedal position. As the throttle is opened, the plates block less and less of the cross-sectional area of the intake, allowing more and more air through. At idle, a throttle body allows very little air to pass, so the engine produces little power and runs at a low RPM. At wide-open throttle, the throttle bodies allow as much air as possible to pass through them, meaning the engine is burning as much fuel as it can, producing maximum possible power at any given instant.

    Throttle body dimensions generally refer to the diameter of the opening through which the air passes. While it may seem that bigger is better — after all, if more air is more power, then much more air is much more power, right? — the truth is, the throttle body size must be carefully matched to the engine and fuel delivery system. The most obvious issue is when the throttle body is too small; in that case, the engine is starved for air at wide-open throttle as it tries to pull in more air than is available, and thus produces less power. Having too large a throttle body is a bit more of a discussion. A throttle body that is too large for the engine airflow capacity can reduce airflow speed, which negatively affects overall power, or provide too much air at lower RPMs, reducing your fine control over the engine. An engine with an oversized throttle body can feel “jumpy” or “peaky”. As a rule of thumb, your throttle bodies should be sized at the minimum diameter necessary to provide maximum possible airflow at wide-open throttle.

    I think that about covers it.

    • Jeremy

      Hey Wes, you should get this guy to write a guest post. Wonderfully articulate and unless I missed it (entirely possible) typo-free. Nice response, friend.

      • Guest

        .oops, thought you replied to me. ;p

      • Robert Phair

        Thank you for the kind words! That’s a very generous compliment.

    • Jeremy

      I read about years ago in a book about tuning VW engines about velocity stacks and fuel standoff. At least in carborated engines, you get fuel atomization that fogs in a way between intake pulses, that can cause fuel spray to stagnate in the air around and above the throttle body. From what I’ve read, the velocity stack should be at least equal length to the length of the throttle body to contain the standoff, or you will encounter a leaning condition at higher RPM. Made sense to me, but I’m not an engineer. Any truth to it?

      • Robert Phair

        It’s definitely plausible to me, although now we’re getting a little past my level of expertise. After all, I’m no engineer. Well, I am, but I work with computers. Point is, I’m just an enthusiast who happens to be handy with a keyboard.

        With something like fuel standoff, though, you can actually have several causes that can contribute to the condition. So, a properly-sized velocity stack can counteract standoff inherent in the engine or carburetor design by providing a positive pressure wave that prevents fuel from being forced back up out of the carburetor by the closing action of an intake valve at high RPM or the cylinder itself if the intake duration is too long. On the other hand, standoff can also be caused by an overly-restrictive exhaust; if there’s too much backpressure in the exhaust, the cylinder won’t be able to completely empty the spent fuel/air mix on the exhaust stroke, and thus excess gas — and pressure — remains in the cylinder. This forces your air/fuel mixture for the next stroke back up the intake path, as there’s nowhere else for it to go. Now, as for the exact ratio of throttle body to velocity stack, I couldn’t really say for sure — I think it depends greatly on engine configuration, maximum airflow, and many other variables — but a stack that is too short will not have the proper resonant frequency to generate that positive pressure, and thus won’t do much of anything to prevent standoff from occurring in an engine that’s prone to it.

      • Mister X

        Interesting, years ago I was in a Corvair race car (3 liter, flat 6) with
        2x3bbl Webers w/velocity stacks and a long overlap cam, the fuel standoff was so
        bad it was clearly visible 2 to 3 inches over the velocity stacks at
        full throttle, and I had a fine wet coating of Av-gas all over me.

    • Dan

      This is an excellent answer. Just to add a few small introductory points for the benefit of people who haven’t seen these parts before:

      - “Velocity stacks” are known by a few names, including “bellmouths” or “intake trumpets.” Choice of term is probably just regional.

      - In the cover photo, the (top) black portions are the velocity stacks, the (lower) silver portions are the throttle bodies (“TBs”). You get one of each per cylinder on the bike.

      - The beige items that are plugged into the (silver) throttle bodies are the fuel injectors – in this case one per throttle body / engine cylinder. The injectors each receive fuel from the green connector (on top), as well as electrical signals from the ECU (via the socket that is facing the camera).

      - At the far left of the photo, you can see a linkage with a spring return. this causes a butterfly valve inside the TBs to open when the throttle cable is twisted, allowing air to proceed into the system.

      - So, it works like this: clean air enters the system from the airbox (not shown) via the top of the (black) bellmouths . Twisting the throttle causes the butterfly valve (not seen) to open, allowing this air to flow into the (silver) TBs. Twisting the cable also causes the ECU (not seen) to send a signal to the (beige) injectors, which inject fuel (received via the green port) downward into the (silver) TB. This mixture of fuel and air is then forced downward out of the TB and into the engine, where delicious things happen.

      For a really good description of all the major components of a motorcycle, including popular variations (such as valvetrains and frame designs), check out the Haynes Motorcycle Basics Techbook. Best $20 you’ll ever spend, I promise.

  • Dave

    In order to make combustion, you need 3 things: Air, fuel, and ignition. Throttle bodies and velocity stacks deal with the first two.

    In simplest terms, throttle bodies are the things through which the air passes from outside the bike and into the engine. In most cases, the fuel is squirted into the throttle body at specific times, where it mixes with the air stream. This mixture is sucked into a cylinder, and the spark plug provides ignition.

    To understand velocity stacks, you need to understand a little more about exactly how that mixture works.

    In the real world, the exact ratio of air to fuel varies for various reasons, but it’s usually close to 14.1 in a decently tuned street bike. No matter how much gas you burn, that ratio stays the same (or close). The throttle bodies (usually) contain the throttle plates (and/or slides). These parts control how much air comes through the throttle body, and therefore into the cylinder. If you draw more air, you add more gas to keep the ratio you want. This is why you use more gas the more “open” your throttle is at the handle.

    With carburetors, the ratio is controlled with a set of various-sized “jets”, which are precisely sized tubes that dip into a pool of gas. The airflow itself sucks gas through these tubes. More air flowing faster means more gas gets sucked up. It’s called the “venturi” effect. You can simulate it by blowing across the top of a straw in a glass of water. Fuel injectors are computer controlled. They monitor various sensors, and based on a set of values in the computer, will inject precise amounts of fuel at precise times.

    Given all that, you might start to see why the bore size matters. Bigger bore means more air, which means more gas, which means more power.

    Obviously, there’s a size limit here. Too big, and you lose pressure and the gas can’t mix and the mixture can’t flow fast enough. This is kind of where the velocity stacks come in. These are (often) tapered doohickeys that cause incoming air to increase in pressure, which causes it to speed up. Hence the “velocity” name. If you have faster moving, higher pressure air, then you have *more* air. Which means you can shove more fuel into it to maintain your ratio, and this gives you more power.
    The exact speed and pressure is tuned by velocity stack length, width, and shape.

    So the trick for tuners is to find the right balance of bore size, stack length, jet size (or computer configurations) to get as much gas burned over as much of the throttle range as possible. Too much gas in the ratio, and you get cylinders full of carbon and all kinds of gummed up stuff. To little, and you lose power due to not enough gas and worse, you start overheating. (The gas in the mix actually cools things down before it burns.)

  • Eric Shay

    Just to simplify everything: throttle bodies are instants jest and needles and the stacks help adjust to the amount of air. With the perfect amount being approx: 15:1

    • Eric Shay

      With 15 parts of oxygen to 1 part of gasoline. The I deal amount is supposed to be 14.7 to one part of fuel at sea level, with computers trying to figure all of this out, it is supposed to work.

  • HoldenL

    Thank you so much, guys! This is clear and concise. (It’s kinda strange when something is rather lengthy, yet concise.) I really appreciate this.

    One thing I wonder about is “resonance.” I used to think that, in the context of velocity stacks, that this somehow referred to vibration — almost as if you wanted a certain tone to the air running into the cylinders, as if the intake system were a musical instrument. But actually, it sounds like resonance refers to something else, having to do with the height of the column of air in the throttle body plus velocity stack. From what Robert Phair and Dan are saying, it sounds like you don’t want that air column to be too high or too short, because either way the airflow could be impeded as the intake valve opens and closes. Right?

    • runnermatt

      This may not be the same thing, but I know that on intake manifolds the ideal length of the intake runners depends on the rpm the engine is designed to operate at. Short runners are ideal for high rpm, long runners for low rpm. Short runners allow a faster rate of air flow permitting a fast intake air flow at high rpm, but conversely cause intake turbulance at low rpm. Long runners smooth the air flow at low rpm, but become a restriction at high rpm.

  • Cory McNair

    I appreciate everyone’s input in this for the informative, basic explanations without any condescending snark. I already knew WHAT velocity stacks and throttle bodies were, but I came away with a little more of the WHY answered. I also think the WHAT part of the answers were clear enough for most people to understand. What is the adage? For every person that asks a seemingly simple question, there are 30 (or 1000) that are afraid to ask.

  • OtisGerald

    If anyone wants custom length or shape velocity stacks, I’ve been making them out of carbon fiber.

    • zombarian

      Definitely going to save this for…..later…