When it comes to bolt-on horsepower for modern vehicles, few upgrades are as tempting as a bigger throttle body. Situated between the air intake and the intake manifold plenum, the throttle body uses a butterfly valve to control how much air enters the engine. As you roll into the throttle, the valve opens to let more air in, while backing off the throttle closes the valve, thus reducing airflow and engine power in turn.

“The more you open that butterfly valve, the more air the is drawn through – or forced into – the engine,” explains Holley Performance Product Manager, Jake Anderson. “And you’ve got some kind of sensor, be it a mass airflow sensor or a manifold pressure sensor, that compensates for the difference in throttle position and will add more fuel as a result.”

Swapping in a larger unit is relatively a quick and affordable procedure that’s easy to tackle in the garage, and that’s made it a popular upgrade for enthusiasts who’re chasing more performance in their fuel-injected vehicles. After all, more airflow sounds like an obvious recipe for greater performance, right?

Well, as it turns out, that’s not always the case. Here we’ll take a closer look at how throttle body sizing, airflow velocity, and other factors play into the equation to help you determine what throttle body size is ideal for your engine.

The Dynamics Of Airflow

“From a psychics standpoint, air is essentially a fluid because it behaves a lot like water,” Anderson tells us. “Like a fluid, air naturally wants to take the path of least resistance, and it will also ‘pile up’ when it encounters a hard turn at high velocity. Meanwhile, the air that engine ingests at a given time is going to be a fixed amount – no matter what, at wide-open throttle it’s going to inhale a fixed amount, or on a turbocharged or supercharged combination, that power adder is going to be forcing a fixed amount of air. And the size of the throttle body is going to affect the air velocity – the smaller you go, the faster the air is going to travel through that throttle body and into the engine in a given amount of time. The larger you go, the slower the air is going to move through that system in the same amount of time. And there’s a critical number for air velocity – about 300 feet per second. Beyond that, you start to have air piling issues, where the air is basically slamming into the back of your intake manifold. That can actually create issues that hinder performance instead of enhancing it.”

This can lead to a lean-running engine, fuel atomization problems, and other issues associated with inconsistent airflow. From the driver’s seat, these problems often manifest as drivability issues under hard acceleration.

“If you install a throttle body that’s much larger than what the engine actually needs, you’ll typically see adverse effects in the way the throttle pedal reacts to inputs,” he notes. “The throttle pedal will become very touchy, almost like an on/off switch. Because now, at 10% throttle, you could be flowing 50 or 60% percent of the engine’s maximum air needs. So at any time you’re not at wide-open throttle, your drivability is going to suffer. And that’s especially problematic with stick-shift cars, where you need to be precise with your throttle applications in order to drive the vehicle smoothly. It can also be really problematic if you’re running a tight (numerically low stall) torque converter in a vehicle with an automatic transmission.”

High-quality throttle bodies like the Holley EFI Billet Series typically have small air dams built into the bottom of the throttle body opening to help mitigate oversizing issues by slowing down the air as it passes through the butterfly valve during throttle tip-in. But there’s only so much that features like these can do if the throttle body is significantly larger than the application calls for.

That said, there are also situations where an undersized throttle body might actually be creating a bottleneck in your engine combination.

When To Upgrade

“The old adage in racing is that smooth is fast, and that concept applies here as well,” Anderson says. “So when your throttle body is correctly sized for the application, not only are you getting all of the power the engine can make, you’re also going to get more precise control over throttle inputs, and that can equate to getting better traction on launches, cleaner shifts, and generally improved driving manners.”

He also points out that there are telltale signs when a throttle body is actually undersized for the engine combination in forced induction setups.

“In those applications, you’ll see a big pressure drop between the charge pipe and the intake manifold. So if you have a pressure sensor in the intake and another in the charge pipe right from the turbo, you might be seeing, say, 25 pounds of boost in your charge pipe, but then only 20 or so in your intake. If you see something like that, it’s a good indication that you have a restriction somewhere in the intake tract.”

With naturally aspirated engine combinations it takes significantly oversized throttle body to really cause serious issues, but Anderson points to some of the symptoms which can indicate that a throttle body is way too big for the naturally aspirated engine it’s bolted to.

“In those situations you’ll often see good low and mid-range power, but it will fall off prematurely at higher RPM, and the engine will feel “choked” near redline – especially at wide-open throttle. You may also notice that going from, say, 80% throttle application to 100% (wide-open) throttle makes little or no difference in acceleration. In a naturally aspirated engine, you’ll also be able to see that manifold pressure remains noticeably below atmospheric pressure at wide-open throttle. On a naturally aspirated engine, MAP (Manifold Absolute Pressure) should approach barometric pressure as airflow demand peaks. In this situation you’ll often also see that measurable vacuum remains in the intake manifold at wide-open throttle, and it may become greater as RPM increases. In severe cases, different cylinders may even receive uneven airflow because the incoming air velocity and pressure drop through the throttle body are so excessive.”

While insight from other enthusiasts and racers running similar combinations can provide a broad guideline when choosing a throttle body for a naturally aspirated combination, Anderson cites the following equation as a way to calculate the minimum throttle body size needed for a given setup:

Diameter is measured here in millimeters, while engine displacement is measured in liters. 0.8 is the average volumetric efficiency of most naturally aspirated engines. Note that this is specifically for naturally aspirated engines – boosted engines will have volumetric efficiency numbers over 100%.

He says that despite potential pitfalls at the extreme end of things, aftermarket throttle bodies are generally a bit bigger than what a particular engine actually needs, even when correctly matched. This ensures that the engine won’t be underfed, while the engineering tweaks on well-designed throttle bodies are on hand to compensate for a slight overabundance of airflow to ensure that the engine maintains good low-RPM drivability.

“If it’s a turbocharged car, or you’re running a centrifugal supercharger, I’d recommend matching the throttle body size to the intercooler piping size. If you have three-inch intercooler piping, for example, there’s really no point in going larger than a 75mm throttle body. You certainly can if you want to, but the intercooler piping itself is going to be your limiting factor in that setup. If you have four-inch intercooler piping, then a 90 or 100mm throttle body would be more appropriate. To give you an idea of how big you need to go here, my personal race car has four-inch piping and I’m running a Holley EFI 95mm billet throttle body. I make right around 2,000 horsepower at 45 pounds of boost, and that throttle body works great with this combination. I could step up to a 105mm if I wanted to, but I’m not having any issues and the drivability is great.”

If you’re using a positive displacement supercharger, he recommends going as big as you can. “The supercharger itself is going to control how much air the engine digests, so you want the rotors in that supercharger to control that air speed – you don’t want the throttle body to be your limiting factor.”

It’s also worth noting that packages like the Holley Performance Top End Kits include throttle bodies and use vetted combinations of parts to ensure that everything works together properly. Going this route can take a lot of the guesswork out of the equation while also saving you money in the process.