Throttle body upgrades are popular modification for anyone looking to improve airflow in their intake system. Whether you’re tuning for better throttle response or aiming for more power at higher RPMs, choosing the right throttle body size is crucial. It is possible to induce turbulence by going with too large of a throttle body. In this post, we’ll break down the significance of effective area, CFM (Cubic Feet per Minute), and calculated horsepower to help you decide which throttle body is best for your engine setup.
Effective Area: What Does it Mean for Performance?
The effective area refers to the actual usable area through which air can flow in a throttle body. The diameter of the throttle body gives us a starting point, but not the full picture. The thickness of the throttle blade and shaft can block airflow and need to be accounted for. For example, a 60mm throttle body may have a larger diameter than the stock 52mm throttle body, but after accounting for obstructions like the throttle blade and rod, the effective area (and thus airflow) will be smaller than the raw area.
We calculate the effective area using the formula for the area of a circle:
Let’s compare the effective areas of different throttle bodies for a more precise look:
| Throttle Body | Diameter | Effective Area (mm²) |
|---|---|---|
| Stock 1.6L Throttle Body | 52mm | 1,635 mm² |
| Focus ST/RS | 57mm | 2,138 mm² |
| 2.3L Ecoboost Mustang | 60mm | 2,617 mm² |
| Stock 1.6L Ecoboost Intake Manifold | — | 2,160 mm² |
As you can see, the 52mm stock throttle body is the most restrictive, while the 57mm RS and 60mm throttle bodies provide a significant increase in effective area and airflow capacity.
How Effective Area Relates to CFM
Now that we know the effective areas, let’s look into CFM (Cubic Feet per Minute). A larger throttle body with a higher effective area can flow more air at a given RPM, which can improve performance throughout the power band.
To estimate the CFM, we can use the following formula for a naturally aspirated engine:
So, the CFM for this engine at 4,000 RPM and 85% VE is approximately 97 CFM.
Now that we have the engine’s baseline CFM, we can compare how each throttle body performs in terms of airflow.
Throttle Body Airflow at 4,000 RPM
Here’s a comparison of the CFM flow at 4,000 RPM for the stock 52mm throttle body, the 57mm RS, and the 60mm throttle body, based on their effective area:
| Throttle Body | Effective Area (mm²) | Flow @ 4k RPM |
|---|---|---|
| 52mm Stock | 1,635 mm² | 95.9 CFM |
| 57mm RS | 2,138 mm² | 126.1 CFM |
| 60mm | 2,617 mm² | 154.4 CFM |
At 4,000 RPM, we can see that while the stock throttle body provides just enough flow for the engine’s needs, the 57mm RS and 60mm throttle bodies can flow significantly more air, which translates to better potential for higher performance, particularly at higher RPMs.
Throttle Body Airflow at 7,000 RPM
Now, let’s estimate the airflow at 7,000 RPM, where airflow demand is much higher.
| Throttle Body | Effective Area (mm²) | Flow @ 7k RPM |
|---|---|---|
| 52mm Stock | 1,635 mm² | 168.6 CFM |
| 57mm RS | 2,138 mm² | 220.6 CFM |
| 60mm | 2,617 mm² | 269.8 CFM |
How Much Power Can Each Throttle Body Support?
Now, let’s calculate the theoretical horsepower each throttle body can support at 7,000 RPM by using the rule of thumb that 1 HP = 0.5 CFM:
| Throttle Body | Flow @ 7k RPM | Theoretical HP (CFM ÷ 0.5) |
|---|---|---|
| 52mm Stock | 168.6 CFM | ~337 HP |
| 57mm RS | 220.6 CFM | ~441 HP |
| 60mm | 269.8 CFM | ~540 HP |
As you can see, while the 52mm stock throttle body can theoretically support up to 337 HP, it’s well below the potential of the 57mm RS or 60mm throttle bodies, which can support 441 HP and 540 HP, respectively.
*Each calculation was done based on a naturally aspirated engine, even though these engines are forced induction. This was done to eliminate a variable to get a true apples to apples comparison.*
Conclusion: Which Throttle Body is Right for You?
The right throttle body is going to be the one the best suits your power goals.
52mm Stock Throttle Body: Good for a stock engine or mild performance mods. It can support up to 337 HP but will be restrictive at higher RPMs or boost levels.
57mm RS Throttle Body: A great option for larger turbocharger setups. It flows 220.6 CFM at 7,000 RPM and can support up to 441 HP. This is the best matched option for the stock 1.6L intake manifold in terms of effective area.
60mm Throttle Body: Best for high horsepower builds, especially for turbochargers that make their power at high RPMs. It can support 540 HP and flows 269.8 CFM at 7,000 RPM on the 1.6L Ecoboost. The best case use of this throttle body would be with an upgraded intake manifold that can be port matched to the larger throttle body.