There was recently a thread about larger throttle bodies for the ST. I wanted to address this in a comprehensive way for those interested (Must be more than just me!)
First, let's talk about airflow in terms of Cubic Feet per Minute (CFM) vs Mass Airflow. CFM is a VOLUME measurement; it doesn't address density/mass of the air which is a product of pressure and temperature. When we talk about Volumetric Efficiency (VE) of an engine, we are assigning a value in terms of percent to theoretical volume of air an engine will ingest at a given RPM vs the actual amount consumed. Let's first calculate the ST engine's CFM. We'll use a 3 Liter displacement at 6000 rpm and 100% VE. Doing the math and converting to imperial units, our ST engine will consume approximately 318 CFM.
Employing the calculation of CFM through a pipe and using 100 m/s for the variable of velocity (this number has been used for port design for 30+ years as the maximum speed for inertia tuning while keeping the velocity low enough to minimize boundary layer disruption) we can calculate CFM flow rates through various throttle body sizes. Note that this is the internal diameter of the bore and doesn't address restrictions like throttle plates, etc.
40mm = 266 cfm Obviously too small for our engine
50mm = 416 cfm Probably a good size, and still leaves us 98 cfm leeway for throttle plate interference.
60mm = 600 cfm It's pretty clear that this would be more than enough and would represent about a 2.4 inch bore.
70mm = 815 cfm No problem!
Now I don't know the exact size of the ST throttle body, but this gives you an indication of whether a larger one is needed. Bear in mind, the intake manifold would also have be able to have this same diameter from the throttle body transition. I'd suspect the ST is probably in the 60mm category and therefore any enlargement is unnecessary.
First, let's talk about airflow in terms of Cubic Feet per Minute (CFM) vs Mass Airflow. CFM is a VOLUME measurement; it doesn't address density/mass of the air which is a product of pressure and temperature. When we talk about Volumetric Efficiency (VE) of an engine, we are assigning a value in terms of percent to theoretical volume of air an engine will ingest at a given RPM vs the actual amount consumed. Let's first calculate the ST engine's CFM. We'll use a 3 Liter displacement at 6000 rpm and 100% VE. Doing the math and converting to imperial units, our ST engine will consume approximately 318 CFM.
Employing the calculation of CFM through a pipe and using 100 m/s for the variable of velocity (this number has been used for port design for 30+ years as the maximum speed for inertia tuning while keeping the velocity low enough to minimize boundary layer disruption) we can calculate CFM flow rates through various throttle body sizes. Note that this is the internal diameter of the bore and doesn't address restrictions like throttle plates, etc.
40mm = 266 cfm Obviously too small for our engine
50mm = 416 cfm Probably a good size, and still leaves us 98 cfm leeway for throttle plate interference.
60mm = 600 cfm It's pretty clear that this would be more than enough and would represent about a 2.4 inch bore.
70mm = 815 cfm No problem!
Now I don't know the exact size of the ST throttle body, but this gives you an indication of whether a larger one is needed. Bear in mind, the intake manifold would also have be able to have this same diameter from the throttle body transition. I'd suspect the ST is probably in the 60mm category and therefore any enlargement is unnecessary.
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