Why use a bigger throttle body/Intake plenum?
There is a great deal of discussion on the need for an improved intake manifold or larger throttle body on a Forced induction vehicle. In order to better explain the need for this I have pieced together this step by step look at why it is necessary. My personal knowledge of this subject is in its early stages so special thanks to those that assisted for explaining it to me and performing the calculations so I can pass the info on.
Corky bell is world renowned for his knowledge on turbo charged vehicles and states that “A maximum air velocity of approximately 300 ft/sec thru a throttle body will keep flow losses acceptable” This is because at speeds above this aerodynamic drag will create unacceptable flow loss (ie Pressure Drop)
In order to work out the air velocity for any engines intake there are a set group of formula’s and information required. To illustrate this point I am using the details on a highly modified 3TGTE engine as owned by a Toymods forum member.
Please keep in mind this is a small area of things we are looking at and needs to be considered as part of the big picture.
Airflow Rate
Airflow rate = (cid x rpm x 0.5 x Ev) / 1728
cid = Cubic Inch Displacement
rpm = Revs Per minute
0.5 = This is due to the fact that a 4 stroke cycle engine only fills its cylinders only on one half the revolutions.
Ev = the volumetric efficiency of the engine ( I am using 85% as a average figure of a well designed system)
1728 converts cubic inches to cubic feet.
That being the case the 3TGTE in question yields the following equation:
Airflow = (110 x 7200 x 0.5 x .85) / 1728
Base airflow or cfm = 195
Pressure Ratio
The Pressure Ratio is the total absolute pressure produced by the turbo divided by atmospheric pressure. Therefore atmospheric is 14.7 absolute pressure.
Pressure Ratio = (Boost + 14.7) / 14.7
In this example we are using and extreme boost level of 38 PSI
Pressure Ratio = (38 + 14.7) / 14.7
Pressure Ratio = 3.6
Please keep in mind this is an ideal ratio assuming the ideal filter element and pipework etc is used. Any other areas of increased aerodynamic drag will add pressure to the atmospheric division value used. For example with an Oil filled pod type filter is not uncommon to have a pressure drop of up to 2 PSI thus producing a value of 12.7 on the base of the equation and an actual pressure ratio of 4.14
Now that we know the Pressure ratio and base cfm we can calculate the engines flow rate under boost. This is quite simply: Boost Airflow rate = Basic cfm x pressure ratio Boost airflow rate in this case = 195 x 3.6 = 702 cfm !!!!
Air Velocity
OK now we can get to the juicy bit. The stock 3TGTE throttle body is 2 inch in diameter. (So is the 2TGEU)
Air velocity = airflow rate / Area of section
The airflow rate is simple the airflow rate expressed in second (The cfm/60)
The area of section is the area of the throttle body divided by the area in inches of a foot to convert it to ft per second ( the Throttle body area in inches / 144)
To calculate the area of a circle use the equation pr2
(Note: a 2 inch diameter (50mm) throttle body yields an area of 3.14 inches 2)
Therefore air flow rate in this case = 702 / 60 = 11.7
Area of Section in this case = 3.14 / 144
Area of section = 0.021805555
Air Velocity = 11.7 / 0.021805555
Air Velocity = 536.56 ft/sec !
By comparison this exact same set up as fitted with a 70mm Throttle body (2.8 inches) would yield an Air Velocity of 273.50 ft/sec
Conclusion
No-one ever hesitates at the suggestion of fitting a sports exhaust or air filter. Hopefully this will help to show the benefits achieved by installing a custom intake plenum or more importantly a larger throttle body.
Rod Try