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Useable surface area for a throttle body (valve), is this correct?

  1. Oct 30, 2007 #1
    Here is some information I have for throttle bodys:

    60 millimeters is equal to 2.362205 inches = 17.52 square inches
    65 millimeters is equal to 2.559055 inches = 20.56 square inches - (17%)
    70 millimeters is equal to 2.755906 inches = 23.85 square inches - (36%)
    75 millimeters is equal to 2.952756 inches = 27.37 square inches - (56%)
    90 millimeters is equal to 3.543307 inches = 39.42 square inches - (125%)

    Okay, so my question:

    If I take the 75mm throttle body and open the blade up 10% I get (27.37 sq. in. x .10 = 2.74 sq. in.).

    Now what does the 2.74 sq. in. answer that I got mean? Is that 2.74 sq. in. of air that pass by the blade into the engine?

    So for a 60mm throttle body, my 10% number is, 1.75 sq. in.

    So does this prove that more air passes the blade, therefore potentially causing a touchier gas pedal with less depression of the pedal?

    That is a 56% increase of air at that particular valve angle right?

    I hope this made sense, I am a newbie:)
  2. jcsd
  3. Aug 4, 2010 #2
    Your assumption that the performance of a "blade" valve is incorrect. A "blade" style throttle, like any blade style valve, is NON-LINEAR in performance. If you look at the true flow characteristics, you will see that the airflow around a blade reaches more than 70% full capacity (for a given static pressure) at just about 30% open. So in your case, 10% blade movement may correspond to something like 25+% change in flow!!!!!!!!
  4. Aug 6, 2010 #3


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    The 2.74 in^2 answer that you got is mostly worthless.

    Reasons why:
    1) Like mentioned, the flow through the throttle body is not linear.

    2) Even trying to base 10% off of the TPS (Throttle Position Sensor), you can't assume the relationship to simply be that percentage of the total opening.

    3) Most throttle body blade openings are progressive, meaning that the amount the blade(s) open per amount of pedal input changes with different throttle positions. Unless you know the rate at which your throttle body opens/closes it is very difficult to estimate much about the surface area your have available (plus the manufacturer went to great lengths designing the progressive TB and measuring the flow to the engine, so it's much easier to snag that data from them than calculate it yourself).

    As for your question about having a touchier gas pedal the answer is mostly yes, with some qualifiers. If you keep everything the same except for the throttle body opening, then at the same degree of rotation of the blade(s) there will be more surface area available to flow through with the larger blade(s). This could cause a more responsive throttle at very low openings, but there are other things to consider as well. The fuel injection system may or may not have corrections based on TPS position or rate of change of TPS. In either case, it is possible that the computer will over or under correct, due to the fact that the change it predicts is not proportional to the change that it will actually see. Typically this isn't seen except in the extreme ends of the spectrum (rapid opening on the throttle body).
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