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Help me understand intake taper

  1. Jan 8, 2008 #1
    Often I see talks about intake taper. This has brought on a few questions from me:)

    I am trying to keep this on a Fuel Injection level. The intakes seem to have longer runners that range from 10-15 inches. Compared to carb intakes that are in the 4-6" range. How can they both perform in the same rpm range?

    Okay anyways, this is the text I have questions from...

    To further help fill the cylinder, it helps to have a high velocity at the back of the valve. To aid in this the intake port can be tapered. To be effective, there should be between 1.7 and 2.5% increase in intake runner area per inch of runner, which represents a 1-1.5° taper. For an example, let’s say you're looking for a 2% increase per inch taper on the 2.02" valve we discussed earlier. We already came up with a port area of 2.56 square inches at just before the valve. Now let’s say the total runner is 10 inches from the valve to the plenum and we're looking for a 2% per inch taper. This turns out to be a total of 3.12 square inches where the port meets the plenum. As you get near the 2.5% per inch taper point, you are pretty much at the limit of helping airflow. A larger taper will only hurt signal strength at the carburetor.

    I see that intake taper is talked about in percentages and in degrees.

    What is the difference?

    On an intake, how can you measure intake taper in percentage or in degrees?

    Thanks and I hope I did not make it too confusing to follow.
  2. jcsd
  3. Jan 8, 2008 #2


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    You can't measure them in either. You have to assume (if you don't know for sure) that the walls are straight and measure different diameters at different locations (usually at the two ends) along the tube. Then the angle of taper or the percentage area increase can be calculated.

    This is analogous to pipe threads, which are also tapered. They are tapered at .75" per foot , or 1°47'. The way that they are checked for acceptance is by checking diameters at gauge points along the axis of the thread.
  4. Jan 8, 2008 #3
    Thanks for the quick response Fred Garvin.

    So, let us say we have an entrance measurement of 2" x 1.75", and the exit measurements are 1.90" x 1.50".

    Entrance area is 3.5" (squared) and the Exit area is 2.85" (squared).

    Then let us say there is a 15" runner.

    Can any taper percentage or degree measurements be made or assumed with the above information?
  5. Jan 8, 2008 #4


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    Unless I'm missing something huge, the taper angle can be obtained from simple trigonometry of measurements, and the percentage as defined in your original post, the amount of increase of runner area per inch of runner length.
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