Direct Measurement of "4-Square" Intake Runner Length
by Clay Autery

We've been having periodic discussions about intake design over at TurboFord, and 67King piqued my interest in tuned-port stuff.... So, since I had the entire intake sitting on my desk 6" away, I decided to measure them.  Please read the entire page including my notes, so that you can appreciate the value AND limitations of the information below. Feel free to share the following information as you please, but please include a source reference like: "Info. provided by C. Autery on THIS page."  The HTML code for the above credit is located at the bottom of the page.

Enjoy,
Clay

Runner Number (min/max/avg)

1 (min)

1 (max)

1 (avg)

2 (min)

2 (max)

2 (avg)

3 ( min)

3 (max)

3 (avg)

4 ( min)

4 (max)

4 (avg)

Note: Measurements do not include head face to intake port

Measurement Procedure:

Using a relatively stiff solid core copper wire, hemostats, and assortment of bright halogen lights, and a ruler, I took two measurements on each cylinder runner. I measured the minimum air path possible by pulling the wire tight around the runner transitions/loop. Then I measured the max possible air path by setting the start point at the runner entry that is furthest from the runner exit on the lower face. Then I pulled it tight to make sure the wire curve was "straight". Then I carefully introduced the wire into the runner until it was riding on the "far" face of the runner all the way to exit and marked.

Wire was marked at measurement distance and then extracted from the runner. Ruler was used to measure wire distance from hemostat nose to wire mark, making sure that to keep wire fixed and straight.

Measurement conventions:

Measurement starts at the first perceptible point of transition from plenum to runner in the casting and ends at the lower intake face.

Tolerances:

I have no idea... The measurements suggest that I am within about +/- 0.25" of exact measurements... procedure and start/end point choice could create significant differences.

Note regarding arithmetic average: 

Using the arithmetic average as the "true" runner length is only an approximation (as are the raw measurements). There are actually two runner lengths. There is the the "true" path which is defined by the line described by the circular/elliptical cross sectional centers (or bisected line between centers in the case of an ellipse) throughout the runner. Unless you have the formulae for creating the runner cross-sections and are really good at calculus, you ain't gonna find this value exactly...

The other path is the "effective true path" which is the average distance an air molecule travels from runner start to end... this value is determined (or not) by calculating the effects of boundary layer influences, turbulence at transitions, pressure differentials across cross-sections, etc... It would be virtually impossible to calculate this value in your lifetime... BUT it IS the most accurate,

Personally, I'm satisfied with my measurement technique as being "close enough for government work" as we used to say.

HTML code for source reference:

Info. provided by C. Autery on <a href="http://www.montac.com/mustang/intake/runner_calcs.htm" target="_blank"><b>MTB Madness</b></a>