Intake Designs, Airflow Testing, and Theories

Dec 14, 2003

Much debate still lingers about the standard CAI box vs. a FWI system. It is well known that maximum performance out of a combustion engine requires cool air. This means designing an intake system that brings in as close to ambient as possible.

With these thoughts in mind, many people are drawn to a FWI. People tend to favor them because of three apparent benefits, low cost, simple & clean installation, and it forces the engine to draw air from the fender where the air temps should be near ambient. I must admit that we ran a FWI on our 97 GTP all the way into the 11’s believing that we had a perfect solution. Little did we know how much we were holding our car back with the intake system.

After speaking to Jim Wierzbicki, it was brought to our attention that the FWI may have some inherent flaws by nature of its design. He explained some basic physics principals that got us thinking about our intake system. After acquiring a few tools we set out to do some testing on the flow bench. The data we collected is below. The numbers do not represent anything more than comparative information. They do not correlate to any specific HP amount or cfm in the car. We simply set up the flow bench to flow at various vacuum levels and tested a variety of intake pieces to determine where most of the restriction in an intake system comes from. Below is a small sample of the data collected:

CFM using 1 inch of vacuum (higher numbers indicate more flow)
238 5″ of straight 4″ PCV
220 26″ of straight 4″ PVC
183 Above tube with added 90 deg bend
205 9″ long 4″ inlet dirty K&N filter
227 9″ long 4″ inlet clean K&N
181 7″ long 3.5″ inlet clean K&N
168 LS6 MAF with screen in
191 LS6 MAF w/o screen
159 26″ PVC, 90 deg bend, LS6 MAF (no screen), 9″ K&N
94 Stock GT TB with screen
104 Stock GT TB w/o screen
107 Stock GTP TB with screen
119 Stock GTP TB w/o screen
123 S1 GT HVTB w/o screen
138 S1 GTP HVTB w/o screen

What we learned in our testing was very clear. So much so that dyno testing, in car testing, and track testing was not needed.

  • We learned that adding a K&N filter to an intake tube will not decrease flow. In fact, at times, flow went up from an open tube (using clean large K&N).
  • We learned that small filters do not flow very well even if they are clean. A 4″ inlet filter seems to be needed for maximizing flow.
  • We learned that MAF screens slow down air considerably and removing them will provide more air increase than a larger TB will at times. This does not always equal more HP because the MAF can read improperly but does make for interesting possibilities.
  • A ZZP Stage 1 GT TB will outflow a stock GTP TB even though the GTP is 1mm larger in diameter. More airflow with a smaller diameter means drastically increased velocity.
  • The LS6 MAF outflows the TB by quite a bit in all applications making a good argument for doing a custom setup with the stock MAF removed and an LS6 MAF placed inline. By removing the screen from the LS6 there is potential for more airflow gains but these might be negated by the inaccurate reading of the MAF. The LS6 MAF with screen in outflows the stock MAF setups with screen out. This gives us the potential for full streetability and high flowing performance by using a stock TB (MAF removed) and an LS6 MAF sensor.
  • Adding a 90 deg bend to any intake setup is not beneficial. No intake setup with performance in mind should incorporate a bend if at all possible. Using the largest intake tube available (4 inch) still shows a drastic reduction in airflow from the bend. Based on airflow theory and the mechanisms at work that cause this flow reduction we can conclude that an intake utilizing a 3.5″ or even worse 3″ would be so drastically deficient after adding a 90 deg bend that it could possibly be the most restrictive portion of the entire intake on most Grand Prix.(see Fig.1)



By combining multiple portions of the intake system it’s interesting to see how much total flow drops. At the beginning of the experiments we thought that the most restrictive parts of a mechanism would be the determining factor in total flow. While they do exhibit the most effect on lowering airflow, the numbers allow us to conclude that each part of the system plays a factor in determining total flow. We can safely say that there is no such thing as ‘too much flow’ unless it drastically affects velocity. Believing that your intake system ‘flows enough’  because you are not drawing in a large amount of air is a false. Designing an intake based on ‘enough’ rather than ‘optimum’ is obviously not the way to go.