Ever wonder if that new air filter makes a difference?
Ever wonder if that new CAI is an improvement?
Thinking about rigging up some type of ram air system?
Need a little more information than a butt-dyno?

Using a simple tool we can actually see and measure the amount of resistance in your air inlet system. You can spend more money and buy a dial gauge that will do this but for a few bucks you can build your own.......

The most common method requires some clear tubing, a bottle to hold water and a piece of wood to attach everything to.

Basically you are going to run a piece of clear hose from the point of measurement on the air inlet, attach it to something that will be marked off in inches and down into a reservoir of water.

Here is how I built mine.

Building the manometer

  1. I used an 18 inch piece of 1" PVC pipe and capped the bottom.
  2. I drilled a hole near the bottom and glued in a 90 degree fitting that fit the plastic tubing.
  3. Installed a short piece of the tubing to the fitting.
  4. Installed a 16" piece of rigid clear plastic tubing to the tubing connected to the fitting.
  5. Installed another short piece of tubing at the top of the rigid tube and installed a straight connector. You don't have to have the straight fitting, I just wanted to use it to make it easy to connect and disconnect the hose from under the hood.
  6. I then took a 1/4" tape measure from the dollar store and cut it to fit under the rigid plastic tube.
  7. I then held everything together with rubber bands. You can permanently attach the tape measure to the tube if you want, just remember when you use the meter you will need to fill the reservoir to only come up to the zero mark. Using the rubber bands I can slide the tape measure up and down to align with the water line.
  8. I took water and added red food coloring so it would show up easy and then poured that into the pipe until it reached the zero mark on the tape measure.

That is it for the meter.

Now we need to put it to use

There are lots of places you can measure to see the resistance in your air intake.

I wanted to see what was my total resistance was so I needed a place to tap in between the MAF and the Throttle Body. On the Bullitt there are two holes in the air inlet just above the TB. One is for the Idle Air Control valve and the other is the fresh air intake for the PCV system. I decided to borrow the port from the PCV system. I took a cheap PCV filter and just pressed it onto the PCV supply tube. I used a rubber stopper plug and drilled a hole through it and inserted about a 4" piece of rigid tubing. Then I inserted the plug/tube assembly into the air intake. I left about 2 1/2" sticking through so as to measure the center of the intake air.

As long as you keep the placement and depth of the tube the same then you will have a good reference to compare to if you make changes. My experience was that I could vary the reading by about 10% by playing with how far in the measurement is taken.

But seriously how accurate is this? This is a very sensitive instrument that will accurately measure pressure drops. To give you an idea, 1 inch of water equals .03612 of one PSI.

I ran the tubing around the corner of the front right fender by the windshield and just ran it into the open passenger window. You really need to have a passenger ride along to take the measurements. The meter needs to be held in a vertical position and the only time you can get the most valuable information is when the engine is under a load at full throttle.

I did my measurements in second gear and took the readings at 4K, 5K and 5500 rpms.

NOTE: The amount of resistance is at its maximum when you are at wide open throttle. You will be surprised to see how low the readings are when you are just driving steady down the road.

NOTE: I originally started with a 36" meter but after taking the readings I was able to keep cutting it down.

What are we measuring?

When you use a straw in a drink you are creating a vacuum in your mouth and the fluid rises up the straw. If you use enough suction to raise the water 2 inches then you have created a negative pressure of 2 inches of water. (27.68 inches of water equals 1 psi) It is atmospheric pressure (14.7 pounds per square inch) that actually pushes the water up the straw.

Same deal with the engine, as the pistons go down and create a void of air, atmospheric pressure pushes the air into the engine. The resistance to air flow is what we are measuring with the Manometer.

The total resistance was less than 10" of water, or less than .36 of one PSI.

You could just simply use this one point of measurement to compare any modifications you make.
OR you may want to dig a little deeper and make some more measurement points.

I had picked up an extra air box to donate to the project so I drilled some more holes.
I drilled one in the air box and another one just after the air filter and before the MAF/Screen.
I used a grommet and inline connection and simply capped the connection when not in use.

So we are set to measure

  • Inside the air box
  • After the air filter
  • Total resistance just above the TB

Below are the measurements I got (inches of water)

Using the measurements we can see how much resistance of some of the components.

Resistance across the Air Filter.

Resistance across the MAF and MAF Screen.

Seeing as there has always been big debates on the restriction of the MAF screen I wanted to see what it was. I removed the screen and measured my Total Resistance again.

NOTE: I had no drivability problems or issues with the MAF Screen removed. Had it been a huge resistance I would have kept it out but seeing as the drop was pretty small to me I decided to run the MAF Screen installed.

Just a side note. Remember I said that the most resistance is at WOT and while you are driving at a steady speed there is very little resistance. I took measurements from the air box at 40 MPH and 65 MPH with the cruise control on. ALL measurements were right at a negative 3/4". Coasting to a stop would allow the meter to go back to zero. So the air flow of the moving vehicle creates a high pressure in the fender well. For people that are looking to change or modify their CAI may want to keep that in mind.

Oil crankcase pressure

Part of my wanting to take these measurements was some vendors were claiming that Ford overflows their crankcase producing more oil vapors in the intake than needed. They claimed that their Oil Separators were matched to the engine. Well you are not going to be able to increase air flow so they must be increasing the restriction.

Ford lists a different PCV valve (not just the heated PCV option either) for the 2001 Bullitt than they do for the 2001 GT or Cobra. Seems Ford is pretty particular about the PCV flow to me. Because of this I decided that I wanted to see some numbers before and after the mods.

I decided to use the manometer to measure the affects of a couple of mods that are used to reduce these oil vapors from entering the intake.

JLT sells a filter that installs between the rocker cover and the PCV valve. This same filter screen was used in earlier Mustangs.

There are many different types of Oil Separators from vendors or many people make their own from ones available at their local hardware store. I am not going to go into the design but only take the measurements from the hardware available oil separator.

I used the manometer to read the pressure of the crankcase at the Oil Dipstick Tube.

Below are the measurements I got.

I used 4000 and 5000 RPMs for my measurements.

The base line for the stock setup was 3" of water at 4000 RPMs and 7" of water at 5000 RPMs.
Since the PCV system flow is not linear, the jump from 3" to 7" with only a 1000 RPMs more is not unreasonable.

After a couple of measurements to see what the stock system was reading I installed the JLT screen in between the rocker cover and the PCV valve.

The measurement at 4000 RPMs went from 3" of water to 2.75" of water. And the measurement at 5000 RPMs went from 7" of water to 6.75" of water. At first glance this looks backwards. But we added a restriction to the suction side of the air flow through the crankcase so the lower numbers reflect the affects of the restriction in the system.

Looks like the JLT screens are worth a flat .25" of water restriction. Or if you are thinking PSI, that is a drop of .00903 PSI.

Next I removed the JLT screen and installed the Oil Separator.
At 4000 RPMs we had the same reading as the base line of 3" of water.
At 5000 RPMs we dropped from 7" to 6.75" of water.

If you take the oil separator and just blow through it as hard as you can, you can feel the restriction it produces. But at only 7" of water it is such a low pressure, the restriction is almost invisible.

After looking at the numbers I plan on running the Oil Separator for a while and see what kind of oil I collect.

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