Checking Air FIlter And Air Box Flow

Many see the air filter as a major impediemnt to flow, but in a road car, before tossing the filter element and replacing it with something that is supposed to flow better, it is wise to do a little investigation. If the stock element isnt blocked by dirt,insects and other rubbish, changing to a so-called high performance element probably wont do much. Frequently it is found that it is the air box, or other sound muffling devices n the inlet duct that are the real culprits in strangling air flow.



The easy was to evaluate the inlet system is by means of a flow bench, but an inexpensive and viable alternative is to use a water manometer (ill draw a pic tomorrow). While this test can be a bit of a fiddle it will help you discover a good deal about the science of engine breathing, and you will feel challenged to consider flow restrictions possibly existing in the whole inlet system and not just the air filter itself.



The areas we want to check are shown in Fig. 14.2 (ill post those up tomorrow). In naturally aspirated and nitrous assisted engines we can se the manometer to test all the way through to the inlet manifold. However, when a blower or turbo is fitted we cannot check out the pressure side of the inlet tract using this method.



Table 14.1 (see bottom of post) shows the results of tests done on a subaru liberty rs turbo while it was running on a wheel dyno. The pressure drop, which can also be referred to as the vacum or depression, in the intake system ws measured using a 72in water manometer. When you do this test in a car you will be limited to about 42in column height, so past 5000RPM this car would have been off the scale!



When using the water manometer, set your assistant up with it in the back seat. Using full throttle, accelerate from 4K RPM to 7K RPM in 3rd, or preferrably 4th, gear up a hill. As you approach each thousand RPM call out to your rear seat passanger. He will mentally record the depression at each call and write the figures down at the end of a run. Extreme accuracy isnt important. For exapmle, if the water level was less than halfway between 20in and 30in record it as 23in. Again, do three runs at each test point to check for errors. Note that a video camera can be used to record this test, but unless you have another tacho that can be duct taped close to the manometer you may have problems getting both the manometer and tach on focus.



Now what do these figures mean? STandard air pressure at sea level is 14.7psi which equals 29.92in mercury or 406.9in water. Therefore 1in mercury equals about .5psi and 1in water equals about .036psi. Clearly this Subaru has a problem which wont be solved by replacing the air filter arrangement. At 7K rpm there is enough flow resistance cause by the air muffler before the air filter to cause a depression of 36in water. Thats a restriction of 1.3psi even before you get to the filter! Again at 7k rpm the filter restriction is only 8in water (44 - 36=8) or .29psi The other major restriction is the little resonater box stuck in the air duct right after the air meter. It is causing a depression of 12in water (60 - 48=12) or .43[si. In total the system has a pressure drop of 60in before you get to the throttle body (and thats another story). That is almost 2.2psi, so instead of air pressure of 14.7psi to force air into the turbo, this car is working with only 12.5psi. Is that costing horsepower? Well, lets put it this way, by getting rid of that flow restriction its like getting 2.2psi of free supercharging without adding any heat to the air! In hp terms it means that a 200hp engine when restricted like this is going to be limited to around 175-180hp.

Table 14.1. Pressure drops in Subaru intake

RPM point 1 point 2 point 3 point 4

4000 12 17 17 20

5000 20 24 26 30

6000 27 32 34 41

7000 36 44 48 60



point 1 - In the air box before the filter element.

point 2 - In the air box after the filter element.

point 3 - In the air duct after the air flow meter.

point 4 - In the air duct after the resonator box.