[Hi Volt]

I was reading on this site last night about a mod that Tundraholic did on a stock airbox. I thought this over and ran some actual tests on the flow of the stock airbox. For the engine to make full power under WOT engine vacuum should be zero or very close. Being Zero means that the engine is getting all the air it requires, if there is however vacuum, then the air intake system is causing a restriction. This is common place when chassis dynoing engines, if there is still vacuum, then you can put on a bigger carb. The same holds true for fuel injected engines. I define the air intake system as the throttle body, intake hose runner, air filter and airbox. NOTE: engines will require different amounts of air depending on mods such as exhaust.

First of all I wanted to know just how well the stock airbox supplied air to my engine. My truck has JBA headers and a custom dual in single outlet 2 1/2" cat back system, K&N drop in filter. For the tests I had a nice stretch of country road I could repeat the tests on all day long. I mounted 2 hose nipples into my stock airbox, one below the filter and one above. I also located a vacuum port right behind the throttle body. The first test was conducted with my K&N drop in filter installed. I placed a vacuum gauge on the lower nipple which would measure any restriction of flow from the inlet tube ( from the fender ). I ran at WOT to 4000 RPM and monitored the vacuum gauge (any vacuum reading would indicate a restriction). Next I did the same on the upper nipple which would measure air filter/inlet tube restriction. I finally used the engine vaccum port, this would give me throttlebody, air filter, inlet tube restriction. I did the same test with a stock toyota filter as well. I repeated the test a few times and got consistant results. To determine WOT I used the lowest spike in the vaccum readings. O.K. Here are the results:

K&N Filter

Intake 0.75" vacuum
Air Filter 0.75" vacuum
Engine 0.75" vacuum

Stock Filter
Intake 0.25" vacuum
Air Filter 1" Vacuum
Engine 1 " Vacuum

To understand what these numbers mean we must look at the differences in vacuum along the way: No difference between the air intake and filter numbers tells us that the K&N filter flows adequately for the engine. As well, no difference between the filter and engine vacuum tells us the throttle body size is adequate for the engine. The restriction is in the air box inlet !

Now if we look at the stock filter numbers we see something a little different. We can see the throttlebody is still fine ( no restriction ) as it should be. But the stock filter has 0.75" of vacuum over the inlet tube vacuum (restriction). The reason for the inlet tube's number to be lower is that the air filter's restriction causes less draw from the airbox and the inlet tube can keep up better during the reduced flow.

These results showed that there was indeed room for improvement so I did a similar mod as described by Tundraholic. From that post I understand that the air box mod is yet someone else's idea, however I am sorry I don't know who, but hats off, good job !

I crunched some numbers and calculated that an 1 1/2" ID second inlet should be sufficient (preferred instead of trying to modify the OE inlet). I used 1 1\4 " PVC fittings which are 1.7" ID. I used a Gates # 20523 2" rad hose ( cut a section) to connect the airbox to supplemental inlet tube. I have posted some pictures to show the installation. I chose to place the inlet behind the right headlight as it is protected yet gets air. The back of the headlight was very clean after 200,000 mi so it is a good place for clean air. All the parts cost $23.00 and took me a couple of hours to do.

After the install I re-ran all the tests, here are the results:

K&N Filter

Intake 0" vacuum
Air Filter 0" vacuum
Engine 0" vacuum

Stock Filter
Intake 0" vacuum
Air Filter 0.75" Vacuum
Engine 0.75 " Vacuum

As you can see, the K&N combination is just what we want to see, the stock filter, while better, only gives some improvement. The numbers show the quantitaive results, as far as my qualitative results... YES there is a noticeable improvement on my truck. Even at lower speeds it helps, here is why, when watching the vacuum gauge during a quick hit of the throttle, there is a vacuum drop to almost zero at 2400 RPM which indicates WOT as commanded by the ECU. CAI's should do the same thing, however the issue is how the intake runners affect the MAF. With this mod, the stock MAF characteristics are maintained as we are only modifying before the air filter. Well worth the $23.00 ! I may just abandon my CAI research from what I have just seen and felt.


Hope this is of some interest