Stock Air Flow Numbers and Mod

[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

[Big_Al1]

Good research dude thumbs up.

[cleanb13]

Great job!!! I'll have to give it a try.

[apimpdad]

great write up. I will have to give this a try also!

[Burger Steak & Eggs]

Congratulations on proper hot rodding in 2007. Quantified measurements and real performance potential for negligible dollars. A nice change from the hearsay, untested SOTP claims, and ripoffs that fill the internet.

[Hi Volt]

Thanks for the feedback ! I would like to hear how it works out for others.

I should also add that you can go bigger than the size I used, but from the numbers I recorded, it shouldn't make any additional improvement. The exception would be supercharged applications, then bigger may help as long as you also have a free flowing air filter. Even though SC's are pumps, you can't pump what isn't there.

(In case some are wondering, I did consider simply upsizing the intake hose from the fender, but decided against it because it looked to difficult to get in there and cut the inner fender neatly, and then the big question... was some of the restriction caused by the fender cavity as it does get mostly sealed up with the plastic inner guard , and cutting holes in that just lets dirt and water in.)

Good luck with your mods !

[Hi Volt]

Well it has been a while since the mod, 1200 miles and one drag race. Obseravtions on my truck; first, this does pick up bottom end performance, no question. Top end performance is a given with more air flow, highway travel rocks ! Extended highway mileage seems up, city seems the same.
I regularily drag race this truck at the track and have logged over 200 passes. I have a good feel for what the truck will do and can usually make consistant passes within 1 or 2 hundreths of a second. I just came back from a race this last weekend and have calculated the improvements. I was able to find records of passes made in very similar weather conditions ( temp, humidity, barometric, all used to calculate corrected altitude ) and came up with the following repeatable numbers. My 60' times dropped by a close average of .147 seconds ( quite substancial) and the ET picked up by an average of 0.41 seconds with a trap speed improvement of 1.48 MPH. All these numbers were averaged over 4 passes and did not vary on any pass more than 5 %. The average corrected altitude was 1800', so more would be present at sea level air conditions. For those who do not race, these numbers represent bottom end improvements as seen by the 60' et's while the higher trap speed represents top end gains. The overall ET gains are a combination of bottom end and some top end gains. Using a HP factoring program, it is calculated that a gain of 17 HP was required for what was recorded. I admit the numbers sound large, but on a cool evening slamming the throttle at 40 MPH will set my head firmly into the headrest ( never felt it that much before). Now keep in mind that these improvements happened on my truck as I definately needed more air from my measurements. On another note, I am not really heavily modified either, K&N drop in, headers and a 2 1/2" single output cat back exhaust. I am confident that the exhaust modifcations caused more scavenging of the cylinders and the resultant undesired WOT engine vacuum. Those trucks with bigger exhaust may require even more air than I do ( there will finally be an upper limit ), as well as any SC vehicle will definately benefit from less air restriction. The CAI systems provide more air, but as some have seen effect the MAF in a negative way, this mod definately does not mess up the MAF. I used a 2" hose for more air, perhaps a larger one would have a bit more improvement ? however the vacuum gauge said it shouldn't. I may put in a second one for a test. It is really easy to go back to the factory restriction as well, just plug the hose! This will allow back and forth tests to really see the difference and discount any other possible effects. This turned out to be one of those rewarding mods that cost next to nothing ( less than a buck and a half per HP, wish I could spend a couple of hundred this way ! but in all reality this is most likely the practical limit for this mod.)

[Burger Steak & Eggs]

Hi dude. Thanks again for your excellent work. I am doing the mod right now as time permits. I have one plastic tube attached in the front rad support and will attach another in the airbox as soon as I can.

One curiousity....did you remove the "HC" screen just above the main filter?

Also, have you considered using one of the URD boxes to try reoptimizing the air fuel mixture? You might recall the fellow with the Forerunner who found that such mods tend to cause the engine controller to over richen the mixture, not necessarily due to sensor miscalibration you mention, but rather due to safety margin related behavior in the engine controller. He realized gains similar to yours when he installed a CAI, but found huge additional gains when he used a URD gizmo to lean out his mixture.

[Hi Volt]

Hey,

There was no screen above my filter, my truck is an '01 perhaps they were used on later years ? What does this screen look like exactly ? Is it fine mesh or more open ? Part of the airbox or filter ?

I have thought about some possible fuel alterations, but first I am purchasing an Auto Tap so I can watch the fuel trims, O2's and other sensors to get the whole picture. From the economy point of view, the truck certainly is not running over rich as economy is up. Then I am not sure whether I will buy a controller or build one based on what I feel the engine wants.

Did not see the Forerunner post, but unless we are dealing with the same engine then not all mods will necessarily react the same. For example, the shape of the air intake, or location of the MAF on two different engines can get two different results from a CAI, one may work, the other may lose power. The fuel map strategies can also vary between engine types thus rendering a whole new set of fuel trims for a given change in air flow.

I am planning on trying dual exhaust, and then will measure my airbox again to see if there has been any additonal scavenging to cause vacuum to form again at WOT. I am going to restist the temptation to throw on a number of things at once, I want to do one thing, optimise it and move on ( at least that is the plan ! )

[Burger Steak & Eggs]

The HC filter is in the top half of the airbox. When the airbox is put together, the HC filter would sit just above the main filter. The HC filter is the same length and width as the main filter, but only about 1/4" thick. The HC filter media is flat and unpleated. "Filter" might not even be a good word for it, as apparently it is designed to block emissions from leaving the engine to the atmosphere when the engine is off. My reference is a VVTi engine, so your comments about potential different responses to mods is valid.

[Hi Volt]

Filter is probably correct, this media wouldn't happen to be something like a charcoal filter would it ? Your airbox would be a perfect candidate for the vacuum test I ran before and after the mod. If you feel inclined all it takes is a vacum gauge, 10 ft of hose and 2 threaded 1/8" hose barbs. If you are interested I will give you the full details. You can also measure the restriction of the HC filter to see what effect it has.

Anyway the mod can't hurt anything the way it is done.

[apimpdad]

i completed this mod about 2 weeks ago, and so far everything is working correctly, its hard to tell if there is any noticeable gain from low and high end, because of the 38's i have. Also, i have not yet been able to indicate if there is any mileage gain, i have had to disconnect my battery a couple of times since the mod and my mileage was erased, but i hope to get a good reading within the next couple of weeks.

[Burger Steak & Eggs]

I will try to take a photo of the HC and post it tomorrow. I appreciate your test with the vacuum gauge, but since the HC is very easy to remove or put back, I will try it at the drag strip instead.

I have sort of used the gauge testing before, when I was trying to find a low restriction exhaust for my Raptor ATV. I used a low pressure tire gauge and shop vac . I found one from a CBR600 crotch rocket that flowed almost as good as an open pipe but was as quiet as the stock exhaust.

[Hi Volt]

Originally Posted by apimpdad

i completed this mod about 2 weeks ago, and so far everything is working correctly, its hard to tell if there is any noticeable gain from low and high end, because of the 38's i have. Also, i have not yet been able to indicate if there is any mileage gain, i have had to disconnect my battery a couple of times since the mod and my mileage was erased, but i hope to get a good reading within the next couple of weeks.

I am not sure what other mods you have, but I am using a K&N filter, I tested the stock type and still had 1/2" of vacuum even after the mod. Also if you haven't cleaned your MAF lately, do it, this will ensure the MAF is seeing all the added air and give the fuel accordingly.

Nice stance with the 38's !

[Hi Volt]

Originally Posted by MikeN.

I will try to take a photo of the HC and post it tomorrow. I appreciate your test with the vacuum gauge, but since the HC is very easy to remove or put back, I will try it at the drag strip instead.

I have sort of used the gauge testing before, when I was trying to find a low restriction exhaust for my Raptor ATV. I used a low pressure tire gauge and shop vac . I found one from a CBR600 crotch rocket that flowed almost as good as an open pipe but was as quiet as the stock exhaust.

The drag strip will work , I once ran a late test and tune where I could run 3 passes back to back ( I mean as fast as I could get off the track and to the head of staging ) the truck although hot, ran within 5 thou on all 3 passes. That kind of scenario would be perfect for your HC filter test, in one pass, out the next, and no weather changes to worry about. Good Luck !