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Old 11-16-2005, 11:03 PM   #1
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How to build your own Warm Air Intake (WAI)

Author:Harrell, Rick


The buzzword in today's engine performance is the Cold Air Intake. The rationale for this device is that the colder/denser the air coming into the engine, the richer the mix, creating more horsepower (and more fuel consumption). This device generally consists of drawing air from a cool source, someplace outside of the engine compartment and then into the engine's air intake.
With this in mind and studies done independently by this author and others, warmer/dense air allows the car's computer to cean-out the mixture, meaning it takes less fuel to complete a combustion stroke and fuel economy is increased. The common denominator is dense air, which is an independent variable of this experiment. Humidity determines air density. Humidity, unfortunately, was not recorded during this experiment. Lastly, air is a poor conductor of heat, meaning that it looses heat rapidly. This experiment is the groundwork for a heater-core system which will be described at a later date.
In this author's opinion, further experimentation was indicated, as there has been the ability to trick the intake air temperature sensor into thinking that the air is actually very hot. This implementation utilizes resistors input into the air intake temp. sensor to mimic a particular current, which translates into a higher temperature reading. This author has reservations using this technique, as the air may be too cold for a lean-mixture, and create detonation, pre-ignition, or spark knock, which can take a toll on the life-span of cylinder heads and related components.
Implementation of a rudimentary device using foil dryer vent pulling air from the Y junction of the exhaust manifold yielded a first-run fuel economy of 31.1 mpg on 268 miles of my normal driving style, which is pretty aggressive. The second run was over 33 mpg. The baseline was 26.0 mpg from previous tanks. This 15%+ increase has been considered significant, and further testing should be performed to support this increase. As the air at night became approximately 40-50 deg. F., it became increasing difficult to maintain a warm-air transition. During the test, it should be noted that the duct broke loose from the header and pulled cooler air into the engine as displayed by the data-logger collecting information on vehicle speed, engine speed, Long-Term Fuel Trim, and intake air temperature. My hypothesis, is that a easy-to-implement warm air intake will increase fuel economy.
You are assuming your own risk by performing modifications to a vehicle that is either stock from the factory, or has been modified within the scope of local laws; furthermore, this author and/or this website cannot be held responsible for damage resulting from experimentation.
  • Assuming you comply with the disclaimer, let's get started.
  • This author's vehicle is a 1998 Acura Integra 3-door, with the 1.8-Litre Dual-Overhead Cam, 16-Valve, Non-VTEC, 4-cylinder. It was equipped with a home-made cold air intake that drew air from a point behind the passenger-side turn signal, into the airbox, with a K&N-brand, low-resistance cone filter. (see picture 1) Also, the intake air silencer box was removed.
  • Basically find where your air filter is located. Mine is located on the passenger-side of the engine compartment with the air input from the bottom of the box and with filtered air running through a tube into the intake throttle body (see picture 2). Every vehicle is different – some may be out in the open others may be hidden. If you're not sure, consult your owner's manual. Take the filter out and see if you can remove the airbox (if necessary). Also, many models have anair silencer. This is usually large plastic box of some sort that quiets the sound of your engine's operation. My recommendation is to completely remove it, as it adds additional weight, and with our new setup, may not be needed to quiet operation, and free up some space in a cramped engine compartment. Consult Internet sources for your particular model to remove this, as models vary.
  • Secondly, find a source of considerable heat that can be sourced using ducting that won't choke-off the flow of air, and can be run without impeding the operation of fans, belts, etc. In my case it was between the heat shield and exhaust header (see picture 3). I used a foil-type clothes dryer-vent duct for my ductwork. Further experimentation of using a heater core is in the works, which will provide the potential for additional consistence (more on that later). Your ideal location may be near the radiator, catalytic converter, exhaust header, etc. Bear in mind that the longer the air has to travel, the more heat it loses.
  • Next put together an idea of where your ductwork will run. As you can see in this photo (see picture 4) outside air does not pass over the duct, which would cool the air; instead, the plastic baffle was forced down to accommodate the width of the duct. This, in turn, has created a situation where the plastic baffle easily bottoms-out on uneven road surfaces. The key here is to get a robust connection on the end of the tube, so when it does come in contact with obstacles, it doesn't break loose. Also, it may be necessary to remove or re-locate components to allow ductwork to fit. As you can see, the header's heat shield needed bending. (see picture 5).
  • The next step is to figure out how to get the duct into the airbox without sucking air from the engine compartment: a complete seal is needed from the airbox to the end of the tube's hot air source. As you can see here (see picture 6), a PVC pipe was slightly heated and bent to conform to the oval shape of the airbox's inlet. Since the pipe kept falling off, a screw was set into the box to keep it from falling off. The inlet has a rubber seal that aids in keeping our setup sealed from cooler engine compartment air. Again, your setup may vary. (see picture 7)
  • Route your ductwork. As this is technically a duct, duct tape was used to seal the PVC pipe to the dryer vent. (see picture 8). I know it's not pretty, but it does the trick to ensure a sealed system.
  • After running your ductwork to a hot source, ensure a solid connection. As my resources are limited, I used a coat hanger and hobby wire (similar to what is used to wrap bows on holiday wreaths) to secure the duct end to the header. The hobby wire was used primarily to angle the end to draw air directly off of the header.
Using my datalogger (OBD-II), I noted increases in temperature from outside air temperature/engine compartment temp (40-90 degrees) to a range in the 80-150 deg. F. range. I noted on my logger that as air intake temp increased, Long-Term Fuel Trim decreased, indicating a leaner mixture = fuel savings. This was especially helpful on my 80% highway travel. Bear in mind that power loss is to be expected. Lower-RPM power wasn't affected as much as mid-range to near redline power diminished. This small sacrifice, in this author's opinion, is worth the fuel savings. Good luck in testing, and let us know your results here at Please use the scientific method whenever possible, and do try to not let bias enter the equation (maybe have someone else drive the car after the mod., as this may effect mileage by using a more fuel efficient driving style). Happy motoring. </td>


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Old 01-12-2006, 12:27 PM   #2
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more discussion here:

more discussion here:

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Old 05-11-2006, 10:07 AM   #3
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Are you sure ?

Hello Matt

With all due respect, I suppose there is an error in the article concerning the warm air intake...

I have studied physics at college (thermodynamics etc) and worked as a flight dispatcher for 11 years (closely related to meteorology) during all these years I learnt that cold air is much denser than warm, both by means of oxygen and humidity (remember; dew point where humidity densens and becomes fog, is lower than ambient temperature)

I have been told by several saver/misers that cold air intakes improve economy. Furthermore I have been told that chip tuning improves economy given that you do not get tempted to "step on it"

I chenged my filter with a twin venturi cold air direct intake removing the filter box, removed the two mufflers (not the cat) and mounted a Remus glasspack.

Since I habitually do not step on the accelerator pedal, there is no excessive noise and I noticed thru the trip computer that the vehicle started reporting 4.8-5.2 liters / 100 km. from the usual 6.2-6.7 liters/100km. (from 35.10 MPG to 49 MPG) That is an average of a gallon saved every 200 kms (approx 130 miles)

I did not try any flush wheels, skirts and other aerodynamic aides as yet. I plan on getting a complete ground effect kit, lower the vehicle so drag is decreased. I want to prove that fuel saving can also go along with good looks of a car...

Best regards

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Old 05-11-2006, 10:52 AM   #4
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Cold vs warm air intakes

Cold air intakes will produce more power, and will save gas if the engine operates only at Wide Open Throttle.

However, under 90% of driving, Warm Air Intakes are more efficient because the engine is throttled and under less than full load. WAI does two things:

1) Fuel evaporates more quickly, increasing combustion speed. Fast burn engines improves the amount of pressure generated and expansion work available for a given amount of fuel.

2) With oxygen sensors, engines run at stoichiometric air/fuel ratios. For any given amount of fuel, a corresponding mass of oxygen is admitted. WAI lowers the density of air, meaning that a higher volume of air must be let into the engine for a given fuel input. So with WAI, the throttle must be opened further for a given output, and this lowers the "pumping" work of pulling air past the throttle.

WAI is better than CAI for fuel efficiency imho.
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Old 05-11-2006, 09:08 PM   #5
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man i want one! too hard

man i want one! too hard can't i just pay some money and someone do it for me?
If your reading this, then good for you, your saving some gas because your here.
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Old 05-12-2006, 04:16 AM   #6
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Quote:man i want one! too

man i want one! too hard can't i just pay some money and someone do it for me?
Haha, I'll do it for free for ya.

See if my write up this evening or next makes it seem a little easier.
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Old 05-12-2006, 10:37 AM   #7
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I See

Well I guess you should know better than I do since you are further involved in fuel savings than me.

So I will follow your word...

Will test out your theory for a while...
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Old 05-12-2006, 10:55 AM   #8
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Hot air is probally seems

Hot air is probally seems like the strangest mod to the the typical car tuner. Someone who pays +$300 for a cold air intake probly would think you we are insane.

Hot air=better fuel atomization= better fuel economy (or so the theory goes)

2008 EPA adjusted:

Distance traveled by bicycle in 2007= 1,830ish miles
Average commute speed=25mph (yes, that's in a car)
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Old 05-12-2006, 01:06 PM   #9
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Less Power = Less Fuel

I've had luck with the HAI (hot-air intake) for several months. It reduces power, and consequently uses less fuel. It takes more throttle input, so if you accelerate at a pre-determined level, say 20% throttle (from the ScanGauge), then you're doing so more slowly as well - then it's a driving style change = more economy.

Also, I have a new design from what's pictured above. The dryer vent was bottoming out and also deteriorating. I bought some hot-air (carb) flex-tube as reccommended by krousdb, cut a hole in the airbox, and ran it directly. (I may post some pics later...) I need to block the air flow as highway speeds cool things down. I have to use Premium fuel or I get continuous detonation, though -- It pays for itself.
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Old 05-12-2006, 01:18 PM   #10
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Has anyone compared the HAI

Has anyone compared the HAI with the IAT mod. I tried the IAT mod using 260, 160, 125 with no luck.:? The OBDII just retarded the timing and my FE was actually worst. I'm thinking that the HAI might work because the temps would be lower and the air acutally thiner. I've got some dryer hose I think I'll rig it up and see what happens.

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