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Intakes 101


What's the difference between a stock airbox, a short ram system (open intake filter), or a CAI (Cold Air Intake)? Isn't the CAI the best since it draws cold air? Won't the short ram system just draw hot air and cause my engine to lose power?


A short ram system usually refers to a open filter (cylindrical or conical shaped) attached directly to the mass air flow meter, or attached via a short velocity tube. In some applications, a heat shield surrounds the filter to reduce the effects of radiant heat coming from the engine. A cold air intake usually refers to either a heat shield that seals to the hood and surrounds the sides and bottom of the filter, or a long extension tube that relocates the filter in the front fenderwell.

The Science Behind an Air Intake

You've probably heard this before on the internet, "...colder air is denser air, and that means more horsepower." In fact, you've probably heard this enough times that you're convinced it's true, right? Well this might throw you for a loop then: It's entirely possible to gain horsepower with just an open element intake. Drawing in air that is warmer than the outside air does not automatically equate to a horsepower loss when the stock airbox has been removed. Now here is the science behind that statement.

When looking at the properties of gases (and air is a gas) the actual scientific equation for density is:

Density = mass / volume

A quick review of our science book tells us to increase air density you could increase the mass, or reduce the volume. To increase mass, you could increase pressure or reduce temperature. To reduce volume you could increase pressure or reduce temperature. So air pressure and temperature are the two common variables that we have to work with. Everyone likes to talk about temperature, but very few people ever address pressure. Pressure and temperature are equally important.

Pressure, Temperature, and SAE J1349

The Society of Automotive Engineers (SAE) has established a test standard that helps standardized engine horsepower testing and results so that the variable effects of barometric pressure, altitude, and intake air temperatures do not bias the test results. The SAE J1349 test procedure includes an engine horsepower correction factor so that, for example, dyno readings taken at 3500 feet on a 40 degree day can be compared with dyno readings taken at sea level on a 77 degree day.

This correction factor is used for Normally Aspirated Engines, not forced induction engines. Once the correction factor is determined, it can be applied to the actual dyno readings so they can be adjusted back to simulate a test conducted at sea level, on a 77 degree day, with 1% humidity.

The SAE correction factor can be approximated using this equation:

CF = 1.18 * (29.235 / Bdo) * ((square root (To + 460) / 537) - 0.153)

where CF = the correction factor, Bdo = the dry ambient barometric pressure in inches of mercury (in/Hg), and To = the intake air temperature in degrees Fahrenheit.

Test 1: The Baseline Test

Let's test this equation with a hypothetical engine that dynos at 100HP. We test this engine on a 77 degree day, at sea level. So, we set Bdo = to 29.235 in/Hg and To = to 77F. When we solve the equation for CF, the correction factor equals 1. That means according to SAE, our dyno reading does not require a correction factor for temperature or barometric pressure. It is a true 100HP engine.

Test 2: Temperature = 87 degrees F, Pressure = 29.235 in/Hg

What happens when the temperature climbs by 10 degrees, but pressure stays constant? Plugging in 87 for To and 29.235 for Bdo, we can calculate the value of CF. CF = 1.0104. Working our correction factor equation backwards, we take:

100hp / 1.0104 = 98.97hp.

So, according to the SAE correction factor, a 10 degree increase in temp should result in a loss of 1.03% of rated horsepower, or 1hp on our engine.

Test 3: Temperature = 77 degrees F, Pressure = 28.235 in/Hg

What happens when the pressure drops by 1.0 in/Hg, but temperature stays constant? Plugging in 77 for To and 28.235 for Bdo, we calculate CF and find it equals 1.042.

100hp / 1.042 = 95.96hp.

So, according to the SAE correction factor, a 1 in/Hg drop in air pressure should result in a loss of 4.04% of rated horsepower, or 4hp on our engine.

Wow! A drop of 1.0 in/Hg in air pressure is roughly equivalent to climbing approximately 1,000 feet in altitude. That's not very high. The Sear's Tower in Chicago is 1353 feet tall. So, if we put our car in the Sear's Tower freight elevator and take it to the roof, now we have an SAE correction factor of approximately 1.060. We lost almost 6hp just going from the ground floor to the roof level !!

What Does It All Mean?

When designing a P-Flo intake, it means we are concerned about how much pressure loss is caused by the stock airbox. For example, barometric pressure at sea level may be 29.235 in/Hg, but air pressure drops as air enters the factory airbox and passes through the filter. So the pressure below the air filter element (on engine side of the intake system) is going to be less than 29.235. How do we know this? There is another SAE test, J726, that is used to calculate the efficiency of air filters. One of the variables measured in this test is the air pressure drop caused by the factory airbox and filter element. We call this pressure drop "Delta P" or differential pressure.

Would you be surprised to find that during the SAE J726 test, the stock airbox can cause a Delta P of anywhere from 15-20psi, depending on the CFM moving through the intake tract? And that just changing the filter element material can result in a 1-5 psi difference? Want to test this in real life? Take a normally aspirated car like the Golf VR6 or the Acura RSX and run two back-to-back dyno tests: One test with the airbox on, one with the airbox totally removed. Did you find a horsepower gain at the higher RPMs without the airbox? You gained HP, and yet the intake air temperature stayed the same or maybe even went higher. So part of the Delta P is caused by restrictions in your factory air box.

However, during the SAE J726 tests we conducted, we found the Delta P to be lowest when an oiled cotton gauze filter material (K&N style) is used. Synthetic foam filters have the highest Delta P due to the lubricating polymer they are typically coated with. So the other component to lowering Delta P is choosing the air filter element that flows without much restriction, yet still traps dirt.

So, in summary it is entirely possible to gain horsepower just by moving to a short ram intake. Reducing the Delta P will be an improvement on its own. Of course, if you can reduce intake air temperatures AND reduce Delta P, then you have the best of both worlds. But given a choice between the two, Delta P is more important for most vehicles. NEUSPEED makes short ram systems (with and without heat shields) and "CAI" cold air intakes. So the type of filter kit offered for your car is based on what worked the best during our design and testing.

Be advised that CAI systems that place the air filter down into the front fenderwell should not be used in wet, rainy conditions, especially in areas that experience excessive puddling or patches of standing water. If you operate your car frequently in these conditions, we recommend that you use our short ram systems instead to avoid ingesting water into your engine. Water ingestion is a very serious problem and can lead to extensive engine damage.

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