Rick's Free Auto Repair Advice

More power with a cold air intake?

Do you really get more horsepower with a cold air intake?

Every day I see questions posted on auto forums from guys wanting to mod their cars by adding a cold air intake. They’re convinced they’ll get more horsepower with a cold air intake systems on a late model car are a total rip off. You’re delusional if you think you’ll see any improvement in performance. And, in many cases you’ll actually see a decrease.

Cold air intake kits work on carbureted engines, not on fuel injected engines with MAF sensors

Back in the days of carburetors, a cold air intake offered verifiable increases in performance. Even then however, you couldn’t get both a performance boost AND an increase in MPG – because they’re mutually exclusive. But the technology changed with fuel injection and the use of a mass airflow sensor (MAF). When car makers introduced the MAF sensors, they also moved the air intake snorkel to ingest cold air from outside the engine compartment. That’s because, for the first time, the engine control systems could actually measure air density. Prior to MAF sensors, engine sensors only measured ambient air temperature and barometric pressure and had to impute air density.

Factory air intakes are tuned to provide laminar flow

Car makers also finely tuned the factory intakes to reduce turbulence and provide a laminar flow of air into the mass airflow sensor.Once you replace the factory air intake system with an aftermarket cold air intake, you get a more throaty sound, but that sound is caused by turbulence—the last thing you want to do to air entering the MAF sensor. In effect, an aftermarket cold air intake literally screws up all the engineering that went into the factory intake.

Aftermarket cold air intake kits DON’T increase performance or gas mileage

Cold air intake manufacturers continue to make performance and MPG promises they can’t keep.  Don’t think for a second you’re going to get more horsepower or better gas mileage on a naturally aspirated engine using an aftermarket cold air intake. Think I’m kidding. Pull up the cold air intake manufacturer’s specs and see for yourself. The horsepower improvements they advertise only kick it when you’re at or near red line RPM–and even those increases have been debunked by independent testing. Even if it did work, how often do you drive with your foot to the floor? I didn’t think so.

Cold air intakes rely on a restricted airflow theory

Cold air intake manufacturers want you to believe that if you provide a larger pipe for the intake, you’ll get less airflow restriction so your engine can take in more air. That sounds good in theory, but it doesn’t hold up in practice. Here’s why.

An internal combustion engine works like a doctor’s syringe. Pull the piston back and it sucks in air. If the syringe is a .5-liter syringe, it will suck in .5-liter of air. Five liters of cold air is denser than warm air, so it contains more oxygen than .5-liters of hot air.

Factory engineers aren’t stupid. They know all about cold air and air density. I mean, contrary to what the cold air intake manufacturers want you to believe, they didn’t invent the theory that cold air is more dense than hot air. To the contrary, car makers started using cold air intakes long ago. Most fuel injected vehicle built since the 90’s come from the factory with a cold air intake.

Now let’s talk about what is and is not a true cold air intake. If you mod your car and install a “cold air intake” with a large cone filter, and that filter draws in air from INSIDE the engine compartment, that is NOT a COLD AIR INTAKE. It may be a shiny air intake, it may be a brown or blue intake, but it’s not a cold air intake. If you install one of those systems, you’re actually taking a step backwards to the days when carburetors sucked in air from inside the engine compartment. To be a true cold air intake, it must draw outside air from IN FRONT of the radiator, under the front bumper, or from inside the fender/wheel well.

Factory tuned air intakes perform better than an aftermarket cold air intake

More than just drawing in cold air, factory intakes are tuned to reduce air turbulence and get the air to flow in a laminar fashion as it hits the MAF so the computer can measure the air’s mass. Air turbulence causes drag and noise. So the second you remove the factory intake you defeat that engineering. You may like the noise factor of an aftermarket cold air intake, but you’re getting that noise by creating turbulence in front of  both the MAF and throttle plate. Ever see people write in on car forums saying they got a check engine light after installing a cold air intake? Duh. It’s disturbing the laminar airflow into the MAF. Whatever increase in airflow you may pick up with the larger cone filter, you lose with increased airflow drag caused by turbulence. Turbulence SLOWS down the airflow.

Understanding intake air restriction

Keep in mind that the theory behind an aftermarket CAI system is two-fold:

1) It’s supposed to take in colder and more dense outside air than the factory intake. Well, if it really pulls in air from outside the engine compartment, it qualifies as a CAI, but whether that air is actually colder than the air the factory intake pulls in is debatable.

2) The larger cone filter is supposed to provide less airflow restriction than a factory filter due to its larger surface area.

But what does less airflow restriction really mean? The amount of air a cylinder receives during the intake stroke is determined by

• Cylinder volume, which is fixed.

• Manifold absolute pressure—the difference between the suction created by the intake stroke and the pushing effect of barometric pressure. Intake stroke suction is fixed and barometric pressure is the same for both a factory and aftermarket intake. You can’t change barometric pressure.

• The restriction created by the air filter, ducting and throttle plate and throttle body.

The biggest assumption made by aftermarket CAI’s is that the factory air filter is more restrictive than a CAI cone filter. Let’s assume for a second that the CAI manufacturers are correct. Here’s a scenario; a CAI air filler allows a cylinder to pull in a full .5-liter of air and the more restrictive factory only allows it to pull in .489-liters – in other words, less than a full air charge. What’s the difference in performance?

The air density is the same in both cases because both intakes are pulling in air from outside the engine compartment. So only the volume changes. But the MAF will see measure .011-liter difference and provide exactly the right amount of fuel for the .489-liter fill. On the CAI system, the MAF will provide exactly the right amount of fuel for a .5-liter fill. You could draw the conclusion that since the CAI equipped engine is getting it’s full fill, that it’s producing more power. Great theory. Unfortunately, it doesn’t work that way in real life. If it did, the HP improvement curves on CAI systems would show improvement at all RPM ranges instead of just at full open throttle and 5,00+ RPMS. They don’t. Here’s why

1) A clean factory air filter doesn’t restrict airflow enough to degrade performance.

2) Factory intake ducts are tuned to reduce turbulence and noise and increase airflow

3) Aftermarket CAI systems aren’t tuned for airflow efficiency. They’re tuned to produce turbulence specifically to produce a throaty rumble that sounds like you’re getting more power. That turbulence interferes with the laminar airflow the MAF expects, so it misreads the incoming air mass.

2) The largest restriction in the entire intake assembly is the throttle plate, not the air filter. Read that again, because it’s REALLY important: The largest restriction in the entire intake assembly is the throttle plate, not the air filter.

Here’s a snippet from a recent article by Gary Goms, Technical Contributor

Import Car Magazine

“Modern air filtration systems combine a small cross-sectional area in the air inlet with a larger cross-sectional area in the air filter housing. The difference in cross-sectional area decreases the velocity of intake air as it enters the air filter housing. This decrease in air velocity causes air-borne debris to fall out of the air to the bottom of the housing. [Note, most aftermarket cold air intakes are a cone filter and don’t have this housing]. This change in air velocity increases the life of the air filter and tends to dampen the throaty resonance of air rushing into the engine. Last, and most important, this reduction in air velocity helps increase the accuracy of the MAF sensor by removing turbulence from the incoming air. [remember this part]

Unfortunately, many “tuner” or “hot-rod” technicians who replace OE systems with performance aftermarket systems might not understand that modern MAF sensors basically measure the volume and density of the incoming air by measuring the change in current flowing through a resistor or wire as it is cooled by a smooth, non-turbulent flow of incoming air. This operating principle provides the MAF sensor with the ability to precisely measure air flow in grams-per-second increments. It’s important to know that dirty MAF-sensing elements miscalculate the amount of air flowing into the engine, which results in an erratic air/fuel ratio calculation and a subsequently erroneous calculated engine load or calculated barometric pressure data value on a scan tool. In most cases, miscalculations caused by dirt and air turbulence actually decrease, rather than increase, horsepower and fuel economy.

Of course, the popular perception is that increasing air filter capacity will increase an engine’s horsepower and fuel economy. The reality is that, because an engine is an air pump, increasing the air flow beyond the capacity of the engine’s peak demand will do nothing to enhance performance. In practically all cases, the OE air filtration system is, by far, the most efficient and the least maintenance-intensive system on the market simply because its design matches the engine’s original intake air flow requirements.”

Here’s the proof  

Let’s look at the power ratio curve supplied by one manufacturer. I will not identify the manufacturer, but their actual dyno test speaks volumes about the true effectiveness (or worthlessness) of their system. This power curve is for a 1998 Chevrolet Corvette. The sales literature brags about an 18 horsepower increase. But if you look at the graph, you’ll see that they don’t achieve that increase until the engine is screaming at 5,500 RPM.

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How often do you drive at 5,500 RPM? Let’s get real here, look at the graph at 4,000 RPM. See much of an increase in HP? It’s miniscule. If the CAI were that much more efficient than the factory system, we should see an increase in HP at LESS than full throttle opening. In other words, unless you’re planning on spending your days at the racetrack, you will NEVER SEE that increase in horsepower.

So the cold air intake manufacturers are really playing words games with you when they imply that a larger pipe and filter automatically means you’ll get more air. You can install a 20” diameter pipe and an air filter with 4,000 sq/inches of filter media, but if the throttle bore is 3” and the throttle plate is only ¼ open, that extra capacity is worthless. Think back to the power curve above. Remember that the manufacturer claims an increase of HP at or near red line? Why there? Well, the throttle plate is fully open at that point. As soon as close the throttle plate, you drop airflow. So the less restrictive air filter and ducting doesn’t really offer any benefit at all at partial throttle.

Which leaves us with tuning the intake tube. Car makers never cared about tuning the intake air duct for carbureted engines because the air fuel mixture was performed in the carburetor. Cold air intake manufacturers imply that modern factory intake systems are just as poorly designed. They’re wrong. Car makers actually put TREMENDOUS effort into fine tuning the intakes on late model cars. In fact, car makers do such a good job of tuning the air ducts that tests now show that aftermarket cold air intakes actually reduce performance by creating turbulence prior to the MAF and before the throttle plate

Finally, let’s talk about fuel economy. You cannot get more horsepower from a cold air intake system and still get better fuel economy. No stock engine is designed for maximum fuel economy at 5,500 RPM, period! Remember, if the MAF sees denser air, it will add more fuel. There is simply NO FREE LUNCH with a cold air intake.
Read this updated post on cold air intake systems

Don’t believe any of this? Watch this demo on youtube and see actual dyno tests
© 2012 Rick Muscoplat

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Posted on by Rick Muscoplat



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