AEM’s patented Air Bypass Valve virtually eliminates the chance of hydro-lock in the event the filter of your AEM Cold Air induction system becomes submerged in water. On most systems, the valve installs along the upper portion of the inlet pipe and shuts down induction at the filter when it becomes submerged, rerouting air through its external diaphragm to keep water out and air flowing in. Check with your dealer or call AEM for more information on air bypass valve installation locations. The AEM Air Bypass Valve is C.A.R.B. exempt on AEM Cold Air induction systems only and is NOT for use on forced induction vehicles. Patent #6,394,128.

Virtually eliminates chance of intake system ingesting water if filter becomes submerged

Installs along upper portion of AEM intake systems with minor modifications (you will be required to cut your AEM Air Intake System with a hacksaw or bandsaw. Instructions are included).

Patent #6,394,128

Complete hardware and installation instructions included

If you have an AEM V2 Intake System, or your vehicle is turbocharged or supercharged you should NOT use this product.

Click the "Expert Review" tab below to read about the testing of this item that was done by Modified Magazine!

If you are not sure what the diameter of your air intake system is, you can either measure it with a ruler, or check the "APPLICATION GUIDE" tab above.

20-400 20-401 20-402 20-403 20-400S 20-401S 20-402S 20-403S

From Modified Magazine Author Dave Coleman:


It doesn't happen often, but it doesn't have to. One heavy rain, one deep puddle, and you can have one serious engine failure. It has been happening ever since cold-air intakes first started taking hold in the mid '90s. Slurp, BANG. End of engine.

Cold air makes more power--nobody can argue with thermodynamics--but cold air often comes at a risk. Most cold air intakes pull air from a filter placed very close to the ground, where air passing under the car can be used to feed the engine, rather than air that has already passed through the radiator and bounced off various hot engine parts. This location leaves the filter vulnerable to water.

Water is dangerous for one simple reason. It isn't compressible. An engine works simply as an air pump, drawing air in, compressing it, burning some fuel, and pumping it out. And it does so with a vengance. With the throttle closed and the engine idling, the cylinders are trying so hard to fill themselves with air that they pump the manifold down to a vacuum of over 20 inches of mercury. What's that mean? Quite simply, vacuum is measured in terms of how far up a tube that vacuum would suck a liquid. Mercury is an extremely dense liquid, so sucking it 20 inches up a tube takes a very strong vacuum. If you replace that mercury with water, that same vacuum would draw water over 22 feet straight up! What that means is, no matter how long your intake pipe is, if you stick the end of it under water, that water will get sucked into your engine.

OK, so what's the big deal? Think about what happens next. Say you have a 2.0-liter engine, so each cylinder displaces 500cc. If just one of those cylinders sucked in just 100cc of water (100cc is only 5 percent of what's in a 2-liter bottle, if you are having problems thinking metrically) the end would be very, very near.If this engine has 10.0:1 compression, for example, the combustion chamber would be reduced to just 55.5cc when the piston is at top dead center. As the piston goes up the compression stroke with 100cc of water in the cylinder, impending doom is near. Air will happily compress as the piston moves up, but water will not. As soon as the combustion chamber is reduced to 100cc, the piston will stop. No matter what. And then the engine will stop... if you are lucky.

Sometimes, if the engine is simply idling, the flywheel is relatively light, and the connecting rods, pistons, block, head, and head gasket are very strong, the engine will simply stop dead and can be revived by removing all the spark plugs and pumping out the water. Usually, however, the rotating assembly will have too much inertia and when the water tries to stop the piston, all hell breaks loose. Either the connecting rod buckles, or the piston breaks, or the cylinder wall cracks, or the head gasket blows, or the cylinder head gets lifted off the block, or any combination of catastrophic failures will occur. The only thing guaranteed is that the water will not compress.

Most cold air intake manufacturers, AEM included, have offered the following solution in the past: When it rains, simply remove the lower section of the intake, remove the air filter, and attach the filter to the upper section of the intake where it can act as a conventional underhood intake until the rain stops.

Show me one person willing to do this every time it rains and I'll show you somebody who just missed the prom. The real solution is to avoid puddles like the plague and never drive fast in the rain.

This is why we have traditionally been timid about using cold-air intakes on our project cars. We have done it many times, since the power benefits are indisputable, but it always makes us nervous. AEM has just released an air-bypass valve that calms our nerves considerably.

Surprisingly simple in construction, but quite sophisticated in design, the AEM air-bypass valve sits upstream of the air filter and normally does nothing. If the filter gets submerged in water, however, there will be a slight vacuum in the pipe as the engine tries to suck the water up the pipe. This slight vacuum opens up 12 rubber flaps in the air-bypass valve, allowing the engine to breathe air through the bypass valve's small foam filter. When the water level drops, the vacuum goes away and the main filter supplies air again. No panic, no blown engine. How nice.

When we heard about the bypass valve, we immediately wanted to test it, but none of us were willing to volunteer our cars. So we hatched an idea. John Concialdi designed the bypass valve, so let's use his car. We asked (knowing that his car is an NSX), and to our surprise, he agreed without hesitation.

To be clear, AEM doesn't make an intake with an air-bypass valve for an NSX, and, in fact, the valve isn't considered big enough to supply air for such a large engine, but Concialdi was confident that there was enough headroom in his design to allow his car to survive unscathed.

We cooked up a test that would replicate the absolute worst case senario: an underwater drag race. Dipping your filter into a puddle while idling into a driveway is one thing, but fully submerging it and then opening up the throttle and making full power is something else entirely. And if that full power happens to be 250 hp at the wheels, well... it doesn't get much worse than that.

AEM constructed a custom-made intake that came out of the engine compartment, went over the fender and down to the ground next to the rear wheel. We put the NSX on the dyno at R&D Dyno service with this intake in place and put the filter in a fish tank full of water.

To our delight, the dyno pull was quite uneventful. AEM was thoughtful enough to make the first few feet of intake piping from clear plastic, allowing us to see what happened with the water. At first, the water rose just a few inches up the pipe, but when the big VTEC cam engaged and the engine's appetite for air increased, water was sucked approximately 18 inches up the pipe. Still, no water reached the engine.

Next, we had Concialdi do a third-gear dyno pull while we held the filter above the fish tank and dunked it randomly in, and out of, the water. This was intended to simulate... well, an absolute idiot trying desperately to destroy their engine. Dropping the filter in the water caused the water to rise in the pipe again, but again, nothing happened. Pulling the filter out of the water with the engine still pulling hard, however, caused something rather alarming to happen. When the filter was pulled out of the water, air immediately started going through the filter, even though there was still about a foot of water in the pipe. The trapped water frothed and thrashed about in a most alarming way, raising the eyebrows on even Concialdi's staid face, but he kept his foot down and still nothing happened.

The moral of the story? Use an AEM air-bypass valve (They can be purchased separately and inserted into existing cold air intakes if you wish.) and when you pull out of the puddle, don't start racing until you have given the water enough time to drain out of the bottom of your intake.