I'm planning a modification to my 1998 Toyota Corolla, replacing the stock air intake with something along the lines of forced air induction via a cone air filter. Air will be drawn in to the car with a scoop situated on the bonnet near the windscreen (where the air is at it's highest pressure), brought in to the engine bay along a pipe and in to the air cone filter.

I know that air scoops that sit on the bonnet don't tend to feed air in to air systems like I'm intending to do. They're either cooling down intercoolers or feeding air through to turbos.

What I don't understand though is that, given the position of the air scoop on the body, how do manufacturers deal with water ingress when it's raining? Is it a case that, unless you're driving in near typhoon type of wet weather conditions water ingress doesn't actually matter too much? Or it is a concern and they have a special way to deal with the water?

  • The filter gets wet. Usually not enough to be a problem. Or, some systems aren't sealed completely, so there's room for water to get out. The real question is why you would do this? You aren't going to see an improvement in performance, and as you suspect, you may run into troubles.
    – cory
    May 9, 2017 at 15:43
  • It's not so much for performance I'm doing it, it might improve it or it might not. There's an improvement to the sound by using the cone and forced air, that's what I'm after. I haven't cut anything out yet, this is all very much done on the back of a fag packet at the moment. May 9, 2017 at 15:46

5 Answers 5


The way manufacturers get around the problem of the wet is usually through creating a dead zone where the water will fall out of suspension, then the water is collected and allowed to drain through a tube out onto the ground. If the filter gets a little wet, there's no real issue. A little water getting in through the intake is not going to create an issue. Your engine can ingest and "burn" (used tongue-in-cheek) a little water without causing issue one.

To create your own "dead zone", ensure you have a box around your cone filter (if that's what you're using). The box provides plenty of space where the air can slow down for a second. This will allow for the water to drop out of suspension as I've suggested.


Water intake from rain will be minimal, unless it is a monsoon.

The water will hit the back of the ducting and end up on the sides of the ducting. Swirling air will push some of the water along the ducting. Assuming you are using an air box where the filter is at, some pooling of the water may occur at the bottom since water is so heavy.

If are not using an air box and you think there is a chance of issue with rain water, you can create a "catch box" inline with the duct. These are just a box in the duct that allows the water to get out of the air stream. They typically have a small drain hole.

Any water vapor sucked into the engine will be converted to steam and exhausted thru the combustion process.


enter image description here This is an exhaust component used in jetskis referred to as a water box. It's essentially the exact design step you're moving towards. You could find plastic versions that would likely retain less heat. They come in a wide variety of shapes and have been made for decades so I'm sure you could find one cheap, but I'm not sure that's your best option. There are several issues with piping air from that location and method; it's moving directly over all the heat from the engine, the air is taking numerous turns and being deflected and agitated, and a box like this will require at least somewhat blocking off the standard sources of additional airflow to your intake. Chances are that the cooling effect of the scoop alone will net greater advantages than any possible design you create to channel air directly to the intake. If you look at comprehensive tests of simple cone filters or cold air piping parts you'll find drastic differences caused by things as simple as coin sized cone tips, or single degree variations in a pipes bend that cause interruptions in airflow. Check out some of the air intake debates about the suburu wrx top mounted intake that sits directly inside the base of a hood scoop. They've spent more time in research and development with benefits like wind tunnel testing then you and I could ever do in a lifetime and the results are far from paradigm shifting intake technology. Maybe something like a ground spoiler or carbing skid plate to direct air up into your engine bay and out a rear facing hood scoop would strip some hot air and allow you to do some more effective experimenting with direct air from a scoop, but I think your best results will be all the way to the side of the engine bay or better yet through the wheel well, and/or next to a headlight


Long tuned intake on a high lift dual overhead cam 2.0L zetec Focus. The large K&N cone filter at the entry to the induction system sits behind a large funnel port on the driver's side front edge of the bumper. Recently even with a hydrophobic sock over the filter the high revving engine sucked in enough rainwater to nearly stall the engine. I assume the steam may have interutted the spark and fuel air combustion. While likely steam cleaning the combustion chamber and exhaust the next afternoon the engine began to stumble and lose power. Pulled the plugs and all looked great in the combustion chamber side but went ahead and upgraded to an MSD coil, MSD plug wires and change to some autolite racing copper core spark plugs(another story about the increase in voltage and the ignition flame kernel size over the previous iridium plugs, etc..). Turned out despite the smoother RPM sweep and idle it was still missing at same 2k-3k rpm under load. Turned out the mass air flow sensor (MAF)placed in the last leg of the long intake runner just before the throttle body and intake manifold had been compromised by the water ingestion. I replaced the MAF and for good measure will be replacing the original idle air control valve that had given issues about 11K miles earlier which were resolved by cleaning it and reinstalling. The point is water ingestion even for a few seconds while won't likely hurt your reciprocating parts of your engine might play havoc with sensors and even light corrosion that could lead to problems over time. While this induction system was designed by a tuner facility, they are located in southern California and those cars likely don't see the weather this car does. Time to reengineer a little to keep from getting too much rain sucked up. The low front intake, then long runner does make real power with this engine, exhaust and high lift cams.


There are benefits to forced air induction, gas mileage, horse power. The forced air, forces gas into the cylinders much faster, and all that is available at the time. Thus, increasing performance. As far as moister, you create a low area, for moister to escape by drilling holes in induction tubes at lowest point, we see that turbos and superchargers increase hp 100% ram air increases as the car speeds up, as the rpm increases. Not being sucked in. But forced. Ram air should come from under the front of the car. Not the hood. There you will have moister problems in a heavy rain. Also this will allow a low area to drain moister as the forced air by drilling holes but not big enough to loose air. I'm a big believer in ramair

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