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Air-box modification and coasting with the gearbox in neutral

Our tech guru has the answers

Hi, Ferman. I drive an automatic. Lately, I've been having this habit of putting the car in neutral when going downhill and while the car is stopped. I thought it might be able to save gas, but I read on some forums that this might be harmful to the car. Is it safe to put an automatic car in neutral when coasting?

Joseph Amurao


Hello, Joseph. It's actually more potentially harmful to the driver and the car's occupants than to the car itself! It's not safe to be in neutral and coast in general because when you do this, you're giving up a good degree of car control.

When you're in neutral, you can't accelerate or decelerate as well as you should since your engine is disconnected from your wheels. There's at least a one- to three-second delay from the time you see something that prompts you to engage your transmission, to the time the car will start responding. If you're going at say 100kph, that three-second delay is a pretty long distance to go without control of the car. And when you find yourself needing to suddenly engage your transmission in a corner or on a curve, you'll find that you may end up facing the wrong side of the road real quick. This is because the engagement will more than likely be abrupt--abrupt enough to cause your car to become out-of-balance, resulting in unpredictable handling characteristics (either the tail will swing around or you'll go in an unwanted direction).


Moreover, you'll also give your brakes a harder time. There is a risk of overheating the brake pads or boiling the brake fluid. When either of these happens, you will lose your brakes, and I can tell you with certainty that it's not something you or anyone else wants to happen.

If you drive a modern car that's built after a certain date, it would be more fuel-efficient to keep it in drive when you are coasting. Most modern cars will actually shut off the fuel injectors when you're off the gas pedal and the engine is above idle rpm speed. This is done to save fuel and lower emissions level. I don't know when exactly each manufacturer started doing this, but on some Japanese makes it happened as early as their 2003 models.


I read your article about modifying the car air box by cutting open the sides facing the lower half of the air box. I'd appreciate it very much if you could send me an illustration of how to do it, especially the size of the hole to be cut.

Jonie A. Uy 

Hi, Jonie. Just to avoid confusion, by lower half I meant the section of the air box that's on the "dirty air" side of the air filter and not the "clean air" or engine side of it. If I were to cut open the factory air box, I would cut away either the side that's farthest from the engine or the side or bottom from where the factory air duct comes from. This may also mean that you might have to remove the water trap and (sometimes) the resonator section of the air intake system.

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The size of the area you cut out isn't that critical if your alternative is installing a cone-type intake that has not been given proper R&D or dyno-tested. There's quite a bit of what I call generic "performance" intakes out there that don't really do much other than add cosmetic appeal to the engine bay--and more often than not, they'll be using air filters that have been made to look like K&N air filters. I've come across enough car owners who were misled into thinking that they would benefit from such air filters and ended up spending more than they should have.

A good starting point would be to enlarge the stock opening by about 10-15% in area. Anything bigger may or may not produce the desired result. Personally, I've had good results on some of my own cars with the entire side taken out, but not all vehicles will respond well to that, so err on the conservative side.

I normally start off by drilling out the perimeter of the area I want to cut out, and finish off with an angle grinder before sanding down the edges. Remember to avoid taking out the mounting points, or you will have to figure out how to keep your air box in place.

A final note: There will be cars that won't benefit from this kind of mod, so if you decide to do so, do it at your own risk.


Best regards,

Ferman Lao
Technical Editor


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