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Thrifty Man's Cold Air Intake (stock air box mod)

By Steven

[Rob Robinette adds: This is a great mod that keeps the stock air box and the baffling that keeps the air pump, pre-control and blow-off valves quiet. It also keeps your air pump happy with cool air. In a recent article for the SAE, Jack Yamaguchui describes how Mazda engineers blew an engine while testing and discovered that when the 3rd gen RX-7 is pushed hard it will actually suck hot air backwards through the intercooler. This is caused by the air box intake and intercooler intake merging into one intake duct. On the 99 RX-7 there are now two separate ducts. By doing Steven's mod described here you will prevent the reverse intercooler air flow from happening while still getting 100% cold air to the turbos.]
I've had several people ask me about my "custom cool air intake with K&N in stock air box". I finally decided to write up a good description. Here it goes.
I heard too many bad things on the list about the open air intakes sucking in hot engine air and didn't want to pay $400-$500 for a good cool air intake yet (no one could agree on the best one anyway), so I decided to make my own. You may have heard everyone on the list talking about the opening below the air box and next to the radiator. It's actually only about 1.5 inches wide, but maybe 7 inches long. There's a piece of "spongy stuff" closing this gap that you can just pull out to get to fresh cool air from below the car. I used a Dremmel power tool to cut 2 round 2" diameter holes (actually continuous, like a squashed number 8) in the bottom of the stock air box that is above the gap. I later found I could have done 3 and will do the third one later. Three 1.75" might work better. You want to cut the holes as far to the passenger side (American model) and forward as possible. Notice the coolant hose and location of gap when you pull out the air box. I used the same Dremmel tool (with a different head) to cut grooves in the air box to accommodate for the lips on the K&N filter that caused everyone so much grief. (It may seem easier, but don't cut off the lips on the K&N, or you'll get unfiltered air in when the filter pulls in from the massive suction at full boost.) I then put 2" aluminum tubing in the cut air box holes and bent them forward and squeezed (elongated) them to 1.5" to fit into the radiator gap. [Don't try to use rubber hose, it will collapse under vacuum.] You keep using the original intake setup, so this is just in addition to what you already had before while still being all cool air. Voila, Steven's custom cool air intake. What do you think?
[Rob Adds: You can seal the air box/tubing junction with silicone sealant and consider plugging any gaps around the tubes at the radiator gap]
To tell you the truth, my intake is already more opened up for 10 hp more than the U.S. model, so I really didn't feel much of a difference. I felt more when I put on my cat-back before. I bought a US version air box and cover from a list member (Spic Racer Carlos, thanks again) to do my mods on, so I could keep my original stock one unchanged. I noticed some of the differences between the US and Japan after minor change versions. The first intake area in front of the intercooler was switched from the passenger (US) side to the driver's side of the intercooler stay bar and was increased in width from 57mm to 90mm. Length remained the same at 88mm. And the intake area in front of the air box no longer has the rubber stuff on the cover that reduces the size of the hole. The cover now fits AROUND the tube coming from the air box. I would recommend cutting as much of the inside part of the rubber as you see fit, maybe even pulling all the rubber off. With the K&N (already a big filter) and the increased intake area of all cool air, I would guess this to be almost as good (if not just as good) as the expensive after-market intakes. Even buying the Dremmel tool for just this job, the total is somewhere around $100 and a few hours of labor, some of which you'd have to do installing the expensive ones anyway. Keeping the stock air box also hides all the grunts and groans from the air pump. It's also satisfying to know that you're definitely getting more and all cool air and by your own labor of love.
I also thought of getting a headlight cover with an intake opening and running a couple of hoses from the side of the air box to that area, but those run about 100 bucks (not thrifty anymore) and the difference might be negligible. They're a must have for people with open intakes though. I'm surprised an after-market company hasn't used them to make a better cool air intake system. Well, they ARE useless at night with the headlights up.

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