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.
Steven
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