Facts Or Fiction
There is a
very common belief that getting rid of all the restriction from the air box is
about the best thing you can do.
Wow! Really. Just think of all the
horsepower, and it's practically free.
Isn't it strange then, that all the manufacturers build model after
model, year after year with those restrictive air boxes? Don't they read the same magazines as we
do? If they did, they would certainly
learn about all of the ways to get rid of all that nasty old restriction,
right?
Maybe
not. Let's look at the physics of how
fuel enters your intake system. The
atmospheric pressure exerts force on the fuel in your float bowl, forcing it
through your jetting circuits, into the air stream passing through your
carburetor. In order for this process to
work, the internal pressure in your carburetor must be lower than
atmospheric. Pretty simple. If you have removed your air box, cut a
number of holes in it or are running without a lid, you may be in for a
surprise. Your internal pressure is now
nearly the same as atmospheric, making fuel delivery difficult.
Engineers call the difference in pressure a pressure drop
differential. A particular amount of
restriction (some call it vacuum) provided by your air box keeps the internal
pressure lower than atmospheric, insuring consistent fuel delivery over a wide
range of temperatures and altitude.
Without an acceptable pressure drop differential, the engine will be
fussy about changes in jetting to maintain performance.
Another aspect that affects pressure drop is air speed
(velocity). Simply put, the higher the
air velocity in the carburetor, the lower the internal pressure. When you attempt to operate with a wide open
air box or with the lid off, the air velocity is actually slower (yes slower)
due to the enlargement of the intake opening.
You now have plenty of air; however you also have poor air velocity. Velocity is needed to atomize the fuel and
produce quick throttle response. This
slower air speed promotes higher internal pressures, closer to atmospheric,
hindering fuel delivery.
Yes, it is possible to get an increase in performance by
running with an open air box or even removing the air box. This may be OK if you always ride at the same
temperature and altitude, say Southern California
for example. However, if you are dealing
with temperature and altitude changes, expect to lose horsepower and
performance unless you are a jetting whiz.
The same jetting set up is not right for everyone or everywhere. Temperature, altitude, air density,
atmospheric pressure, fuel density, engine pressure drop and humidity are all
part of the total equation for air/fuel delivery. This is why it is important to have
adjustability of air as well as fuel.
Is the factory air box restrictive? Yes, it is usually somewhat restrictive. To insure optimum performance, it is
necessary to arrive at the correct amount of pressure drop (restriction) that
works for your riding requirements. This
always requires some experimentation.
Rather than drilling holes, cutting holes, and taping or gluing holes
shut, there is an easier and more scientific method.
There is a product called a TPI Valve (Total
Performance Intake), search google about it.
The
TPI Valve looks like a small picture frame with a slide door that opens or
closes the frame.
There is an air filter built into the TPI Valve.
You cut approximately a 2" x 2"
hole in your air box, usually in the lid.
Install the TPI Valve.
Now you
can regulate the size of the opening, controlling the pressure drop and air
speed of your air box. This allows you
to quickly and easily determine the right amount of opening by trying a number
of different openings in a matter of minutes.
You may find your machine runs best at 1/4, 1/2, 3/4 or maybe full
open.
The amount of opening in your TPI
Valve will give you the equivalent of one to three main jet sizes depending on
how restrictive your air box is.
We call
this "air jetting".
The
correct amount of opening can be judged by the machine's performance. Good performance from zero to top end
indicates the proper amount of opening.
You can also return to a totally stock air box by closing the TPI
Valve.
Before you run out and spend all of your hard earned money on some super intake system that gets rid of all the restriction and claims to
make tons of horsepower, you may want to ask a few questions.
Note: I did'nt bought a TPI valve, i actually replicate it to see if it works. Well I guess it did. After all these years, my 10 year old 125cc commuter bike still runs at top condition. With that old, who will tell that my bike is a junk.
Credits to the owner of this article, I cannot find the original, it has been years i had known this trick.
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