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Modification Number One: Index Your Plugs
The next time you are doing a tune-up on your ride, be sure to
take note of this modification. By simply installing your plugs
in a particular way, you will pick up roughly 1 whp, dyno
proven time and time again.
In order to perform this mod, mark the side
on which the "open end" of the spark plug faces. Proceed to install
them as you normally would, except when the plug is hand-tight
(that is, you've screwed it in without a ratchet as far as it'll go), use
your ratchet to tighten the plug until the open end of the plug,
faces the intake side of your cylinder head. That's it!

Modification Number Two: Insulate Fuel Lines
Under-hood engine heat is a serious performance robber in almost
all cars. Not only does the under-hood heat cause performance
loss by heating up the intake air, but it also causes
performance loss by heating up the fuel lines. Cooler fuel will
help cool the intake charge, as well as provide for a better
overall mixture.
In order to get this stolen power back, simply go down to your
local hardware store and purchase a roll of refrigerator and
air conditioning insulation. Wrap all of your under-hood fuel
lines with the insulation to keep the cool in, and the hot out.

Modification Number Three: Relocate IAT Sensor
In most fuel injected vehicles you will find a sensor that
measures intake air temperature. You will need to refer to your
factory service manual to find its location.
At any rate, it is very typical to find the IAT sensor mounted
inside the intake plenum that is very often heated heavily by
coolant and the cylinder head. This is fine of course, since
the original equipment manufacturer designed it to work this
way. However, there is a way to "trick" the engine computer
into thinking that the incoming air is a little cooler than it
really is, and therefore get the ECU to advance the timing a
small amount and increase fuel supply at the same time.
In most mildly modified vehicles, this will create a more
desirable fuel and ignition map and create a few extra
horsepower.
In order to perform this mod, simply locate the sensor and
remove it from the intake manifold. Fill in the hole with JB
weld and proceed to remount the sensor somewhere in the intake
arm. Seal everything up well, and you're done.

Modification Number Four: Synthetic Oil
In the past 3 years I have been running various types of oils
through my engines and have found that for the most part -
all oils are the same, power wise. However, Synthetic oils
definitely make more power over standard "dino" oils. In fact,
I have consistently seen anywhere from 2-3hp across the rev
range from using a true synthetic versus a standard oil.
Therefore, next time you change oils, switch to synthetic.
Even on a high mileage engine, it works wonders. It is also a
far better lubricant and protector of your engine.

Modification Number Five: Increase H2O Ratio In Coolant
If it weren't for corrosion and freezing concerns, automotive makers
would use pure water to cool your vehicle. Straight water cools
better than coolant any day of the week, and a cooler engine is
always going to produce more horsepower.
Instead of a typical half and half ratio, try 40/60. However,
NEVER use pure water, as this may cause premature corrosion which
will cause a costly repair bill. You may also try water wetter
to further enhance the effect, but be cautioned that water wetter
should only be used for track purposes.

Modification Number Six: Throttle Body Coolant Bypass
With this tip you will usually findalmost 10ft-lbs of torque over
the entire rev range. 10ft-lbs that was present in the engine
when completely cool, but once at operating temperature was
no longer available. This is good for about 0.2 seconds in
the quarter mile.All you need to do on any vehicle is to take twocoolant lines, disconnect them from the throttle body and
connect them with a coupler available in the vacuum hose
section of your auto parts store. It's basically two nipples
connected to each other that allow you to connect two pieces
of coolant hose.
If you have problems with erratic idle afterwards. You will
need to find your fast idle (or idle air control valve),
and disable it by blocking it off or somehow keeping it closed.
This occurs mostly in Hondas to my knowledge, and it's pretty
hard to explain in a "universal" manor.

P.S. The obvious ones:Shed some weight ... take out the seats and what not. 100 lbs = 10hp = .10 off your 1/4. Not really adding HP, but you'll be faster. Alsoyou can save some weight by running with a 1/4 tank or less. 16 gallons of fuel weighs about 130 lbs. Maybe drill some holes in your airbox.

That's about all the mods I can think of that you might be able to do for free or very little money.Hope this helps!