Some aren't free, but really cheap.
Simply disconnect the two heater hoses from the TB. There is one on the passenger side and one underneath. Then use a small piece of metal tubing to connect the two together or buy one long piece to use. This mod is supposed to keep the TB cooler becuase it does not have the hot water running through it. Keeping it cooler keeps the incoming air cooler which results in more power. This mod should not cost a thing unless you buy new heater hose ($3) or need to buy a small piece of metal tubing and a hose clamp($1). I used a piece of tubing off of my smog equipment that I had already removed. I would say two minutes to do this.
Intake Air Temperature Mod -
This can be done a few different ways. The idea is to give the computer a cooler air reading so that it will richen the fuel mixture. Some people relocate the IAT, or MAT, sensor to the air ducting in front of the throttle body. This can be done in two ways. First you can reomove the sensor from under the plenum, plug the hole, lengthen the wires, and mount the sensor in the intake tubing. Second, you can buy the relocation kit ($45). The easiest way to do this mod is to disconnect the sensor all together and run a 4.7k ohm resistor ($0.49 for a pack of 5) in-line. Take the resistor and stick each of the two wires on each end into the two holes on the plug, then tape it all up. Wiring in this resistor should make the computer see a temperature of 57 degrees all the time. No more than two minutes to get this one done. The IAT (intake air temperature) and MAT (manifold air temperature) are the same thing.
Fan Switch -
This will give you control to cut your electric fan on at the flip of a switch while retaining complete computer controll also. Connect a wire to the fan switch sensor in the passenger side head, run this to a switch, run other side of switch to a ground, mount switch in desired location. A little wire and a switch should cost about $3. This might take fifteen minutes to accomplish.
160 Degree Thermostat -
Cars come from the factory with 195 degree thermostats. This is good for emissions, but not for horsepower. Switching to a lower temperature thermostat keeps the car cooler resulting in more horsepower. A thermostat costs about $3 and another $1 for a gasket. Remove the upper radiator hose, unbolt the waterneck, and switch the thermostats. This should take about five minutes to do. Don't worry about open/closed loop. More than enough people have done this and used diagnostic equipment to prove that everything runs okay.
Bump Timing -
The more advanced your timing is, the more horsepower. You can only go so far though because of detonation (pinging.) There are many factors that play a role in this since the higher octane gas, cooler air, and other things can enable you to advance more timing. Some people advance their timing until they hear pinging and then back off a couple of degrees. Eight degrees is a good setting. Hook up timing light, loosen the distributor bolt, turn distributor for proper setting, tighten bolt. To get a proper timing reading there is a brown wire on the passenger side firewall that needs to be disconnected and a jumper will need to be installed in the diagnostic connector under the dash to bypass the computer so it will not change timing back to its original setting. This mod is absolutely free unless you have to buy a timing light ($20), but try and borrow one, and it should only take around five minutes.
TPS & IAC -
It is imperative that the TPS (throttle position sensor) and IAC (idle air control) must be at the proper settings. Usually over time with normal wear and tear these will become slightly off, but that can make a big difference. Some also say that increasing these two can improve perfomance by giving a lot quicker throttle response. I recommend setting the TPS at 0.6 volts at idle and setting the IAC at 900 rpms. This is totally free and should take five or six minutes.
MAF mods -
The MAF (mass air flow) sensor can be modded ina few different ways. First remove the screens on both ends. The easiest way is to start with a knife and cut around the outside, then use some pliers to pull out any leftover strands of metal. Second use a file or something to grind down the fins inside the MAF. Be extremely careful through all these mods not to damage the sensor part itself inside the MAF. Last, you can use a Dremel or something to port out the inside a little. All these mods result in better airflow which means more power. This mod is free. Fifteen minutes should be enough, but maybe longer depending on what kind of tools you use and how far you go with this.
Synthetic Fluids -
Synthetic fluids are definitely better than conventional. Synthetics are available for every time of fluid in your car (brake fluid, oil, transmission fluid, gear oil, etc.) They are little more costly than other stuff, but well worth it. Redline makes some very good products, like their Water Wetter and racing oil, that have been proven to work. Putting synthetic oil into an old car can cause small leaks and putting it in a new engine can stop proper ring seating. This is definitely not free, but switching fluids one at a time will not put any kind of a damper on anyone's pocket.
Custom Air Intake -
Depending on the type of your car this is done differently. On Camaros the lower portion of the air box below the filters is cut off for more air flow. On Firebirds there are different ways depending on the intake. On newer ones you can remove the bottom of the cylinder all together and just bolt down the lid on top of the filter and make it like an open element. On the older ones the top on the lid can be removed and the holes enlarged as wel as making holes in the bottom. Also the ugly box thingy should be removed and replaced with some 3" pvc pipe. Everything is free except for the pvc pipe to replace the box thingy. I have three parts to this on my '85 TA. There is a 3" 90 degree elbow ($5), a short piece of 3" pipe ($10 for a 5-foot section), and a rubber flex coupling ($5) to connect to the MAF sensor. I could send pics of this set up if you would like.
Ram/Cold Air Induction -
There are many ways to do this. The idea is to get more air and colder air into the engine for more power. Look around some and you will find a setup for your car. They can usually be made in under an hour for less than $15. I will post more on this later.
Adjustable Fuel Pressure Regulator -
The stock fule pressure regulator can easily be made adjustable. Remove it, drill a hole in the top, attach a nut somehow, and screw in a bolt. Make sure that the bolt is secured good and that the bolt has tape on it. This might cost $3 and take fifteen minutes. I could send pics of one I made if you would like.
Gutting Catalytic Converter -
This frees up a restriction in the exhaust. It is absolutely free and takes ten minutes. Just unbolt the catalytic converter, smash out the inside, and reinstall. Also you could just cut it out and replace it with a piece of straight pipe.
Removing Weight -
There are so many possibilities here. It all depends on how far you want to go. I have seen people gut the entire interior of their daily driven car just for a little more of an edge. For around town driving I leave out my spare tire, jack, and realted items. If I ever go out of town then I put them back in. I have also taken out all my smog equipment since there are no smog laws here in SC and no inspection. I also have removed my a/c condenser, but will be reinstallig it next summer. Fiberglass hoods are a good way to lose weight as wells as aluminum heads. It is all up to you.
Tune-Up -
Keeping your car in good condition with regular tune-ups is a necessity. Using premium parts in these tune-ups is a very good idea. An Accel cap & rotor is a good investment at only $20. Synthetic fluids, high octane gas, cooler properly working thermostat, and keeping the timing, TPS, and IAC at proper settings are all good ideas that I have previously discussed. A K&N air filter seems to be the most popular thing for everyone with any type of car. They retail for $35 or so and it is money well spent. Everyone should have a good set of spark plugs such as Rapidfire or Bosch Platinum. Also make sure not to neglect changing your fuel filter.
Porting -
This is where you will most likely see the most improvements, but takes the most time. Porting will only cost you a new set of gaskets if you have the tools, but is very time consuming. I worked at least 45 hours on porting my intake and heads. Porting a TPI intake is fairly simple. Start with the pleum by grinding down the egr walls right behind the throttle body. Then open up the two throttle body openings. Open up all the runner openings and the both ends of the runners. On the base, just get rid of as much metal as you can. Make sure to port-match and gasket-match everything. Pay special attention to the intake and exhaust ports on the heads. They are considerably smaller than the inatke ports on the base and the exhaust ports on your headers or manifolds. These ports definitely need to be matched for proper air flow. The throttle body can be ported also. Stock is 48mm and it can easily be ported out to 52mm. The hard part is making new butterflies to fit into the new 52mm holes. I will say again that this is very time consuming and requires a lot of patience. I could send pics of some porting I have done if you would like.