Here is a guide of how to gain speed in your Acura RSX and what each part does.
One of the first things that most people do to their cars are their basic bolt-ons. I will list below what is useful and what affects your speed. For further information, you may visit the Clubrsx forums to get assistance from the members of the community.
The "Cold Air Intake" is one of the first mods most people get. This is healthier for your engine because it sucks in cool air into your engine and causing it to be about 7 horsepower faster. It is very close to the ground, so you should avoid puddles, because there is a chance that your intake will suck up water from the ground instead of just the air (although the water would have to be a foot high for that to really happen). Avoid Short Ram Intakes, they actually make your car slower in the hot weather and has less performance gains as compared to the Cold Air Intake anyway.
Recommended brand: Injen
The second would be catless race headers. These will give you up to 20 horsepower to the wheel. You should get the ceramic coated ones, because the stainless steel headers tend to overheat and does not give as good performance. These are connected to the exhaust. So if you are a base model who would like to use a type S race header, you would have to get a Type S exhaust system.
Recommended brands: DC, Buddyclub
Third step is exhaust. It gives a small gain in horsepower, not significant enough. But it is extremely appealing and gives the car a nice aggressive sound. Try to get an exhaust system with bigger piping for the midpipes (the bigger the piping, the better).
Recommended brands: Greddy, Buddyclub, HKS.
Weight reduction is extremely important although most people do not think that. There are many ways to reduce weight, but here are the most reasonable ways to do so without having to strip everything. Get fiber glass / carbon fiber body parts for the car. They drop a lot of weight. Empty your trunk and kick out the passengers.
Make sure your rims aren't gigantic, because if they are, it slows your car down as well. 17" - 19" are fine.
If you have extra money, you can change other small things such as pulleys, intake manifold, etc... You will be able to find information on these miscellaneous mods on clubrsx.
K-PRO: This is an ECU mod that is available for every RSX except for the 05-06 base model. It is very effective and has seen huge gains in performance. The only problem is that it is very expensive. It basically pushes your car to it's maximum potential by editing the map settings within the car's computer system.
Last but not least, if you have the money, you can turbocharge your car. These are quite expensive including the labor, and is bad for your engine overall. You can actually push your RSX to 400-500 horse power this way.
Recommended brand: Greddy
