Many Sport Compact Tuners begin with simply finding the right information. If that's what you're interested in, you're in the correct spot.

Most imports are simply classified as being built overseas, however in the field I work in, imports are defined as not only shipped in, but also the sport compact vehicles of Domestic manufacturers, with examples like Dodge SRT4's, Chevy Cobalt and Pontiac Solstice.

Tuning for many beginners is trying to gather each available bit of HP from a car, and for some of the underinformed, this also means limiting engine longevity and reliability (Seriously, what good is an extra 5whp when it breaks 60k miles sooner).

Import tuning is no different then the hot rod building of the old days. Tiny lightweight cars with big motors stuffed into them. Getting the power/weight ratio as high as possible and still be dependable.

Import tuning's most fundamental tweak hasn't changed. Fuel pressure. Simply raising or decreasing fuel pressure at the fuel rail has a dramatic effect on air/fuel ratios, as well as STFT and Long term fuel tables. Enlarging your fuel system to support changes is very important. Most items like a S-AFC Neo or a D-tec simply change Mass Air Flow and Manifold Absolute pressure sensor readings going into the ECM to increase or lower fuel injector duty. While this works in practice, if you're adding 5% across the board to add fuel, leaving that at 0 and simply raising fuel pressure is a much safer way to do things, and lets the ECM work how it was originally intended.

Let's get back to the fuel pressure. As the most basic and fundamental sport compact tuning point on the car, it is vital to get this right before any other adjustments are made. Research informs us how Fuel injector output is derived off of a fixed pressure value of a standard viscosity fluid. Most companies rate them differently, but the most accepted rating for the aftermarket is 43psi, while some vehicle manufacturers test them much higher. The reason this is

important is because the fuel pressure behind the fuel injector itself will increase it's basic output. A 450cc injector supplied at 37psi at the fuel rail is putting out far less fuel then a 450cc injector supplied at 55psi at the fuel rail.

After doing a few runs with your car, look at your total air/fuel value for the length of your test. If you've modified anything to increase the VE (volumetric efficiency of the motor, the powerplant will be leaner (Air/fuel ratio is higher then before) then when you began. If there is a noticeable deficiency overall in the ratio, then simple fuel pressure adjustments can remedy that. At cruising speeds, the air/fuel should always be 14.7:1, an all-motor car under full acceleration should be about 13.5:1, and a forced induction car under full load should aim for 11.2-11.9:1. The higher the number, the leaner (less fuel) the car is. To richen the car to make the the ideal points, increase fuel pressure, and if you need to lean the car out to lower the air/fuel ratio, lower fuel pressure.

Some autos will not show a change, but need further tuning via the short term and long term fuel trims. Many sport compacts these days will let these values go +/- 25%, but realistically the fuel pressure should be adjusted so that they are all +/- 5% so that the maps are open to the car if it needs adjustment based on weather conditions. Drive the car a few more times with an obd-2 datalogger on board. Look at the fuel trim values, and note their values. If the auto is adding a lot of fuel trim (increasing the numbers) then adding a few more psi of fuel pressure (generally 1-2psi and recheck) can greatly lower that number. Likewise, if you see it pulling fuel trim, decreasing the fuel pressure will off-set this. This is very crucial to allow the car it's full range in making adjustments for such things as air density.