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Import Car Performance Building - The Basics Of Fuel Pressure | A Automotive Article
Many Sport Compact Modifiers start out with simply finding the right information. If that's what you're looking for, you're in the holy land.
Most imports are simply classified as being built overseas,[link widoczny dla zalogowanych], however in the industry 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 every available bit of power from their car, and for some of the underinformed,[link widoczny dla zalogowanych], this also means sacrificing engine longevity and dependability (Seriously, what good is an extra 5whp when it fails 30k miles earlier).
Import tuning is no different then the hot rod building of the old days. Small lightweight autos with monstrous power plants squeezed into them. Getting the power/weight ratio as high as possible and still maintain reliability.
Import tuning's most fundamental tweak hasn't changed. Fuel pressure. Simply raising or lowering fuel pressure at the fuel rail has a dramatic effect on air/fuel ratios, as well as Short term and Long term fuel trims. Changing your fuel system to support modifications is very important. Most items like a V-AFC or an e-manage simply increase Mass Air Flow and Manifold Absolute pressure sensor values going into the computer to increase or decrease fuel injector duty. While this works in practice, if you're adding 5% at least at all points 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 designed.
But back to the fuel pressure. As the most important and fundamental import tuning point on the car, it is very important to get this correct before any other adjustments are made. Various information informs us how Fuel injector flow is based off of a standard value of a standard viscosity fluid. Most manufacturers rate them differently, but the most used rating for the aftermarket is 43psi, while some vehicle distributors test them much higher. The reason this is
crucial is because the fuel pressure behind the fuel injector itself will multiply it's basic output. A 450cc injector sitting at 37psi at the rail is putting out far less fuel then a 450cc injector sitting at 55psi at the rail.
Drive your car for a bit, and then, look at your total air/fuel value for the extent of your test. If you've changed anything to increase the volumetric efficiency of the engine, the engine will be starving for fuel (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 fix that. At normal driving speeds,[link widoczny dla zalogowanych], the air/fuel should always be 14. 7: 1, an all-motor car under full load 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 vehicle 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,[link widoczny dla zalogowanych], decrease fuel pressure.
Some sport compacts will not show a difference, 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 ideally the fuel pressure should be adjusted so that they are all +/- 5% so that the maps are open to the vehicle 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 see where they are
resting. If the auto is adding a lot of fuel trim (increasing the numbers) then increasing a few more psi of fuel pressure (generally 1-2psi and recheck) can immensely reduce that number. Likewise, if you see it pulling fuel trim, decreasing the fuel pressure will correct this. This is very important to allow the car it's full range in making adjustments for such things as weather.
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