sebbs
06-29-2007, 06:19 AM
In case there are some newbies on this forum like me, that were wondering what exactly is engine PSI? Here is a good explanation I found on another website.
Down here at "sea-level" we are under the weight of a column of air 100-kilometers so high. That air, due to gravity, weighs down on us to the tune of about 14.7 pounds per square inch or "PSI". When we breath, our diaphram moves downward in our chest cavity increasing the volume in the thoracic cavity. The air around us, under a pressure of 14.7-PSI rushes in to fill the extra volume, filling our lungs. When we exhale, the diaphram moves back up, decreasing the volume and pushing the breath back out.
And engine is the same way. When the intake valve is open and the piston is moving down, air rushes in through the intake manifold - forced by the pressure of the atmosphere - into the cylinder. This engine - a so called "normally aspirated" engine because it's "breathing" air normally, without any assistance - relies on atmospheric pressure alone to force air into the cylinders. Engines are not perfect machines and air doesn't flow losslessly through the intake piping, manifolding, past the valves and so on. So, even at its best, the engine can likely only inhale about 85% of the air is really should be able to if there were no losses in the intake system and so on. That air is mixed with an amount of fuel (determined by the engine computer) which is then burned in the cylinder.
Forced induction uses an air pump to increase the pressure and volume of air available when the engine's intake valve is open and the piston is moving down. By increasing the pressure in the manifold, the cylinder "inhales" alot more air. If the engine control system is setup right, it will deliver more fuel to match that extra air. More fuel, more air means more expansive pressure and thus more power.
A turbo (a compressor driven by the exhaust gasses) or a supercharger (driven by a belt from the crankshaft) is a big air pump that can actually pressurize the manifold. In simplest terms, the higher the manifold pressure, the more air the engine gets and, if it gets the right amount of fuel, the more power the engine makes.
"PSI", in FI engine terminology, typically refers to the increase in manifold pressure, in pounds per square inch, over the standard atmosphere of 14.7-psi. So if an engine is boosted to "7-psi", it means the intake manifold would read 21.7psia (pounds per square inch absolute, referenced to a perfect vacuum) or 7-psig (pounds per square inch gauge, referenced to 1-atmosphere).
So if an engine is running 7-psi boost and the boost is increased to 12-psi, the engine will be getting something like 23% more air. If mixed with more fuel, this means more power. Further to this topic, if your engine is normally aspirated (i.e. no turbo, no super), your options for increasing performance are largely limited to reducing losses in the intake and exhaust systems and/or switching certain components (e.g. camshafts) to breathe better at different RPM ranges. By doing this, you increase the VE - volumetric efficiency which what that 85% value I was talking about above is all about. This increases the effective "PSI" seen at the open intake valve (that is, there are less losses throughout the system from the air intake to the valve port) and thus admits more air into the cylinder.
Not all turbo engines respond the same to boost increases. A manifold pressure increase of, say, 3-psig may not produce the same power increase on two different engines of similar sizes because, even though the manifolds are pressurized to the same value, the ability of the intake port and valve (including lift, seat angles and so on), valve timing and so on to actually "aspirate" the cylinder varies between engines. Manifold boost pressures therefore, do not tell the whole story.
Further still, a small turbo may be able to produce 12-psig of boost on an engine that doesn't breathe particularly well whereas that same turbo may only be able to move enough air to produce 7-psig on a well-breathing engine. And simply trying to increase the boost pressure by driving a turbo "harder" (e.g. wastegate settings) will usually, depending on the turbo, result more in heating of the air than moving more of it.
It's a pretty complex system with many variables. Compressors must be sized properly (to move the right amount of air at the desired pressure and not be laggy) and, if the system is modified to produce more boost, steps should be taken to ensure the compressor is still operating with the bounds of its performance map.
So now we know why hackish makes the big bucks! ahah.
Down here at "sea-level" we are under the weight of a column of air 100-kilometers so high. That air, due to gravity, weighs down on us to the tune of about 14.7 pounds per square inch or "PSI". When we breath, our diaphram moves downward in our chest cavity increasing the volume in the thoracic cavity. The air around us, under a pressure of 14.7-PSI rushes in to fill the extra volume, filling our lungs. When we exhale, the diaphram moves back up, decreasing the volume and pushing the breath back out.
And engine is the same way. When the intake valve is open and the piston is moving down, air rushes in through the intake manifold - forced by the pressure of the atmosphere - into the cylinder. This engine - a so called "normally aspirated" engine because it's "breathing" air normally, without any assistance - relies on atmospheric pressure alone to force air into the cylinders. Engines are not perfect machines and air doesn't flow losslessly through the intake piping, manifolding, past the valves and so on. So, even at its best, the engine can likely only inhale about 85% of the air is really should be able to if there were no losses in the intake system and so on. That air is mixed with an amount of fuel (determined by the engine computer) which is then burned in the cylinder.
Forced induction uses an air pump to increase the pressure and volume of air available when the engine's intake valve is open and the piston is moving down. By increasing the pressure in the manifold, the cylinder "inhales" alot more air. If the engine control system is setup right, it will deliver more fuel to match that extra air. More fuel, more air means more expansive pressure and thus more power.
A turbo (a compressor driven by the exhaust gasses) or a supercharger (driven by a belt from the crankshaft) is a big air pump that can actually pressurize the manifold. In simplest terms, the higher the manifold pressure, the more air the engine gets and, if it gets the right amount of fuel, the more power the engine makes.
"PSI", in FI engine terminology, typically refers to the increase in manifold pressure, in pounds per square inch, over the standard atmosphere of 14.7-psi. So if an engine is boosted to "7-psi", it means the intake manifold would read 21.7psia (pounds per square inch absolute, referenced to a perfect vacuum) or 7-psig (pounds per square inch gauge, referenced to 1-atmosphere).
So if an engine is running 7-psi boost and the boost is increased to 12-psi, the engine will be getting something like 23% more air. If mixed with more fuel, this means more power. Further to this topic, if your engine is normally aspirated (i.e. no turbo, no super), your options for increasing performance are largely limited to reducing losses in the intake and exhaust systems and/or switching certain components (e.g. camshafts) to breathe better at different RPM ranges. By doing this, you increase the VE - volumetric efficiency which what that 85% value I was talking about above is all about. This increases the effective "PSI" seen at the open intake valve (that is, there are less losses throughout the system from the air intake to the valve port) and thus admits more air into the cylinder.
Not all turbo engines respond the same to boost increases. A manifold pressure increase of, say, 3-psig may not produce the same power increase on two different engines of similar sizes because, even though the manifolds are pressurized to the same value, the ability of the intake port and valve (including lift, seat angles and so on), valve timing and so on to actually "aspirate" the cylinder varies between engines. Manifold boost pressures therefore, do not tell the whole story.
Further still, a small turbo may be able to produce 12-psig of boost on an engine that doesn't breathe particularly well whereas that same turbo may only be able to move enough air to produce 7-psig on a well-breathing engine. And simply trying to increase the boost pressure by driving a turbo "harder" (e.g. wastegate settings) will usually, depending on the turbo, result more in heating of the air than moving more of it.
It's a pretty complex system with many variables. Compressors must be sized properly (to move the right amount of air at the desired pressure and not be laggy) and, if the system is modified to produce more boost, steps should be taken to ensure the compressor is still operating with the bounds of its performance map.
So now we know why hackish makes the big bucks! ahah.