zero should be relitively no diferential in pressure between the ambient atmospheric pressure and pressure inside the intake manifold. hence why the guage sits at zero when the motor is off.
To get a good feel for what vacuum readings mean on your boost gauge you need to first understand how naturally aspirated engines work, and where boost/vac reading are taken from on your car (at the intake manifold, not the turbo).
In case you're unclear n/a engines I'll review it. It's actually fairly simple... As you may know, gas (air) always tries to equalize pressure differences, and air always travels towards low pressure areas. The speed at which the air mass travels to a low pressure area is based on the pressure differential of the adjoining air masses.
Normal atmospheric pressure is 14.7psi (at sea level) so the max amount of air a n/a engine can take in is limited by this factor. When the cam activates the intake valve to open as the piston travels downward in the cylinder it causes a low pressure area (vacuum) in the cyl and draws air in through the open valve from the intake manifold. The valve is only open for a fraction of a second, so only so much air will get through, the greater the pressure in the intake manifold the more air will get into the cyl. They can also utilize the slight vacuum caused by escaping exhaust gases out the other valve to help increase the efficiency of this system. Turbo car tend not to do this because the air pressure in the exhaust manifold far exceeds that of the intake manifold (due to backpressure) and you can guess what would happen if both valves were open at the same time (reversion into the intake).
If there is no restriction in air flow feeding the intake manifold all the way from the throttle body, on up to the air filter, and beyond to where the car first draws air in (assuming no CAI) the boost gauge will read near 0psi/0vac because air pressure inside the intake manifold will be roughly the same as normal atmospheric (since air is able to rush in at the same rate as the engine is sucking it out).
However at part-throttle the butterfly valve inside the throttle body (or carb) will create a flow restriction. The engine will continue to attempt to draw in it's desired amount of air, this will cause the pressure in the intake manifold to drop below normal atmospheric because outside air won't be able to rush into the manifold as fast as the engine is sucking it out. At this point the gauge will read on the vac side. Also if there is a restriction (a portion where flow is less than that of the intake valve) caused by a dirty filter or poorly designed intake system the gauge will read on the vac side even at full throttle for the same reason...
Basically all the turbo does is force air into the manifold and pressurize it, this creates an even greater pressure differential between the manifold and the cyl and increases the speed of the air entering the cyl, which means more air (double at 14.7psi on the gauge) is entering the engine within the time the valve allows, so more fuel can be added... more power can be made and everyone lives happily ever after. 0 on the boost gauge is equal to normal atmospheric, and 15psi is roughly double atmospheric pressure. This is why some ppl say at 15psi you effectively double the size of the motor. They mean that since the engine is taking in double the air, it should make power levels near that of say a 4.8L motor in our case (all else being equal).
On a our car vac readings simply mean the turbo isn't supplying the manifold with enough air to match or exceed the engine's demands, and or the throttle valve is cutting off flow. A dirty filter, or poor intake system in front of the turbo has no affect on the gauge reading, which is why some think a CAI on a turbo car is pointless. However these restrictions instead affect the turbo's efficiency (which increases charge temps) but that's another discussion, and you probably stopped reading this a few paragraphs ago anyway...
__________________
Quote: Originally Posted by planetb
The "important gauge" did not stop his engine from going booom!
Last edited by SRT-4_&_NeonACR : 09-22-2006 at 11:04 AM.
I'm currently in college for engineering and am taking a fluid dynamics class. According to the professor, as I understand it, vac pressure can not be greater than the atmospheric pressure. I was wondering how my boost gage can read 21 psi vac at idle when the atmospheric pressure is 14.7 psi.
I'm currently in college for engineering and am taking a fluid dynamics class. According to the professor, as I understand it, vac pressure can not be greater than the atmospheric pressure. I was wondering how my boost gage can read 21 psi vac at idle when the atmospheric pressure is 14.7 psi.
Well, first off, you need to read your guage again. The stock guage reads inHg (inches of mercury) for a vacuum, and psig (psi guage) for positive pressure. I'd suggest you get your fluids book out and review the whole gauge vs. absolute pressure.
I'm a licensed ME and I specialize in thermal fluids design, and I can tell your professor is right IF you assume atmospheric pressure in psia (psi abolsute). If you're using psig, atmospheric pressure is 0. Just make sure you know the difference between psia, psig, and psid (that's absolute, guage, and differential), and you're good to go.
__________________
"Go that way, really fast. If something gets in your way, turn." - Charles De Mar.
To get a good feel for what vacuum readings mean on your boost gauge you need to first understand how naturally aspirated engines work, and where boost/vac reading are taken from on your car (at the intake manifold, not the turbo).
In case you're unclear n/a engines I'll review it. It's actually fairly simple... As you may know, gas (air) always tries to equalize pressure differences, and air always travels towards low pressure areas. The speed at which the air mass travels to a low pressure area is based on the pressure differential of the adjoining air masses.
Normal atmospheric pressure is 14.7psi (at sea level) so the max amount of air a n/a engine can take in is limited by this factor. When the cam activates the intake valve to open as the piston travels downward in the cylinder it causes a low pressure area (vacuum) in the cyl and draws air in through the open valve from the intake manifold. The valve is only open for a fraction of a second, so only so much air will get through, the greater the pressure in the intake manifold the more air will get into the cyl. They can also utilize the slight vacuum caused by escaping exhaust gases out the other valve to help increase the efficiency of this system. Turbo car tend not to do this because the air pressure in the exhaust manifold far exceeds that of the intake manifold (due to backpressure) and you can guess what would happen if both valves were open at the same time (reversion into the intake).
If there is no restriction in air flow feeding the intake manifold all the way from the throttle body, on up to the air filter, and beyond to where the car first draws air in (assuming no CAI) the boost gauge will read near 0psi/0vac because air pressure inside the intake manifold will be roughly the same as normal atmospheric (since air is able to rush in at the same rate as the engine is sucking it out).
However at part-throttle the butterfly valve inside the throttle body (or carb) will create a flow restriction. The engine will continue to attempt to draw in it's desired amount of air, this will cause the pressure in the intake manifold to drop below normal atmospheric because outside air won't be able to rush into the manifold as fast as the engine is sucking it out. At this point the gauge will read on the vac side. Also if there is a restriction (a portion where flow is less than that of the intake valve) caused by a dirty filter or poorly designed intake system the gauge will read on the vac side even at full throttle for the same reason...
Basically all the turbo does is force air into the manifold and pressurize it, this creates an even greater pressure differential between the manifold and the cyl and increases the speed of the air entering the cyl, which means more air (double at 14.7psi on the gauge) is entering the engine within the time the valve allows, so more fuel can be added... more power can be made and everyone lives happily ever after. 0 on the boost gauge is equal to normal atmospheric, and 15psi is roughly double atmospheric pressure. This is why some ppl say at 15psi you effectively double the size of the motor. They mean that since the engine is taking in double the air, it should make power levels near that of say a 4.8L motor in our case (all else being equal).
On a our car vac readings simply mean the turbo isn't supplying the manifold with enough air to match or exceed the engine's demands, and or the throttle valve is cutting off flow. A dirty filter, or poor intake system in front of the turbo has no affect on the gauge reading, which is why some think a CAI on a turbo car is pointless. However these restrictions instead affect the turbo's efficiency (which increases charge temps) but that's another discussion, and you probably stopped reading this a few paragraphs ago anyway...
Very good post stuck
__________________
11.34 @ 128.33 mph. Stock motor, head, cams and ecu
our engine is four stroke engine:
1. air/fuel mixture intake
2. compresion of air/fuel mixture
3. ignite by spark plug and explosion
4. exhaust the waste
in the first stroke, air fuel mixture is being sucked into engine. how did this happen? let's focus on the number 1 cynlinder in our car:
While cynlinder 1 exits the forth stage (exhaust) that will push most the air out of the cynlinder. Then because of the inertia it the crack shaft will keeps on rotating and pulls the piston away from it's highest point. So at this moment it will create a vaccuum in the combustion chamber as well as intake manifold.
If the car is under wide open throttle, which means the throttle plate is wide open and create an equal pressure in the intake manifold compare to atomic pressure (in natural aspirated cars). In turbo/super charged cars, it will create a positive pressure because the intake system is pressurized.
I hope this will clearify why it's under vaccuum. Post up if you have any questions.
Tiger
__________________
MODS:
NOS Octane Booster
Stickers, those 5+ hp ones, lots of them!!!!
RAPOM +33% long throw shifter
K&N steam air intake system, complete
To get a good feel for what vacuum readings mean on your boost gauge you need to first understand how naturally aspirated engines work, and where boost/vac reading are taken from on your car (at the intake manifold, not the turbo).
In case you're unclear n/a engines I'll review it. It's actually fairly simple... As you may know, gas (air) always tries to equalize pressure differences, and air always travels towards low pressure areas. The speed at which the air mass travels to a low pressure area is based on the pressure differential of the adjoining air masses.
Normal atmospheric pressure is 14.7psi (at sea level) so the max amount of air a n/a engine can take in is limited by this factor. When the cam activates the intake valve to open as the piston travels downward in the cylinder it causes a low pressure area (vacuum) in the cyl and draws air in through the open valve from the intake manifold. The valve is only open for a fraction of a second, so only so much air will get through, the greater the pressure in the intake manifold the more air will get into the cyl. They can also utilize the slight vacuum caused by escaping exhaust gases out the other valve to help increase the efficiency of this system. Turbo car tend not to do this because the air pressure in the exhaust manifold far exceeds that of the intake manifold (due to backpressure) and you can guess what would happen if both valves were open at the same time (reversion into the intake).
If there is no restriction in air flow feeding the intake manifold all the way from the throttle body, on up to the air filter, and beyond to where the car first draws air in (assuming no CAI) the boost gauge will read near 0psi/0vac because air pressure inside the intake manifold will be roughly the same as normal atmospheric (since air is able to rush in at the same rate as the engine is sucking it out).
However at part-throttle the butterfly valve inside the throttle body (or carb) will create a flow restriction. The engine will continue to attempt to draw in it's desired amount of air, this will cause the pressure in the intake manifold to drop below normal atmospheric because outside air won't be able to rush into the manifold as fast as the engine is sucking it out. At this point the gauge will read on the vac side. Also if there is a restriction (a portion where flow is less than that of the intake valve) caused by a dirty filter or poorly designed intake system the gauge will read on the vac side even at full throttle for the same reason...
Basically all the turbo does is force air into the manifold and pressurize it, this creates an even greater pressure differential between the manifold and the cyl and increases the speed of the air entering the cyl, which means more air (double at 14.7psi on the gauge) is entering the engine within the time the valve allows, so more fuel can be added... more power can be made and everyone lives happily ever after. 0 on the boost gauge is equal to normal atmospheric, and 15psi is roughly double atmospheric pressure. This is why some ppl say at 15psi you effectively double the size of the motor. They mean that since the engine is taking in double the air, it should make power levels near that of say a 4.8L motor in our case (all else being equal).
On a our car vac readings simply mean the turbo isn't supplying the manifold with enough air to match or exceed the engine's demands, and or the throttle valve is cutting off flow. A dirty filter, or poor intake system in front of the turbo has no affect on the gauge reading, which is why some think a CAI on a turbo car is pointless. However these restrictions instead affect the turbo's efficiency (which increases charge temps) but that's another discussion, and you probably stopped reading this a few paragraphs ago anyway...
To get a good feel for what vacuum readings mean on your boost gauge you need to first understand how naturally aspirated engines work, and where boost/vac reading are taken from on your car (at the intake manifold, not the turbo).
In case you're unclear n/a engines I'll review it. It's actually fairly simple... As you may know, gas (air) always tries to equalize pressure differences, and air always travels towards low pressure areas. The speed at which the air mass travels to a low pressure area is based on the pressure differential of the adjoining air masses.
Normal atmospheric pressure is 14.7psi (at sea level) so the max amount of air a n/a engine can take in is limited by this factor. When the cam activates the intake valve to open as the piston travels downward in the cylinder it causes a low pressure area (vacuum) in the cyl and draws air in through the open valve from the intake manifold. The valve is only open for a fraction of a second, so only so much air will get through, the greater the pressure in the intake manifold the more air will get into the cyl. They can also utilize the slight vacuum caused by escaping exhaust gases out the other valve to help increase the efficiency of this system. Turbo car tend not to do this because the air pressure in the exhaust manifold far exceeds that of the intake manifold (due to backpressure) and you can guess what would happen if both valves were open at the same time (reversion into the intake).
If there is no restriction in air flow feeding the intake manifold all the way from the throttle body, on up to the air filter, and beyond to where the car first draws air in (assuming no CAI) the boost gauge will read near 0psi/0vac because air pressure inside the intake manifold will be roughly the same as normal atmospheric (since air is able to rush in at the same rate as the engine is sucking it out).
However at part-throttle the butterfly valve inside the throttle body (or carb) will create a flow restriction. The engine will continue to attempt to draw in it's desired amount of air, this will cause the pressure in the intake manifold to drop below normal atmospheric because outside air won't be able to rush into the manifold as fast as the engine is sucking it out. At this point the gauge will read on the vac side. Also if there is a restriction (a portion where flow is less than that of the intake valve) caused by a dirty filter or poorly designed intake system the gauge will read on the vac side even at full throttle for the same reason...
Basically all the turbo does is force air into the manifold and pressurize it, this creates an even greater pressure differential between the manifold and the cyl and increases the speed of the air entering the cyl, which means more air (double at 14.7psi on the gauge) is entering the engine within the time the valve allows, so more fuel can be added... more power can be made and everyone lives happily ever after. 0 on the boost gauge is equal to normal atmospheric, and 15psi is roughly double atmospheric pressure. This is why some ppl say at 15psi you effectively double the size of the motor. They mean that since the engine is taking in double the air, it should make power levels near that of say a 4.8L motor in our case (all else being equal).
On a our car vac readings simply mean the turbo isn't supplying the manifold with enough air to match or exceed the engine's demands, and or the throttle valve is cutting off flow. A dirty filter, or poor intake system in front of the turbo has no affect on the gauge reading, which is why some think a CAI on a turbo car is pointless. However these restrictions instead affect the turbo's efficiency (which increases charge temps) but that's another discussion, and you probably stopped reading this a few paragraphs ago anyway...
Great explanation.
So if your at WOT in a F/I car, then vaccum should read -? If 0 vaccum is atmospheric pressure and a turbo buts more pressure in.
Or am I thinking/going about this all wrong? lol
Last edited by SynthRocker : 09-24-2006 at 11:17 PM.
I'm currently in college for engineering and am taking a fluid dynamics class. According to the professor, as I understand it, vac pressure can not be greater than the atmospheric pressure. I was wondering how my boost gage can read 21 psi vac at idle when the atmospheric pressure is 14.7 psi.
Well, first off, you need to read your guage again. The stock guage reads inHg (inches of mercury) for a vacuum, and psig (psi guage) for positive pressure. I'd suggest you get your fluids book out and review the whole gauge vs. absolute pressure.
I'm a licensed ME and I specialize in thermal fluids design, and I can tell your professor is right IF you assume atmospheric pressure in psia (psi abolsute). If you're using psig, atmospheric pressure is 0. Just make sure you know the difference between psia, psig, and psid (that's absolute, guage, and differential), and you're good to go.
FINALLY...........someone who KNOWS what thier talking about
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Explanation Of Vaccuum
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