Any truth to this?
Well this is how it works to my understanding. Given I will make these numbers for easier understanding and I honestly dont know exact numbers or equations.
Keep in mind a boost guage is almost like a back pressure gauge telling you how hard the blower is working to make the power. More efficient the motor is, less hard the blower works to make power. A boost guage is not reading the size of the crank/ supercharger pulley.
Say a stock truck allows 500 particals of air per cylinder.
Lets also say you add a blower to that truck with the crank and S/C pulley diameter it is an "8PSI" pulley set up.The truck is 100% bone stock other then blower. Now lets also say the blower forces up to 1000 particals of air into 1 (each) cylinder upon the intake valve opening and the blower forcing air in. This is the maximum it can flow before the cylinder is "full" and the blower isnt spinning fast enough to compress the air more to force more in the cylinders. At this time the guage on the dash of the truck reads 8PSI. Due to the fact that you have not changed the camshaft to allow more time for exhaust to exit the engine, or open the valves more to allow more out per stroke, exhaust will try to sqeeze out the tiny exhaust valve and into a small exhaust manifold where it will get smashed into the exhaust from other cylinders creating sort of a "traffic" jam. This jam of exhaust actually limits how much air can be pushed out of a cylinder and can actually keep exhaust from exiting the cylinder causing a power loss as the gasses cannot combust again to contribute to the power. At this point the blower will push 850 particals of new fresh air into the cylinder, while 150 particals were left in the cylinder and were not able to esacpe due to a "filled up" exhaust manifold. At this point you make good power and the truck is fun to drive.
You then add longtube headers and full exhaust. The guage on the dash reads "6.5 PSI" where it use to read "8PSI" The truck is still pullied for 8psi, however because the blower still pushes 1000 particals in per intake valve opening the "6.5 PSI" reading is actually correct and incorrect. Now that the engine can exhale easier, the engine can actually force all the air in the cylinder out the exhaust easier. Now with the long tube headers and exhaust, gas is free to flow. Lets say now you get 950 particals of air into the cylinder with only 50 particals not being allowed to escape due to time/short exhaust valve opening. Now you are gaining power because you have 950 particals of air to mix with fuel thus more to burn to make a combustion. Power goes up as the engine is more efficient. However, while the system use to make 8PSI on the boost gauge it now reads 6.5PSI. This is saying that the truck only has to make 6.5 PSI to actually fill the same cylinder to 1000 particals of air. Instead of the blower having to compress the air and work hard to really force it into a cylinder that is already slightly full, it now easier for it to force air into a less full to begin with cylinder. The truck is still allowing 1000 particals of air in, its just more of that air is fresh new air, not exhaust that cannot be burned twice. Now the truck is a beast and nothing short of a GT500 gives you much of a challenge
Finally in a perfect world, you go built motor and pulley for 30PSI. The crank and supercharger will push 9000 particals of air into the motor at maximum. The cylinders are equiped with deep dish pistons to allow space for 9000 particals of air in the combustion chamber at once. The intake valves along with camshaft are designed to allow all 9000 particals of air into the cylinder easily, while the exhaust valve/headers/exhaust is designed to allow all 9000 particals out on each exhaust stroke of the piston. The whole motor works 100% efficient where not one partical of air is not let in the cylinder and not one partical is not pushed out. At wide open throttle the boost guage reads zero. The engine is not making vaccum but because the blower has zero back pressure and the air just constantly flows with ease in and out of the motor and your guage goes from vaccum to 0. Now you have endless woman wanting you for the baddassness that is your truck. Your big pimpin and probably a father to children you dont even know about.
Another way to think of all this. If you had a boost guage in your mouth and say you take a huge breath of air and try to blow all the air through a tiny straw as fast as you can. Because you cannot blow all the air into the straw at once, it causes your cheeks to puff out and you hold more air in your mouth waiting to exit then is actually going through the straw. You would get a back pressuring/boost reading. Now say you take a huge breath of air and open your mouth as wide as you can with no straw. Exhahale from your mouth as fast as you can. All the air instantly is out of your mouth with no puffy cheeks and no back pressure. While you push the same amount of air each time. There is no backpressure to stop you from pushing out the air, and thus no boost but much faster release of air.
Basically what I am trying to make a point about is on a stock truck, the most you can pulley to is 12PSI with a good tune. 10PSI is safer but if your willing to risk it you can do 12. No matter what the guage says, you have to go off the diameter of the Crankshaft and Supercharger pulley
Ok sorry for the really long post.
Keep in mind a boost guage is almost like a back pressure gauge telling you how hard the blower is working to make the power. More efficient the motor is, less hard the blower works to make power. A boost guage is not reading the size of the crank/ supercharger pulley.
Say a stock truck allows 500 particals of air per cylinder.
Lets also say you add a blower to that truck with the crank and S/C pulley diameter it is an "8PSI" pulley set up.The truck is 100% bone stock other then blower. Now lets also say the blower forces up to 1000 particals of air into 1 (each) cylinder upon the intake valve opening and the blower forcing air in. This is the maximum it can flow before the cylinder is "full" and the blower isnt spinning fast enough to compress the air more to force more in the cylinders. At this time the guage on the dash of the truck reads 8PSI. Due to the fact that you have not changed the camshaft to allow more time for exhaust to exit the engine, or open the valves more to allow more out per stroke, exhaust will try to sqeeze out the tiny exhaust valve and into a small exhaust manifold where it will get smashed into the exhaust from other cylinders creating sort of a "traffic" jam. This jam of exhaust actually limits how much air can be pushed out of a cylinder and can actually keep exhaust from exiting the cylinder causing a power loss as the gasses cannot combust again to contribute to the power. At this point the blower will push 850 particals of new fresh air into the cylinder, while 150 particals were left in the cylinder and were not able to esacpe due to a "filled up" exhaust manifold. At this point you make good power and the truck is fun to drive.
You then add longtube headers and full exhaust. The guage on the dash reads "6.5 PSI" where it use to read "8PSI" The truck is still pullied for 8psi, however because the blower still pushes 1000 particals in per intake valve opening the "6.5 PSI" reading is actually correct and incorrect. Now that the engine can exhale easier, the engine can actually force all the air in the cylinder out the exhaust easier. Now with the long tube headers and exhaust, gas is free to flow. Lets say now you get 950 particals of air into the cylinder with only 50 particals not being allowed to escape due to time/short exhaust valve opening. Now you are gaining power because you have 950 particals of air to mix with fuel thus more to burn to make a combustion. Power goes up as the engine is more efficient. However, while the system use to make 8PSI on the boost gauge it now reads 6.5PSI. This is saying that the truck only has to make 6.5 PSI to actually fill the same cylinder to 1000 particals of air. Instead of the blower having to compress the air and work hard to really force it into a cylinder that is already slightly full, it now easier for it to force air into a less full to begin with cylinder. The truck is still allowing 1000 particals of air in, its just more of that air is fresh new air, not exhaust that cannot be burned twice. Now the truck is a beast and nothing short of a GT500 gives you much of a challenge
Finally in a perfect world, you go built motor and pulley for 30PSI. The crank and supercharger will push 9000 particals of air into the motor at maximum. The cylinders are equiped with deep dish pistons to allow space for 9000 particals of air in the combustion chamber at once. The intake valves along with camshaft are designed to allow all 9000 particals of air into the cylinder easily, while the exhaust valve/headers/exhaust is designed to allow all 9000 particals out on each exhaust stroke of the piston. The whole motor works 100% efficient where not one partical of air is not let in the cylinder and not one partical is not pushed out. At wide open throttle the boost guage reads zero. The engine is not making vaccum but because the blower has zero back pressure and the air just constantly flows with ease in and out of the motor and your guage goes from vaccum to 0. Now you have endless woman wanting you for the baddassness that is your truck. Your big pimpin and probably a father to children you dont even know about.
Another way to think of all this. If you had a boost guage in your mouth and say you take a huge breath of air and try to blow all the air through a tiny straw as fast as you can. Because you cannot blow all the air into the straw at once, it causes your cheeks to puff out and you hold more air in your mouth waiting to exit then is actually going through the straw. You would get a back pressuring/boost reading. Now say you take a huge breath of air and open your mouth as wide as you can with no straw. Exhahale from your mouth as fast as you can. All the air instantly is out of your mouth with no puffy cheeks and no back pressure. While you push the same amount of air each time. There is no backpressure to stop you from pushing out the air, and thus no boost but much faster release of air.
Basically what I am trying to make a point about is on a stock truck, the most you can pulley to is 12PSI with a good tune. 10PSI is safer but if your willing to risk it you can do 12. No matter what the guage says, you have to go off the diameter of the Crankshaft and Supercharger pulley
Ok sorry for the really long post.
Senior Member
Joined: Jun 2005
Posts: 2,805
Likes: 2
From: NEVADA
Righto. The air flow is generally expressed as #/minute,thus, mass air meters are measuring weight/mass.
You have to add in the standard weight of air that we are swimming in, 14.7 at sea level. Mine is ~ 13.65 at 1,950 asl, so my gauge reads -1 psi at rest.
So, your 30 psi example is actually 44.7 at sea level, etc.
You have to add in the standard weight of air that we are swimming in, 14.7 at sea level. Mine is ~ 13.65 at 1,950 asl, so my gauge reads -1 psi at rest.
So, your 30 psi example is actually 44.7 at sea level, etc.