BOOST pressure vs. MANIFOLD ABSOLUTE pressure vs. ATMOSPHERIC pressure - BOOST SCHOOL #4

[Music] [Music] so today we'll be covering something pretty simple but something that must be clearly said explained and understood before we dive deeper into the world of boost and tuning boosted engines and that something is the relationship between atmospheric pressure boost pressure and manifold absolute pressure it's a relatively simple relationship but one that tends to confuse many people especially beginners entering into the world of boost and this is why today we'll be explaining this relationship in the simplest possible terms starting from the ground up and working our way up to having a firm grasp of this relationship so let's start with the basics air pressure or the pressure of all the air around us the pressure created by the weight and the mass of all the air in earth's atmosphere we are born and we die in earth's atmosphere and this is why we really can't tell that there's weight and mass to the air around us to us it feels weightless almost non-existent but air definitely has weight and mass in fact all of the air in all of earth's atmosphere weighs an incredible 5 million billion tons so how come we don't get crushed by the weight of the air we don't get crushed because all of the air is evenly distributed across the entire earth's surface and the pressure we feel on our body from the weight of the air is around 14.7 psi or one bar but we don't really feel it because we don't have a reference for what it feels like to be in a space without this pressure unless you've been to outer space you really haven't spent a single moment of your time without feeling air pressure on yourself and this is why you don't have a reference for what it feels like to be without air pressure and this is why you don't feel it so what do you think where is the place you can experience the lowest atmospheric pressure without leaving the earth the answer is at the top of mount everest at the highest point on planet earth the highest point on planet earth has the lowest pressure because it has the least atmosphere above it the least amount of air weight is above it therefore the air pressure or the atmospheric pressure is the lowest at the peak of mount everest where's the where is air pressure the highest it's of course highest at sea level because that's the lowest you can be while still being on the surface of planet earth and this this is where there's the most airweight above you this is why air pressure is the highest but you can actually experience a very low air pressure while still being on earth without climbing mount everest you can do this by contracting a virus like for example covet 19 and and then they stick you into an isolation room you're going to experience very low pressure in an atmosphere in an isolation room because most isolation rooms are actually negative pressure rooms the air the air pressure inside a negative pressure room is artificially maintained at a pressure lower than the air pressure of the air outside the rooms this is usually done by some sort of an exhaust system or another method this is done to prevent the contaminated air from inside the room from escaping the room when you open the door of an isolation room when you open the door of an isolation room because the pressure in the room is lower than the pressure outside it fresh air from the outside is actually going to rush into the room this is because air like all fluids always flows from high pressure areas to all pressure areas do you know what else works on a very similar principle to an isolation room inside a hospital that's right the cylinder inside your engine when a piston moves very quickly down the bore inside your cylinder it creates an empty space or a void and for an extremely brief moment that empty space is actually devoid of air pressure it's a vacuum it's an absence of air pressure and as we said air flows from high to low pressure and this is why air from the outside quickly rushes in to fill the cylinder inside your engine and this is why if you were to put your hand against the throttle body of a running engine you would feel they are being sucked into the engine but it's the engine really isn't sucking air in it's simply the air rushing into the engine from a high pressure area to a low pressure air this means that a naturally aspirated engine one that relies on the natural atmospheric pressure of air to get air into its cylinder is never going to see air pressures inside its intake manifold being significantly higher than the air pressure around it you can use tuning trickster intake manifold to slightly exceed atmospheric pressure inside the intake manifold of a naturally aspirated engine but you're never going to be able to significantly exceed atmospheric air pressure and this means that the higher you climb with your engine the lower the power because when you go to higher altitudes there's less air pressure which means there's less air pressure to shove air into your cylinders which means you're getting less air into your cylinder which means you can add less fuel which means a less powerful combustion so an engine at sea level is always going to make a bit more power than an engine at the top of a mountain but things drastically change when forced induction enters the scene a turbo or a supercharger can add significant air pressure to the engine in fact the air pressure measured inside the intake manifold of a boosted engine can be double or triple that of the atmospheric pressure outside the engine in essence a turbo or a supercharger stuffs a greater air mass or a greater number of air molecules into the same space by compressing the air this of course increases the pressure inside that space but because there's more air mass in the same space we can also add more feel to that space and this creates a more powerful combustion this is essentially how forced induction makes more power than natural aspiration given equal displacement now in all this your ecu has the task of matching the incoming air mass with the correct mass of fuel to do this the ecu of course needs to know how much air is coming into the engine most modern fuel injected engines are relying on either a map which is a manifold absolute pressure sensor or a maf which is a mass airflow sensor or both to measure the air coming into the engine if you're interested in the differences and how they work and the benefits and drawbacks of each of these sensor types you can watch my video on afm versus math versus map sensors but the gist of it is that the map sensor sits in your intake manifold and it measures manifold absolute pressure the pressure inside your intake manifold and it makes air mass calculations based on this pressure on the other hand a maf sits behind your intake air filter and it directly measures air mass now when it comes to boosted engines we're of course interested in the pressure being generated by our turbo or a supercharger but we're also interested in the air pressure inside our intake manifold you will think they're one in the same but they're definitely not when you're looking at a boost gauge inside of a turbocharged or supercharged car what you're actually seeing is only the boost pressure generated by your turbo or supercharger this means that what you're seeing in the boost gauge is not the air pressure inside your intake manifold if you were to take a boost gauge and its sensor and expose them to the atmosphere you would get a value of zero on the boost gauge this is because a boost gauge is referenced to atmospheric pressure this is done to show you only the pressure being added by your turbo or supercharger this is what boost is it's the additional pressure created by your turbo a boost gauge in other words is telling you how hard your turbo or supercharger is actually working but things are different from the perspective of your ecu the ecu needs to know all of the pressure both atmospheric together with boost pressure being added it needs to know all of the air pressure because it wants the most correct possible calculation of the air mass entering the engine it needs to know all of the air mass coming into the engine because it's trying to match off the air mass with the correct amount of fuel this is why the map sensor which is measuring manifold absolute pressure inside your intake manifold when it's exposed to atmospheric pressure it's going to show a value of 14.7 psi because it's referenced to zero to absolute pressure to absence of pressure its reference to this pressure so you can show all the air pressure inside your intake manifold and feed this data to your engine's ecu if you were to drive your forced induction car up to a higher elevation your boost gauge would actually show a higher value at a higher elevation than at sea level but your map sensor feeding data to your ecu from your intake manifold would show the same overall pressure how come well air is less dense there's less pressure at the top of a high mountain this means that the turbocharger or supercharger needs to work harder to generate more air pressure to make up for the reduced atmospheric pressure to trigger boost control and open the wastegate in case of a turbocharge and this makes sense because this was the original intent of turbochargers for example they were developed to help airplanes keep making power at higher altitudes and the task of the turbo was to generate additional pressure to make up for the variable density at very high flying altitudes so manifold absolute pressure is atmospheric pressure plus boost pressure and boost pressure is of course manifold absolute pressure minus atmospheric pressure and this is why aem has a very broad range of sensors both for measuring manifold absolute pressure and feeding this data to your ecu as well as gauge sensors for measuring anything from boost pressure to oil and fuel pressure aem also has pressure sensors in two different kinds of materials stainless steel and brass sensors stainless steel sensors are extremely accurate and boast an accuracy to within one percent of the full scale of the sensor this means that you're going to use a stainless steel sensor for fine measurement and feeding data to your ecu on the other hand if you want a gauge measurement a gauge sensor you can cut costs by getting one of the brass sensors they're still very accurate to within three percent of the full scale of the sensor but they're a bit more economical and they also boast the same wide range of maximum pressures as the stainless steel sensors it's also important to know that you should match the correct sensor with your planned maximum boost when it comes to map sensors for your ecu aem has 15 30 50 75 and 100 psi map sensors the 15 psi is obviously for naturally aspirated applications while the rest are for boosted applications but putting a 75 psi sensor on an engine that will see a maximum of around 25 psi of boost really isn't the equivalent of leaving room for growth it actually means you're robbing your ecu of the correct sensor resolution the sensor resolution is the relationship between the pressure measured and the voltage output and you should always match the correct sensor with the planned maximum boost and if you decide to up the boost significantly later on you should simply replace the sensor also a little rule of thumb when it comes to math versus map sensors and boosted applications if you're tuning an engine that's boosted from the factory and you're increasing the boost a bit by refreshing the ecu and maybe doing some bolt-ons and that engine already has a maf sensor from the factory then you should keep using that map sensor and relying on it for air mass measurements provided to your ecu but if you decide to significantly up the boost or if you're boosting a previously naturally aspirated engine then a map sensor is going to be a better idea because it will make life easier and make tuning a lot more straightforward a mav sensor is calibrated to the stock parameters and the expected airflow from the stock engine when you significantly increase the boost you're going to run out of calibration you are going to start making airflow that is outside of the maximum airflow that the maf sensor can measure and this will make your stock map sensor inaccurate and even a potential source of engine damage in extreme cases on top of this a map sensor is much smaller than a maf sensor also the maf sensor is made from plastic which is why sometimes finding a place a proper place for the maf sensor in a heavily modified engine can be a bit of a pain on top of this a stainless steel map sensor resists the highest of temperatures and it's it's extremely small which means you can fit it pretty much anywhere and it will never melt irregardless of how crazy and hot things get inside your engine bay on top of this a mav sensor is a bit of an airflow restriction a map has a zero airflow restriction and even though the map sensor airflow restriction is kind of old it's still an obstacle in the path of the incoming air and we all know when it comes to squeezing out the maximum possible horsepower from an engine every little bit matters and a little tip when it comes to mounting map sensors although you can mount the map sensor directly into your intake manifold sometimes this isn't the best idea because engines can vibrate a lot especially if they use polyurethane engine mounts or are heavily modified in a way that increases engine vibrations these vibrations can actually reduce the accuracy of your map sensor and this can negatively impact your engine tune this is why in many cases it's a much better idea to mount the sensor away from the engine somewhere where you can insulate it to reduce the vibrations reaching the sensor as much as possible and then you can measure uh intake manifold pressure by using a hose to the map sensor in many cases is going to improve the running of your engine and when it comes to aem map sensors all of them come with the correct fittings to install the map sensor both on and off the intake manifold and that's pretty much it when it comes to the relationship between manifold absolute pressure boost pressure and atmospheric air pressure along with a bit of extra info on math and map sensors in aftermarket boosted tuned applications i hope this video helps you better understand all these things and helps you make better more informed decisions on your boosted belt as always thanks a lot for watching and i'll be seeing you soon with more fun and useful stuff on the d4 channel

Channel: driving 4 answers

Views: 47,650

Rating: 4.9470801 out of 5

Keywords: manifold absolute pressure, boost pressure, atmospheric pressure, barometric pressure, map vs maf sensor, aem electronics, aem map sensor, boost gauge, boost controller, boost control, boost pressure sensor, boost pressure gauge, map sensor, boost pressure vs map pressure, learn to tune, high performance academy, tuning, boost school, d4a, turbo boost pressure, turbo, supercharger, turbo boost pressure explained, turbo explained, manifold absolute pressure vs boost

Id: ytHbFVkWOJg

Length: 15min 37sec (937 seconds)

Published: Sun Nov 22 2020