Is this true. SAE correction for turbo cars is BS?
03-05-2008, 08:22 PM
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Is this true. SAE correction for turbo cars is BS?
Long read, but kinda makes sense. Can anyone find any holes in the theory?
Cliff notes on the bottom.
The point of this is to educate on how altitude plays a role on how much air can enter your engine. You really need to look at the attached spreadsheet to understand how these numbers work. For comparison sake, I am going to just assume 100% efficency and not factor in any losses to backpressure, heat, etc.
The purpose of forced induction is to get more air into the engine so we can make more power. We commonly refer to this as boost. It is typically measured in pounds per square inch (psi). It is a misconception that at sea level we are at 0 psi. This isn’t outer space! We are actually at or near 14.7 psi. This varies a little depending on weather conditions so just assume a perfect day by the ocean. When we refer to boost, we want to know how much pressure we are running over this amount. Therefore 6 psi of boost is 14.7 + 6 = 20.7 psi ambient. Everything is referenced to ambient pressure at sea level.
You need to print the attached chart up and look at it while reading the rest of this.
At sea level as stated above, we have 14.7 psi. If we want 6 psi of boost, we need to have 20.7 psi ambient pressure. This is a 40.8% gain over ambient. Desired boost pressure should not be considered in psi but rather in a % over ambient. If we want 6 psi, we really just want 40.8% gain in pressure. You get the point. The rest of the examples assumes we have a fixed ratio of 40.8% more power than stock at that altitude which equals 6 psi at sea level for comparison sake.
At 1000 ft above sea level we need to figure out what 40.8% greater than ambient (14.18 psi) is. 14.18 X 1.408 (40.8%) = 19.96 psi. That’s a loss of 3.575% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). There is less air to begin with at higher altitudes therefore less compression. The percentage increase stays the same but the boost pressure does not. Your mechanically driven supercharger’s boost gauge will now read only 5.26 psi since it is set to sea level while a turbo’s will still read 6 psi. A turbo has the advantage by .74 psi.
A naturally aspirated engine loses 3.538% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss since the wastegate is calibrated to sea level or a fixed spring pressure.
At 2000 ft above sea level we need to figure out what 40.8% greater than ambient (13.67) is. 13.67 X 1.408 = 19.247 psi absolute. That’s a loss of 7.02% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 4.55 psi while a turbo’s will still read 6 psi. A turbo has the advantage by 1.45 psi. A naturally aspirated engine loses 7.01% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 3000 ft above sea level we need to figure out what 40.8% greater than ambient is. 13.17 X 1.408 = 18.54 psi absolute. That’s a loss of 10.435% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 3.84 psi while the turbo’s will still read 6 psi. A turbo has the advatage by 2.16 psi. A naturally aspirated engine loses 10.41% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 4000 ft above sea level we need to figure out what 40.8% greater than ambient is. 12.7 X1.408 = 17.88 psi absolute. That’s a loss of 13.624% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 3.18 psi while the turbo’s will still read 6 psi. A turbo has the advantage by 2.82 psi. A naturally aspirated engine loses 13.606% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 5000 ft above sea level we need to figure out what 40.8% greater than ambient is. 12.23 X 1.408 = 17.22 psi absolute. That’s a loss of 16.81% pressure from sea level. This is what a mechanically driven supercharger will yield if is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 2.52 psi while a turbo’s will still read 6 psi. A turbo has the advantage by 3.48 psi. A naturally aspirated engine loses 16.8% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
As we can see from this trend, the percentage of power loss between a naturally aspirated engine and a mechanically supercharged engine is close enough to be considered the same. This is what SAE corrections on dyno’s is designed to compensate for. They are basing the results at a certain altitude (and temperature but it won’t be discussed here) and try to get their results back to sea level on a perfect day. This is a set standard and makes numbers from other dyno’s easy to compare. This correction value is based on a set % for altitude and temperature. This is fine for naturally aspirated of mechanically supercharged vehicles but isworthless for exhaust driven turbocharged vehicles. This is because mechanically driven superchargers are boosting to a certain set ratio of air greater than what the engine is actually sucking in. An exhaust driven turbocharged vehicle is set to reference pressure to sea level. At higher altitudes it just works harder to get that pressure back up. It has to work harder since there is less pressure to start with. It’s like climbing a ladder. A supercharger is like a person climbing up a ladder from the bottom. His goal is to only climb a certain way in total distance. The turbocharger is like climbing up a ladder to a fixed elevation only it doesn’t matter if you startedout on the ground or 10 feet under ground. You still climb to the same spot. The total gain is different and calibrated to a fixed, known location. Got it! This is why you use SAE corrections for naturally aspirated and mechanically supercharged engines but not for exhaust driven turbocharged engines. Correction factors for turbocharged vehicles will basically be the same as giving you some free boost. That's cheating the numbers. It may be great way to sell more product but it isn't an accurate representation of how much power you put down. The greater the altitude change, the more inaccurate it becomes.
In reality, there is some differences that offset the effect of turbochargers holding boost at higher altitudes. First off, the turbo is working harder since it has to spin faster. This creates more heat. we also have an average loss in temperature of 3 degrees F over every thousand feet in elevation rise. While these will affect the final numbers from sea level a small amount, they are nowhere near as off as the SAE correction factor for turbocharged engines at altitude. We may also run into the problem of the turbo getting too far outside it's efficiency range spinning at these speeds. Differently sized turbos will have different efficiencies so we can't jsut get a set standard for this either.
The next time someone tells you that you need to use SAE correction for a turbocharged engine because it is the "standard", laugh at them, tell them to go do their homework and to just go ahead and print up your uncorrected dyno sheet (turbo cars) so you can leave.
http://www.rx8club.com/attachment.ph...3&d=1108100716
EDIT for cliff notes: SAE correctio for turbo charged cars is not correct because turbo's run off wastegates and a set boost pressure. This pressure will be reached regardless of elevation. Spring pressures don't change. N/A and supercharged cars WILL lose that power because they are set up and designed to run a certain % of air more than what the car is actually bringing in.
Cliff notes on the bottom.
The point of this is to educate on how altitude plays a role on how much air can enter your engine. You really need to look at the attached spreadsheet to understand how these numbers work. For comparison sake, I am going to just assume 100% efficency and not factor in any losses to backpressure, heat, etc.
The purpose of forced induction is to get more air into the engine so we can make more power. We commonly refer to this as boost. It is typically measured in pounds per square inch (psi). It is a misconception that at sea level we are at 0 psi. This isn’t outer space! We are actually at or near 14.7 psi. This varies a little depending on weather conditions so just assume a perfect day by the ocean. When we refer to boost, we want to know how much pressure we are running over this amount. Therefore 6 psi of boost is 14.7 + 6 = 20.7 psi ambient. Everything is referenced to ambient pressure at sea level.
You need to print the attached chart up and look at it while reading the rest of this.
At sea level as stated above, we have 14.7 psi. If we want 6 psi of boost, we need to have 20.7 psi ambient pressure. This is a 40.8% gain over ambient. Desired boost pressure should not be considered in psi but rather in a % over ambient. If we want 6 psi, we really just want 40.8% gain in pressure. You get the point. The rest of the examples assumes we have a fixed ratio of 40.8% more power than stock at that altitude which equals 6 psi at sea level for comparison sake.
At 1000 ft above sea level we need to figure out what 40.8% greater than ambient (14.18 psi) is. 14.18 X 1.408 (40.8%) = 19.96 psi. That’s a loss of 3.575% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). There is less air to begin with at higher altitudes therefore less compression. The percentage increase stays the same but the boost pressure does not. Your mechanically driven supercharger’s boost gauge will now read only 5.26 psi since it is set to sea level while a turbo’s will still read 6 psi. A turbo has the advantage by .74 psi.
A naturally aspirated engine loses 3.538% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss since the wastegate is calibrated to sea level or a fixed spring pressure.
At 2000 ft above sea level we need to figure out what 40.8% greater than ambient (13.67) is. 13.67 X 1.408 = 19.247 psi absolute. That’s a loss of 7.02% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 4.55 psi while a turbo’s will still read 6 psi. A turbo has the advantage by 1.45 psi. A naturally aspirated engine loses 7.01% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 3000 ft above sea level we need to figure out what 40.8% greater than ambient is. 13.17 X 1.408 = 18.54 psi absolute. That’s a loss of 10.435% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 3.84 psi while the turbo’s will still read 6 psi. A turbo has the advatage by 2.16 psi. A naturally aspirated engine loses 10.41% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 4000 ft above sea level we need to figure out what 40.8% greater than ambient is. 12.7 X1.408 = 17.88 psi absolute. That’s a loss of 13.624% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 3.18 psi while the turbo’s will still read 6 psi. A turbo has the advantage by 2.82 psi. A naturally aspirated engine loses 13.606% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 5000 ft above sea level we need to figure out what 40.8% greater than ambient is. 12.23 X 1.408 = 17.22 psi absolute. That’s a loss of 16.81% pressure from sea level. This is what a mechanically driven supercharger will yield if is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 2.52 psi while a turbo’s will still read 6 psi. A turbo has the advantage by 3.48 psi. A naturally aspirated engine loses 16.8% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
As we can see from this trend, the percentage of power loss between a naturally aspirated engine and a mechanically supercharged engine is close enough to be considered the same. This is what SAE corrections on dyno’s is designed to compensate for. They are basing the results at a certain altitude (and temperature but it won’t be discussed here) and try to get their results back to sea level on a perfect day. This is a set standard and makes numbers from other dyno’s easy to compare. This correction value is based on a set % for altitude and temperature. This is fine for naturally aspirated of mechanically supercharged vehicles but isworthless for exhaust driven turbocharged vehicles. This is because mechanically driven superchargers are boosting to a certain set ratio of air greater than what the engine is actually sucking in. An exhaust driven turbocharged vehicle is set to reference pressure to sea level. At higher altitudes it just works harder to get that pressure back up. It has to work harder since there is less pressure to start with. It’s like climbing a ladder. A supercharger is like a person climbing up a ladder from the bottom. His goal is to only climb a certain way in total distance. The turbocharger is like climbing up a ladder to a fixed elevation only it doesn’t matter if you startedout on the ground or 10 feet under ground. You still climb to the same spot. The total gain is different and calibrated to a fixed, known location. Got it! This is why you use SAE corrections for naturally aspirated and mechanically supercharged engines but not for exhaust driven turbocharged engines. Correction factors for turbocharged vehicles will basically be the same as giving you some free boost. That's cheating the numbers. It may be great way to sell more product but it isn't an accurate representation of how much power you put down. The greater the altitude change, the more inaccurate it becomes.
In reality, there is some differences that offset the effect of turbochargers holding boost at higher altitudes. First off, the turbo is working harder since it has to spin faster. This creates more heat. we also have an average loss in temperature of 3 degrees F over every thousand feet in elevation rise. While these will affect the final numbers from sea level a small amount, they are nowhere near as off as the SAE correction factor for turbocharged engines at altitude. We may also run into the problem of the turbo getting too far outside it's efficiency range spinning at these speeds. Differently sized turbos will have different efficiencies so we can't jsut get a set standard for this either.
The next time someone tells you that you need to use SAE correction for a turbocharged engine because it is the "standard", laugh at them, tell them to go do their homework and to just go ahead and print up your uncorrected dyno sheet (turbo cars) so you can leave.
http://www.rx8club.com/attachment.ph...3&d=1108100716
EDIT for cliff notes: SAE correctio for turbo charged cars is not correct because turbo's run off wastegates and a set boost pressure. This pressure will be reached regardless of elevation. Spring pressures don't change. N/A and supercharged cars WILL lose that power because they are set up and designed to run a certain % of air more than what the car is actually bringing in.
03-06-2008, 10:34 AM
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Re: Is this true. SAE correction for turbo cars is BS?
If anyone read the big post above, you should now read this so you can get the corrections.
So grab a beer, and get comfy. Cliff notes at the bottom, if you really don't care enough to find out why this guy is wrong.
Lol.. Speaking of doing your homework, that guy needs to spend more time on his.
Seriously. I live at 5000 ft (4978 actually), I live this shit every day. That guy sucks at math apparently or doesn't know how a boost gauge works or something like that. He defines boost and then uses it incorrectly, I don't know how that happens..
This guy is wrong. I understand where his head is, but he's got some serious flaws in his explanation.
His physics is fine (well, sort of), his reasoning and logic is screwed up. I can't quite figure out how you would get 20.7 psi absolute (14.7 + 6) into an engine while reading local atmosphere + 6 psi on a boost gauge. Those numbers don't add up except at sea level (where local is 14.7).
Ok, some brief background.. There are three common types of pressure gauges, which are:
Differential pressure gauges measure the difference between two pressure ports (say a pair of tube fittings).. This is what is commonly used to measure pressure drop across an intercooler, etc (with a single gauge).
Gauge pressure is the same principle, but it measures the difference between ambient and a pressure port (tube fitting, etc). This is what boost pressure sensors usually are, whether they are mechanical or electronic (piezo). ECUs sometimes just do math between the barometric sensor and the manifold sensor, but the result is the same. They still usually measure with respect to local pressure, not absolute pressure.
A mechanical boost gauge has the pressure port on one side of the diaphragm (or whatever controls the spring travel) a port for atmosphere (usually just a hole without a fitting) on the other. The boost reading is the difference between the two. Don't believe me? A boost gauge should read 0 psi with the car off, because there is no difference between local atmospheric pressure and the input (boost) reading.
Absolute pressure measures the difference between a vacuum reference and the pressure port. Many barometric pressure sensors work like this, so they actually measure pressure regardless of altitude and can change ECU tuning as you climb the Rockies or whatever.. For example, an electronic (piezo) based absolute pressure sensor would have a pressure port on one side of the piezo, and a reference vacuum on the other side of the piezo. The sensor outputs the difference between the two. A mechanical absolute sensor works the same, but it has the pressure port on one side of the diaphragm (or whatever controls the spring travel) and vacuum (or some other sealed reference pressure) on the other. This is why at sea level (1 atmosphere) an absolute gauge will read 14.7 psi, and a boost gauge will read 0 psi.
Now, for the important stuff..
Where I'm going with this is that most boost gauges are measuring the difference between the local atmospheric pressure and the manifold pressure. Boost controllers are the same way. This guy's post only halfway makes sense if the only boost regulation that you have is a mechanical wastegate with NO external control from the ECU (or boost controller) or otherwise, AND if the wastegate is only referenced to sea level pressure.. If the ECU or boost controller is measuring 6 psi or whatever, it's usually 6PSI in relation to the local pressure, so the ECU or boost controller will regulate boost based on that.
So about the wastegate being referenced to sea level...
A simple mechanical wastegate with no external control only cares about the spring stiffness and nothing else, but there is still no seal on the other side of the diaphragm so they are still referenced to local atmospheric pressure.. So if you have a 6 psi wastegate spring, you need to be 6 psi above ambient pressure to get the actuator to open. If you change altitude, you still need to be 6 psi above that altitude for the wastegate to open. The wastegate really doesn't give a shit about anything other than the differential pressure across it. Some wastegates use another pressure signal, not atmosphere, but they still take a difference between the two. The other half of the equation is the boost gauge though, so keep reading.
So anyway, regardless of what the turbo is putting out for boost, the boost gauge will read that. If the turbo puts out 10 psi, the gauge reads 10 psi. You can't put out 6 psi on the gauge and more than 6 psi at the turbo, or else the gauge isn't actually reading boost (and I don't really know what it would be reading at that point). You can't have one definition of boost for a turbo and another for a gauge; they are both referenced to the same thing.
If you want to know what the turbo is actually putting out in relation to sea level pressure, your boost gauge needs to read absolute pressure, and then it wouldn't be a boost gauge, it would be an absolute pressure gauge instead.
How can you read 6 psi boost and have 20.7 psi into the engine when you're at 12.23 psi (absolute) locally? Something is missing; around 2.47 psi just disappeared into the aether.
20.7 - 6 psi is 14.7 (hey look, sea level pressure) instead of 12.23 (which is the pressure at 5000 ft). For clarity, 12.23 + 6 psi is 18.23 psi into the engine, obviously less power than with 20.7 psi.
Under this guy's logic, say I'm making 20.7 psi absolute at 5000 ft. This means that my boost gauge should actually read (20.7 - local atmo) = (20.7 - 12.23) = 8.47 psi NOT (by his strange arithmetic) 6 psi of boost.. Anyone see a problem there? Simple subtraction anyone? See what I mean about not knowing how a boost gauge works?
In reality (and I'm taking the numbers right from his post) the reason I'm only getting 18.23 psi absolute (which is 12.23 + 6 psi) into the engine is because my ECU regulates to 6psi over ambient, not 6psi with relation to sea level. That's not to say that I can't make the same amount of power that I would at sea level, it just means that now I need to make 8.47 psi of boost at 5000 ft instead of the 6 psi of boost at sea level to get the same 20.7 psi total into the engine. So if you want to make that 20.7 psi, you have to turn up the boost 2.47 psi somehow.
If he was correct, my WRX would make a lot more power than it actually does at the 5000 ft altitude where I live, because I would have the same amount of total pressure into the engine as at sea level, with the same boost pressure.
All of his logic also assumes that a turbo can actually make up the pressure difference at a different altitude, and that it isn't getting outside of it's efficiency range trying to do so. Asking a turbo to make just a few more psi at altitude isn't always going to happen, and it isn't necessarily going to be linear if it does. That's beside the point though, because there are different flavors of turbos on different engines.
So my real problem with his whole explanation:
Does this guy know that pressure at whatever altitude is based on air density (and gravity)? If you have 20.7 psi absolute going into an engine, you'll make the same power regardless of altitude (given the same intake air temp, of course). That just means that every foot above sea level you go, that 20.7 psi is now higher pressure in relation to your local atmospheric pressure. So....
Every psi that you lose from altitude, you have to add back in boost to get the same power output, so that's one more psi on a boost gauge, not the same psi on a boost gauge. Actually, it isn't that linear, but you get the idea.
By the way, I googled and found the original post, just in case anyone wants to see it in the flesh and look at the altitude chart. It's from 2005 though, so maybe he's learned how wrong he is by now. Or maybe he doesn't care and likes spreading misinformation like it's the truth.
http://www.rx8club.com/showthread.ph...ferrerid=23589
Cliff Notes: So which is it? 20.7 psi into the engine or 6 psi on the boost gauge?
Only one can be correct at anything other than sea level. You can't make sea level power at a different altitude with the same amount of boost. Period.
Eventually I'll edit this enough so that it will be longer than the original post..
By the way, it turns out that the SAE knows a few things when it comes to engines and all things automotive. Probably more than some guy on an RX8 forum.
So grab a beer, and get comfy. Cliff notes at the bottom, if you really don't care enough to find out why this guy is wrong.
Lol.. Speaking of doing your homework, that guy needs to spend more time on his.
Seriously. I live at 5000 ft (4978 actually), I live this shit every day. That guy sucks at math apparently or doesn't know how a boost gauge works or something like that. He defines boost and then uses it incorrectly, I don't know how that happens..
This guy is wrong. I understand where his head is, but he's got some serious flaws in his explanation.
His physics is fine (well, sort of), his reasoning and logic is screwed up. I can't quite figure out how you would get 20.7 psi absolute (14.7 + 6) into an engine while reading local atmosphere + 6 psi on a boost gauge. Those numbers don't add up except at sea level (where local is 14.7).
Ok, some brief background.. There are three common types of pressure gauges, which are:
Differential pressure gauges measure the difference between two pressure ports (say a pair of tube fittings).. This is what is commonly used to measure pressure drop across an intercooler, etc (with a single gauge).
Gauge pressure is the same principle, but it measures the difference between ambient and a pressure port (tube fitting, etc). This is what boost pressure sensors usually are, whether they are mechanical or electronic (piezo). ECUs sometimes just do math between the barometric sensor and the manifold sensor, but the result is the same. They still usually measure with respect to local pressure, not absolute pressure.
A mechanical boost gauge has the pressure port on one side of the diaphragm (or whatever controls the spring travel) a port for atmosphere (usually just a hole without a fitting) on the other. The boost reading is the difference between the two. Don't believe me? A boost gauge should read 0 psi with the car off, because there is no difference between local atmospheric pressure and the input (boost) reading.
Absolute pressure measures the difference between a vacuum reference and the pressure port. Many barometric pressure sensors work like this, so they actually measure pressure regardless of altitude and can change ECU tuning as you climb the Rockies or whatever.. For example, an electronic (piezo) based absolute pressure sensor would have a pressure port on one side of the piezo, and a reference vacuum on the other side of the piezo. The sensor outputs the difference between the two. A mechanical absolute sensor works the same, but it has the pressure port on one side of the diaphragm (or whatever controls the spring travel) and vacuum (or some other sealed reference pressure) on the other. This is why at sea level (1 atmosphere) an absolute gauge will read 14.7 psi, and a boost gauge will read 0 psi.
Now, for the important stuff..
Where I'm going with this is that most boost gauges are measuring the difference between the local atmospheric pressure and the manifold pressure. Boost controllers are the same way. This guy's post only halfway makes sense if the only boost regulation that you have is a mechanical wastegate with NO external control from the ECU (or boost controller) or otherwise, AND if the wastegate is only referenced to sea level pressure.. If the ECU or boost controller is measuring 6 psi or whatever, it's usually 6PSI in relation to the local pressure, so the ECU or boost controller will regulate boost based on that.
So about the wastegate being referenced to sea level...
A simple mechanical wastegate with no external control only cares about the spring stiffness and nothing else, but there is still no seal on the other side of the diaphragm so they are still referenced to local atmospheric pressure.. So if you have a 6 psi wastegate spring, you need to be 6 psi above ambient pressure to get the actuator to open. If you change altitude, you still need to be 6 psi above that altitude for the wastegate to open. The wastegate really doesn't give a shit about anything other than the differential pressure across it. Some wastegates use another pressure signal, not atmosphere, but they still take a difference between the two. The other half of the equation is the boost gauge though, so keep reading.
So anyway, regardless of what the turbo is putting out for boost, the boost gauge will read that. If the turbo puts out 10 psi, the gauge reads 10 psi. You can't put out 6 psi on the gauge and more than 6 psi at the turbo, or else the gauge isn't actually reading boost (and I don't really know what it would be reading at that point). You can't have one definition of boost for a turbo and another for a gauge; they are both referenced to the same thing.
If you want to know what the turbo is actually putting out in relation to sea level pressure, your boost gauge needs to read absolute pressure, and then it wouldn't be a boost gauge, it would be an absolute pressure gauge instead.
At 5000 ft above sea level we need to figure out what 40.8% greater than ambient is. 12.23 X 1.408 = 17.22 psi absolute. That’s a loss of 16.81% pressure from sea level. This is what a mechanically driven supercharger will yield if is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 2.52 psi while a turbo’s will still read 6 psi. A turbo has the advantage by 3.48 psi. A naturally aspirated engine loses 16.8% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
since the wastegate is calibrated to sea level or a fixed spring pressure.
20.7 - 6 psi is 14.7 (hey look, sea level pressure) instead of 12.23 (which is the pressure at 5000 ft). For clarity, 12.23 + 6 psi is 18.23 psi into the engine, obviously less power than with 20.7 psi.
Under this guy's logic, say I'm making 20.7 psi absolute at 5000 ft. This means that my boost gauge should actually read (20.7 - local atmo) = (20.7 - 12.23) = 8.47 psi NOT (by his strange arithmetic) 6 psi of boost.. Anyone see a problem there? Simple subtraction anyone? See what I mean about not knowing how a boost gauge works?
In reality (and I'm taking the numbers right from his post) the reason I'm only getting 18.23 psi absolute (which is 12.23 + 6 psi) into the engine is because my ECU regulates to 6psi over ambient, not 6psi with relation to sea level. That's not to say that I can't make the same amount of power that I would at sea level, it just means that now I need to make 8.47 psi of boost at 5000 ft instead of the 6 psi of boost at sea level to get the same 20.7 psi total into the engine. So if you want to make that 20.7 psi, you have to turn up the boost 2.47 psi somehow.
If he was correct, my WRX would make a lot more power than it actually does at the 5000 ft altitude where I live, because I would have the same amount of total pressure into the engine as at sea level, with the same boost pressure.
All of his logic also assumes that a turbo can actually make up the pressure difference at a different altitude, and that it isn't getting outside of it's efficiency range trying to do so. Asking a turbo to make just a few more psi at altitude isn't always going to happen, and it isn't necessarily going to be linear if it does. That's beside the point though, because there are different flavors of turbos on different engines.
So my real problem with his whole explanation:
Does this guy know that pressure at whatever altitude is based on air density (and gravity)? If you have 20.7 psi absolute going into an engine, you'll make the same power regardless of altitude (given the same intake air temp, of course). That just means that every foot above sea level you go, that 20.7 psi is now higher pressure in relation to your local atmospheric pressure. So....
Every psi that you lose from altitude, you have to add back in boost to get the same power output, so that's one more psi on a boost gauge, not the same psi on a boost gauge. Actually, it isn't that linear, but you get the idea.
By the way, I googled and found the original post, just in case anyone wants to see it in the flesh and look at the altitude chart. It's from 2005 though, so maybe he's learned how wrong he is by now. Or maybe he doesn't care and likes spreading misinformation like it's the truth.
http://www.rx8club.com/showthread.ph...ferrerid=23589
Cliff Notes: So which is it? 20.7 psi into the engine or 6 psi on the boost gauge?
Only one can be correct at anything other than sea level. You can't make sea level power at a different altitude with the same amount of boost. Period.
Eventually I'll edit this enough so that it will be longer than the original post..
By the way, it turns out that the SAE knows a few things when it comes to engines and all things automotive. Probably more than some guy on an RX8 forum.
Last edited by Fabrik8; 03-06-2008 at 09:04 PM.
03-06-2008, 01:24 PM
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Re: Is this true. SAE correction for turbo cars is BS?
Designing piezo sensors for work, I was gonna chime in and say the same things but I think fabrik8 said them better than I could have. If anyone has questions to exactly how the sensors work, ask away.
03-06-2008, 01:38 PM
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Re: Is this true. SAE correction for turbo cars is BS?
Ok, lets fix this:
Wastegates aren't referenced to sea level.
Now you can see the effect of altitude on turbo cars, correctly.
I've corrected the important parts of the math so the absolute pressure is what it should be..
Wastegates aren't referenced to sea level.
Now you can see the effect of altitude on turbo cars, correctly.
I've corrected the important parts of the math so the absolute pressure is what it should be..
Originally Posted by that RX8 Forum Guy
The point of this is to educate on how altitude plays a role on how much air can enter your engine. You really need to look at the attached spreadsheet to understand how these numbers work. For comparison sake, I am going to just assume 100% efficency and not factor in any losses to backpressure, heat, etc.
The purpose of forced induction is to get more air into the engine so we can make more power. We commonly refer to this as boost. It is typically measured in pounds per square inch (psi). It is a misconception that at sea level we are at 0 psi. This isn’t outer space! We are actually at or near 14.7 psi. This varies a little depending on weather conditions so just assume a perfect day by the ocean. When we refer to boost, we want to know how much pressure we are running over this amount. Therefore 6 psi of boost is 14.7 + 6 = 20.7 psi ambient. Everything is referenced to ambient pressure at sea level.
At 1000 ft above sea level we need to figure out what 40.8% greater than ambient (14.18 psi) is. 14.18 X 1.408 (40.8%) = 19.96 psi. That’s a loss of 3.575% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). There is less air to begin with at higher altitudes therefore less compression. The percentage increase stays the same but the boost pressure does not. Your mechanically driven supercharger’s boost gauge will now read only 5.26 psi since it is set to sea level while a turbo’s will still read 6 psi. A turbo has the advantage by .74 psi.
A naturally aspirated engine loses 3.538% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.18 psi absolute or a 0% pressure loss since the wastegate is calibrated to sea level or a fixed spring pressure.
At 2000 ft above sea level we need to figure out what 40.8% greater than ambient (13.67) is. 13.67 X 1.408 = 19.247 psi absolute. That’s a loss of 7.02% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 4.55 psi while a turbo’s will still read 6 psi. A turbo has the advantage by 1.45 psi. A naturally aspirated engine loses 7.01% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 19.67 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 3000 ft above sea level we need to figure out what 40.8% greater than ambient is. 13.17 X 1.408 = 18.54 psi absolute. That’s a loss of 10.435% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 3.84 psi while the turbo’s will still read 6 psi. A turbo has the advatage by 2.16 psi. A naturally aspirated engine loses 10.41% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 19.16 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 4000 ft above sea level we need to figure out what 40.8% greater than ambient is. 12.7 X1.408 = 17.88 psi absolute. That’s a loss of 13.624% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 3.18 psi while the turbo’s will still read 6 psi. A turbo has the advantage by 2.82 psi. A naturally aspirated engine loses 13.606% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 18.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 5000 ft above sea level we need to figure out what 40.8% greater than ambient is. 12.23 X 1.408 = 17.22 psi absolute. That’s a loss of 16.81% pressure from sea level. This is what a mechanically driven supercharger will yield if is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 2.52 psi while a turbo’s will still read 6 psi. A turbo has the advantage by 3.48 psi. A naturally aspirated engine loses 16.8% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 18.23 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
The purpose of forced induction is to get more air into the engine so we can make more power. We commonly refer to this as boost. It is typically measured in pounds per square inch (psi). It is a misconception that at sea level we are at 0 psi. This isn’t outer space! We are actually at or near 14.7 psi. This varies a little depending on weather conditions so just assume a perfect day by the ocean. When we refer to boost, we want to know how much pressure we are running over this amount. Therefore 6 psi of boost is 14.7 + 6 = 20.7 psi ambient. Everything is referenced to ambient pressure at sea level.
At 1000 ft above sea level we need to figure out what 40.8% greater than ambient (14.18 psi) is. 14.18 X 1.408 (40.8%) = 19.96 psi. That’s a loss of 3.575% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). There is less air to begin with at higher altitudes therefore less compression. The percentage increase stays the same but the boost pressure does not. Your mechanically driven supercharger’s boost gauge will now read only 5.26 psi since it is set to sea level while a turbo’s will still read 6 psi. A turbo has the advantage by .74 psi.
A naturally aspirated engine loses 3.538% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 20.18 psi absolute or a 0% pressure loss since the wastegate is calibrated to sea level or a fixed spring pressure.
At 2000 ft above sea level we need to figure out what 40.8% greater than ambient (13.67) is. 13.67 X 1.408 = 19.247 psi absolute. That’s a loss of 7.02% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 4.55 psi while a turbo’s will still read 6 psi. A turbo has the advantage by 1.45 psi. A naturally aspirated engine loses 7.01% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 19.67 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 3000 ft above sea level we need to figure out what 40.8% greater than ambient is. 13.17 X 1.408 = 18.54 psi absolute. That’s a loss of 10.435% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 3.84 psi while the turbo’s will still read 6 psi. A turbo has the advatage by 2.16 psi. A naturally aspirated engine loses 10.41% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 19.16 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 4000 ft above sea level we need to figure out what 40.8% greater than ambient is. 12.7 X1.408 = 17.88 psi absolute. That’s a loss of 13.624% pressure from sea level. This is what a mechanically driven supercharger will yield if it is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 3.18 psi while the turbo’s will still read 6 psi. A turbo has the advantage by 2.82 psi. A naturally aspirated engine loses 13.606% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 18.7 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
At 5000 ft above sea level we need to figure out what 40.8% greater than ambient is. 12.23 X 1.408 = 17.22 psi absolute. That’s a loss of 16.81% pressure from sea level. This is what a mechanically driven supercharger will yield if is designed to provide 40.8% more air (6 psi at sea level). Your mechanically driven supercharger’s boost gauge now reads 2.52 psi while a turbo’s will still read 6 psi. A turbo has the advantage by 3.48 psi. A naturally aspirated engine loses 16.8% pressure at this same elevation over sea level. An exhaust driven turbocharged engine will get a total of 18.23 psi absolute or a 0% pressure loss
since the wastegate is calibrated to sea level or a fixed spring pressure.
Last edited by Fabrik8; 03-06-2008 at 03:09 PM.
03-06-2008, 01:52 PM
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03-06-2008, 09:08 PM
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Re: Is this true. SAE correction for turbo cars is BS?
Something to think about..
It is possible to make a turbo setup that would make constant pressure any any altitude (up to a certain point of course). All it takes is for the ECU (or boost controller) to keep increasing boost as the altitude increases, so you always have the same amount of total pressure going into the engine. But then, why not have all that extra power at sea level instead, instead of having all that extra headroom that you're not using when you're at sea level???
Kind of like the M button on the M6, which cuts 100HP off of the engine. Why do you actually need that??? My right foot power limits an engine just fine..
It is possible to make a turbo setup that would make constant pressure any any altitude (up to a certain point of course). All it takes is for the ECU (or boost controller) to keep increasing boost as the altitude increases, so you always have the same amount of total pressure going into the engine. But then, why not have all that extra power at sea level instead, instead of having all that extra headroom that you're not using when you're at sea level???
Kind of like the M button on the M6, which cuts 100HP off of the engine. Why do you actually need that??? My right foot power limits an engine just fine..
Last edited by Fabrik8; 03-06-2008 at 09:10 PM.