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It's time for another RG long crazy time consuming thread!!! Take your restroom breaks and go grab a snack.
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.
Below is the chart that shows the affect that altitude has on air pressure.
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"None of us on our own is as dumb as all of us combined."
Lots of info...The math seems to make sense, however sometimes we forget that extracting HP from cars via whatever method can be as much art as science. :D
I had a friend insist a 2" air intake tube on his KL-powered mazda would allow for increased velocity, and show better gains than my 3.5" intake piping. He presented lots of math to show why his idea would work.
After two weeks of driving, he dumped it for a larger intake. Maybe his math was off...who knows - I certainly do not; I haven't taken math since 10th grade in HS. I do know that sometimes...tuners just sit back and scratch their heads thinking 'how the f. did THAT happen?'.
Altitude effects on intake/boost pressure
:D 
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