AFR effects on power
11-20-08, 07:55 PM
#27
This is a misconception, if your running 12 pounds of boost at sea level you have an absolute pressure of 14.7 + 12 = 26.7psi, at 4500 feet your absolute pressure is 12.5 + 12 = 24.5psi. To make the same power at 4500 feet as at sea level he would have to run 2.2psi more. Likewise his equivalent at sea level is 9.8psi, and i think most of us would be comfortable running at 12.5afr under 10psi of boost.
11-20-08, 08:04 PM
#28
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Wouldn't it depend on your map sensor? or how you are reading boost? Example, I use a GM sensor on my haltech e11v2 that is calibrated for sea level, and without the engine running I get ~-1.4psi. (since I am above sea level quite a bit) My boost gauge from prosport gauges calibrates itself to 0 each time it boots up, and shows 0 vac/boost with the engine off.
Last edited by TweakGames; 11-20-08 at 08:07 PM.
11-21-08, 10:58 AM
#29
Mechanical boost gauges will be affected by altitude. Map sensors will not regardless. The only thing that changes is that your turbo works harder at higher altitudes to make the same pressure but 12 psi = 12 psi on a map sensor which is what matters in tuning/safety. Your boost gauge can say 30 psi for all i care.
thewird
thewird
11-21-08, 12:14 PM
#30
The increase got smaller as the boost went up, but was still easy to pickup 30-40whp going from low 10's to low 11's
11-21-08, 04:19 PM
#31
Mechanical boost gauges will be affected by altitude. Map sensors will not regardless. The only thing that changes is that your turbo works harder at higher altitudes to make the same pressure but 12 psi = 12 psi on a map sensor which is what matters in tuning/safety. Your boost gauge can say 30 psi for all i care.
thewird
thewird
The turbo will work harder to move the same mass of air at higher altitudes, not to make the same pressure.
11-21-08, 07:00 PM
#32
True the MAP doesnt care about the altitude, all it does is read absolute pressure and put out a voltage. How you calibrate your ECU does care about altitude, not everyone calibrates for sea level... If you live at 4500 feet and you ECU is calibrated for 0vac/press with the motor off then when it reads 12psi there is less mass of air than if you have the same scenario at sea level. 12psi isnt always 12 psi, but 26.7 psi absolute is always 26.7psi absolute. My point is depending how you tune and adjust boost the altitude can play a factor.
The turbo will work harder to move the same mass of air at higher altitudes, not to make the same pressure.
The turbo will work harder to move the same mass of air at higher altitudes, not to make the same pressure.
thewird
11-22-08, 12:10 AM
#33
If your map sensor is reading 12 psi at sea level and 12 psi at 5000 feet, your engine will be operating exactly the same with the exception of your turbo spinning a little faster. My point is, its not less or more safe at high altitudes for a turbo boosted car using a map sensor.
thewird
thewird
As an example take a properly setup/calibrated car at sea level with a 12psi wastegate spring as the sole form of boost control, run the car and log boost, you will log 12psi. Now drive the car to 5000 feet and log boost you will log less than 12psi on the ecu even tho the turbo is still making 12psi.
Likewise fill up a ball to a specific pressure at 5000 feet, take the ball down to sea level and the ball will have less pressure.
Depending on your electronics, setup, and tuning, elevation is something to think about.
11-22-08, 08:43 AM
#34
I see where your coming from, there are a bunch of 'ifs'... If you keep the same calibration in your map sensor from sea level to 5000 feet, and only if your boost is controlled by an EBC that targets absolute pressure (or the ECU controlls boost)... If you were to shut off the ignition to read static pressure then you will see a negative reading, so once you overcome the negative reading (which in reality is positive pressure) and push the car up to 12psi the actual pressure is more.
As an example take a properly setup/calibrated car at sea level with a 12psi wastegate spring as the sole form of boost control, run the car and log boost, you will log 12psi. Now drive the car to 5000 feet and log boost you will log less than 12psi on the ecu even tho the turbo is still making 12psi.
Likewise fill up a ball to a specific pressure at 5000 feet, take the ball down to sea level and the ball will have less pressure.
Depending on your electronics, setup, and tuning, elevation is something to think about.
As an example take a properly setup/calibrated car at sea level with a 12psi wastegate spring as the sole form of boost control, run the car and log boost, you will log 12psi. Now drive the car to 5000 feet and log boost you will log less than 12psi on the ecu even tho the turbo is still making 12psi.
Likewise fill up a ball to a specific pressure at 5000 feet, take the ball down to sea level and the ball will have less pressure.
Depending on your electronics, setup, and tuning, elevation is something to think about.
I know there are differences, but I'm just talking specifically about safety in changes in altitude. Now if your change in altitude is causing you to overboost and your ECU was never tuned for that pressure, that would be a problem obviously. But the tunes should usually throw in a bunch of fuel at higher pressures which have not been tuned.
thewird
11-22-08, 01:38 PM
#35
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I agree. Boost gauges read in PSI above ambient atmospheric pressure. So if he is running 12psi at high altitude, this would be 12psi above the atmospheric pressure at 4500 ft which is 12.5 psi. Since atmospheric pressure at sea level is 14.7 psi, which is 2.2 psi higher, 12psi at 4500 ft is equal to 10.2 psi at sea level.
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