Altitude affecting boost pressure?
08-27-2004, 06:45 PM
#1
1.5 BAR
Thread Starter
Join Date: Mar 2003
Posts: 1,190
Altitude affecting boost pressure?
I posted this on the Uberdata board, but it's really a forced induction question and not EM related. Let's see what you think:
EDIT: this might be easier to understand if I say that when my MAP sensor is seeing 10 psi, at my altitude, it means my turbo is operating at 14 psi relative to surrounding air pressure...
I've been thinking recently about why my boost guage and my map sensor readings are so different, and I think I figured it out. Somebody tell me if i'm way off here.
The map sensor gives an absolute pressure reading, and the boost guage (at least the cheapy ones) give a relative pressure reading. I believe the wastegate also operates based off of a relative pressure reading. Our altitude here is over 5000 feet, so that would explain why my unmodified internally wastegated Subaru turbo hits a consistent 7psi, and that's exactly what my boost guage says, but my SAFC mBar reading peaks at 0.33 bar. (I have the SAFC installed just to monitor, it isn't correcting any map values) The wastegate is releasing NOT at 7psi as compared to sea level pressure, it is releasing at 7psi as compared to our ambient air pressure at 5000 feet (which means my engine is only seeing roughly 4.8 psi compared to sea level).
Which leads me to my next hypothesis. If a stock honda 4cyl. D series engine can handle 10-11 psi of boost pressure (if it's tuned well of course), assuming you're talking about 10-11 psi at sea level of course, then us folks up at 5000 feet altitude should be able to run approximately 14 psi, relative to our air pressure up here. This only works if your car will not travel to lower altitudes. If the map sensor is able to read absolute air pressure, then 14 relative psi at our altitude is equivalent to about 0.68 bar at sea level, or about 10 psi at sea level.
What do you guys think?
The map sensor gives an absolute pressure reading, and the boost guage (at least the cheapy ones) give a relative pressure reading. I believe the wastegate also operates based off of a relative pressure reading. Our altitude here is over 5000 feet, so that would explain why my unmodified internally wastegated Subaru turbo hits a consistent 7psi, and that's exactly what my boost guage says, but my SAFC mBar reading peaks at 0.33 bar. (I have the SAFC installed just to monitor, it isn't correcting any map values) The wastegate is releasing NOT at 7psi as compared to sea level pressure, it is releasing at 7psi as compared to our ambient air pressure at 5000 feet (which means my engine is only seeing roughly 4.8 psi compared to sea level).
Which leads me to my next hypothesis. If a stock honda 4cyl. D series engine can handle 10-11 psi of boost pressure (if it's tuned well of course), assuming you're talking about 10-11 psi at sea level of course, then us folks up at 5000 feet altitude should be able to run approximately 14 psi, relative to our air pressure up here. This only works if your car will not travel to lower altitudes. If the map sensor is able to read absolute air pressure, then 14 relative psi at our altitude is equivalent to about 0.68 bar at sea level, or about 10 psi at sea level.
What do you guys think?
08-27-2004, 11:05 PM
#2
1.0 BAR
Join Date: Mar 2003
Posts: 202
Re:Altitude affecting boost pressure?
I'm not exactly sure you're right. The wastegate pressure is determined partially by the internal spring. I'm not sure it's entirely relative. Not sure about the boost gauge. It makes sense that the MAP is absolute, you might be on to something.
Thread
Thread Starter
Forum
Replies
Last Post
