Effects of elevation on boost pressure

The other day I was trying to figure out what the average atmospheric pressure was here in Denver. Originally I was trying to determine how hot I could get methanol before it boiled off (different project, unrelated to my Daytona) and then I realized that it would have an effect on where certain boost and power levels plot on a compressor map.

Let us take the T04E 46 trim compressor map (http://www.squirrelpf.com/turbocalc/...product_id=2&), since it is one I've been considering running on my daytona. At sea level, 14.7 psi of boost is at a 2.0 pressure ratio. Assuming this is flowing 25 lbs/min of air, that puts us right in the middle of the compressor map, where it is the most efficient. Now, after all my searching I have discovered that atmospheric pressure at 5280 ft is 12.1 psi. This probably varies +/- a few tenths of a psi around town due to the variation in elevation depending how close to the mountains you are. But we will assume 12.1 psi in Denver just like 14.7 is assumed for sea level - for the sake of easy math. So at 12.1psi atmospheric pressure, 14.7psi of boost is a higher pressure ratio (2.21), which moves us higher up the map. Still in that center island of efficiency. But this effect probably gets magnified the higher boost goes.

Let us say one is running 24.2 psi of boost, and getting 30 lbs/minute of airflow. That is a pressure ratio of 2.64 at sea level. This puts us right on the edge of that center efficieny island. However, in Denver, that is a pressure ratio of 3.0, which puts us out of the efficient section of the map and uncomfortably close to the surge line. Now since it is my understanding that cars gow slower on the same boost levels here at altitude than they do at sea level, which gives me the impression that not only do pressure ratios go up at altitude, but flow rates also drop because the the less dense air. This would make the shifting of points on the compressor map worse, because now with the point moving to the left instead of just up, it is moving closer to the surge line.

Now am I just getting all worked up about nothing because I don't understand what I am talking about, or is this something I should take into account when selecting a turbo and planning boost and power levels for my daytona? Is is possible that this is a non-issue because I am estimating lower airflow rates for a certain boost level than this turbo is likely to produce on our engines? Could some of the resident turbo gurus chime and and help set me straight, please?

Quote:

---

Originally Posted by Koreth

The other day I was trying to figure out what the average atmospheric pressure was here in Denver. Originally I was trying to determine how hot I could get methanol before it boiled off (different project, unrelated to my Daytona) and then I realized that it would have an effect on where certain boost and power levels plot on a compressor map.

Let us take the T04E 46 trim compressor map (http://www.squirrelpf.com/turbocalc/...product_id=2&), since it is one I've been considering running on my daytona. At sea level, 14.7 psi of boost is at a 2.0 pressure ratio. Assuming this is flowing 25 lbs/min of air, that puts us right in the middle of the compressor map, where it is the most efficient. Now, after all my searching I have discovered that atmospheric pressure at 5280 ft is 12.1 psi. This probably varies +/- a few tenths of a psi around town due to the variation in elevation depending how close to the mountains you are. But we will assume 12.1 psi in Denver just like 14.7 is assumed for sea level - for the sake of easy math. So at 12.1psi atmospheric pressure, 14.7psi of boost is a higher pressure ratio (2.21), which moves us higher up the map. Still in that center island of efficiency. But this effect probably gets magnified the higher boost goes.

Let us say one is running 24.2 psi of boost, and getting 30 lbs/minute of airflow. That is a pressure ratio of 2.64 at sea level. This puts us right on the edge of that center efficieny island. However, in Denver, that is a pressure ratio of 3.0, which puts us out of the efficient section of the map and uncomfortably close to the surge line. Now since it is my understanding that cars gow slower on the same boost levels here at altitude than they do at sea level, which gives me the impression that not only do pressure ratios go up at altitude, but flow rates also drop because the the less dense air. This would make the shifting of points on the compressor map worse, because now with the point moving to the left instead of just up, it is moving closer to the surge line.

Now am I just getting all worked up about nothing because I don't understand what I am talking about, or is this something I should take into account when selecting a turbo and planning boost and power levels for my daytona? Is is possible that this is a non-issue because I am estimating lower airflow rates for a certain boost level than this turbo is likely to produce on our engines? Could some of the resident turbo gurus chime and and help set me straight, please?

---

Your level of elevation will certainly play a key in what turbo you select. You have way less air up there to try and suck in and compress. Not sure what turbo you'd need, but I know if you have less air and are trying to make XXX HP at Y elevation vs the same XXX HP but at Z elevation you'll need something that has to work a little harder produce those same numbers.

Koreth,
I think you are pretty right on the money with the elevation and pressure ratios. However I know that the points do not shift to the left as theorized.


Frank

Well, it is an assumption, I'd have to actually test things on a dyno or something. To be clear, I've never actually observed a turbocharged vehicle be slower at higher altitudes, I've simply been told by others that they have observed such. The increase in pressure ratio obviously increases temperatures, as evidenced by the point moving into less efficient areas of the map. However, as long as the lbs/min of air doesn't change, I don't see how the power out put would be reduced, as there is still the same mass of air needed to burn the same mass of fuel, thus producing X power and Y torque.

Regardless it is good information to know that I should take my elevation into account when selecting a turbo.

Quote:

---

Originally Posted by Koreth

To be clear, I've never actually observed a turbocharged vehicle be slower at higher altitudes, I've simply been told by others that they have observed such.

---

Stock SRT-4s run around 14.0 at sea level right? Here in Mexico city at a little over 7000ft of altitude stock SRT-4s run 14.5-14.7s.

Would the unrelated reason have anything to do with making alky? I know that you would have to spin the turbo faster to make the same boost.

The other side of the HP equation your forgetting is the actual 02 content of the air at your altitude is less than sea level too. That means your have to pack way more available atmosphere into the engine to realize the actual air / fuel ratios you are targeting to make the HP goals your after. Remember that's why they started using turbos and superchargers on piston type military aircraft during the war. Allowed them a higher flight altitude ceiling. As long as the crew had oxygen. ;)

BTW... pressurizing the container your boiling the methanol in will raise the boiling temperature point. Roughly 1 psi per degree F gain. Or placing the container under a vacuum will reduce the required boiling temperature point. Depends on what your trying to accomplish with the experiment. In an open container, the juice will boil at a lower temp at your altitude than at sea level.