Engine/Transmission (1998.5 - 2002) Is All Boost Pressure Really The Same?

[Tom G]

Please Help Me Understand My Diesel A Little Better



I have this silly question about boost pressure. I know there are no silly questions but wait - this may be a first.



My truck normally runs at about 18. 5-19 lbs of boost at Wide Open Throttle (WOT) on steeper hills with our loaded 5er which I think is pretty typical. Exhaust Gas Temperatures (EGT) normally run about 1150-1250 degrees while the Cummins is working away in 3rd gear which I think is also typical. We just got back from a week in Colorado and climbed lots of mountains. Even at 14,000 feet my boost gage still showed 18. 5-19 lbs of boost. I seldom IF EVER hear the turbo at lower elevations but at anything over say 8,000 feet I can hear it happily singing away up front.



My question then is:



Is boost the same at all elevations? I mean is 19 lbs boost at 1000 feet the same as 19 lbs of boost at 14,000 feet AND



If the boost is the same is the fueling rate also the same regardless of altitude?



Really wondering

 

[BarryG]

TomG:

Good question. As someone who lives at 6300ft and travels the mountains frequently I have wondered the same thing. You would think that 19 is 19 (or whatever pressure) but the air is definitely less dense at altitude. It is not uncommon in the Colorado mountains to go over 10000 feet on the main roads in the mountains. Maybe the turbo just has to work harder to achieve the same pressure. You would also think that 19 psi at sealevel would have more air or at least denser air than at say 8000 ft. If the pressure is relative to the altitude then you are getting less air. As far as fueling I think the fueling is the same judging by the smoke I see going up the passes just not enough air to burn all the fuel. One of those makes you go Hmmmm questions. Come on engineer/physics type we know your out there help us clear this up.

 

[kaffine krazed]

No 19psi on a boost gauge at sea level and at 10,000ft are not the same. Gauge pressure is the diffrence between where the gauge is hooked up and atmosphreic pressure surrounding the gauge. To get absolute pressure from gauge pressure you add atmospheric pressure to gauge pressure.

 

[Mark_Kendrick]

Like kaffine krazed said . . if ambient pressure is 13psi at altitude and 14. 7 psi at sea level then 19 psi of boost is really 32 and 33. 7 psi of manifold pressure.

 

[andrewkoperdak]

TomG

I am going to stick my neck out here. Won't be the first time. In my humble opinion, 19 PSI read on your gauge will always be 19 psi whether or not you are at sea level or on K2. The difference between PSIG and PSI absolute is not important to your question. It used to matter when I was in the submarine force. There is a big difference between PSIG and Absolute. Anyway

your turbo must spin faster to make the same boost in the thinner atmosphere at altitude. It is in this way that the pressure is different. Your turbo has denser air at sea level so the boost comes up faster than at altitude. I don't know what the turbo RPM difference is but the turbo must spin faster to get the same boost as you go up in altitude. You do loose performance as you go up. This was a topic of discussion about 3 weeks back on the TDR threads

 

[SOOT]

19 PSIG is the same at sea level or 10,000 feet thats why the turbo is called an alttitude compensator, up to about 10,000 feet. Which means that manifold press will be the same. Anything above that and you run the risk of overspeeding the compressor wheel then it goes all directions:{ If I remember right at sea level the compressor wheel spins around 100,000 RPM's at its most efficient max boost. As the elevation increases the compressor wheel will spin faster to comensate for the less dense air to achieve the same level of boost.

 

[kaffine krazed]

No 19psig is not the same at sea level and at 10,000ft. Gauge pressure is comparing the manifold pressure to the atmoshpere pressure. At 10,000ft there is less atmospheric pressure than there is at sea level. To compare the boost pressures at diffrent altitudes you need to use absolute pressure wich is gauge pressure plus atmospheric pressure. If you where to get a scan tool and watch the boost sensor wich reads absolute pressure it will read lower at 10,000ft than at sea level. Yes a turbo charged engine will loose less power than a naturally aspriated engine at high altitudes because the NA engine is only getting atmosphric air pressure so for every psi drop in atmoshpric pressure from sea level you have a 6. 8% drop in the pressure going to the engine. With a turbo you have boost pressure and atmosphreic pressure so with 19psi boost pressure and 14. 7psi atmoshperic pressure at sea level you have a 2. 97% drop in pressure avalible to the eninge for every psi drop in atmosphric pressure.

 

[Mark_Kendrick]

You also have to consider the turbo maps. at altitude the max pressure of the turbo is reduced because the turbo works in pressure ratios not PSI.



if you make 40psi at sea level and make 40psi at altitude the exit temperature of the air will be much higher because the pressure ratios are different. the higher the ratio the higher the temps.



40/14. 7=2. 72

40/13=3. 08



this difference in ratio can make a 100* difference in ouput air temp. as the temp goes up the air expands so hotter air has considerably less oxygen. this is the reason that a HX35 making 40+ psi of boost has a diminishing HP return as boost increases. after 40psi you start to make more heat which counteracts the greater boost.



Later,

Mark

 

[emjay]

Kaffeine Krazed is absolutely right! Gauge pressure is relative to atmospheric pressure -- here's one way to prove it: Connect a hose to a gauge that has a scale of at least 15 PSI. Now take that same gauge and duct tape if to a shop vac hose covering all but the face and the end of the hose. Turn on the shop vac and watch the gauge rise! If the gauge disappears into the shop vac, try it again with an airtight mechanical means of preventing the gauge from getting sucked in.

 

[Tom G]

Wonderful Education

Thank you all for the responses and education (BarryG, KaffineKrazed, MarkKendrick, AKoperdak, MMclain, and emjay – hope I didn’t miss someone). So what did I learn? Well “Is All Boost Pressure Really The Same” NO IT ISN’T. It’s funny what you forget over time.



I had forgotten how altitude affects a pressure gage reading. BUT even this has its variables doesn’t it. If I go higher in altitude then my turbo is working harder (spinning faster) just to keep up. This of course adds more heat to the air but at least some of that extra is heat removed by the intercooler I would guess. But we must also remember that when you go up in altitude the outside air temperature goes DOWN by about 2½ to 3 degrees per 1,000 feet. At 10,000 feet you can be 30 degrees cooler so does that cancel out the pressure ratio increases at 19 lbs. of boost; I wonder? It makes one wonder; is the extra heat added by the faster spinning turbo about the same as the decrease in outside air temperature since it is cooler the higher you go? Also since the outside air is cooler the intercooler is more efficient at removing the extra heat; but wait; the air outside the intercooler is less dense so there are fewer air molecules going over the intercooler fins to do the cooling; so…? This question has a significant number of variables to consider doesn’t it?



The whole question is still complicated even more by the wastegate and it’s operation. Since it is hardlined does this act somewhat like an altitude compensator? Is the wastegate adding to the problem or giving me back some of my lost boost at altitude? Does the ECM somehow know that it is at 1,000 feet or 10,000 feet and compensates by adding or decreasing fuel?



I am smarter now because of all of your responses and thank you for sharing your knowledge.



Tom Garven



p. s. As recommended by emjay in his post; the gage DID get sucked into my Genie shop vac – broke the glass, stripped the brass threads, bent the needle and I had a really hard time finding it in all the dirt. But the good news is the gage was stuck on 20. 5 so I mounted it on the dash next to my Autometer gage. I did this of course to remind myself to NEVER again ask another silly question, LOL

 

[Whitmore]

Great question

as a comparison I can tell you that altitude makes a huge difference, lets talk about a naturally asperated engine, being an old motocross racer I spent most of my racing career at 5 to 7 k feet , I once went down to 2 k elevation and the bike was amazing there was ALOT of new found power, I bout had to learn how to ride all over again, a wild guess would be about 30% gain in that case... ... ..... Kevin