<br><br>I've had my boost gauge in several weeks now, but i noticed about a 3-4 psi drop depending on the ambient air temp (&amp; maybe humidity). is this normal ? when i leave for work in the mornings (@ 6am), the boost builds without effort. When I come home in the evenings (mid nineties with high humidity lately), it seems to only want to build 20psi (I've gotten 24 psi on a good day, but i don't have to courage to go any further until I install the egt gauge). Is this normal?? thanx 4 your input

 

Cethane,<br><br>It's a normal occurance to build more boost and have much better spool up when it's colder outside. Our trucks get worse and worse the hotter and more humid it gets outside and when pouring in lots of fuel, they smoke a lot worse, too. My truck will see 43-44 psi on a cold day (40 degrees or lower), but I can only manage 40-41 psi when it's hot like you describe. <br><br>BTW, was good to meet you at Claremont.<br><br>John

 

The cool air is more dense and would tend to produce more boost, but thats just my opinion. <br><br> ramguy :

 

[quote author=ramguy link=board=9;threadid=16602;start=0#155783 date=1057162720]<br>The cool air is more dense and would tend to produce more boost, but thats just my opinion. <br><br> ramguy :<br>[/quote]<br><br>You hit the nail on the head there, cooler air will have more oxygen than warm air. This is why these trucks tend to feel doggy in warmer 90+ weather and smoke a little more, compared to nice 60-70* weather.

 

got 36 PSI boost out of mine this spring in 60 degree weather. but now that it is hot I only see 34 at best. I cant wait for this winter when we have a night under 20 degrees so I can see what she will really do. ;D

 

cool air is more dense, and will have more of everything in it, not just more O2.

Boost psi is also related to EGT. If you're EGT's are hotter for a given load and rpm, the gas is expanding more, which will make the turbine is spin faster, which will provide more boost

For those of you who change elevations while hauling with your trucks, what do you see happening to your boost psi? That is really the same issue if the explanation of others above is correct -- thinning air leads to less boost. I'm not sure I agree with that. It would seem to me that a truck should make roughly the same psi for a given load, rpm, and EGT. The density ratio of the charge air should be the thing changing.....

 

no one looking to hash this one out? I like the discussions that delve more into the theory and such

 

I agree, boost would not change. How oxygen rich the air was would change though. So, this leads me to believe that a turbo is working harder to maintain a certain psi at a higher altitude than it would at a lower altitude.

 

I agree with Monty. Did I say that? : ;D<br><br>At higher altitudes the turbo has to spin faster to create the same amount of boost pressure. Since the turbo is spinning faster it heats the incoming air more.

 

It seems to me that with a non-wastegated turbo your boost pressure is directly connected to how much power you're making. If the truck is capable of making 300 hp in the 65 degree morning due to the nice dense air, but only 280 hp in the 90 degree less dense air of the afternoon, it stands to reason that the afternoon boost reading would be lower. I think with altitude the chages will be similar. Add a wastegate to this and all of the previous statements go out the window since it's the wastegate controlling the boost. Then, like Stake says, the turbo will just spin faster to make the same boost as at a lower altitude. Obviously, there's a limit to this, go too high and you'll no longer be able to get enough pressure to open the wastegate. On my work truck (see sig) that's probably around 10,000 feet, because the engine is rated to make 300 hp at altitudes below 10,000 feet. The lowered boost should cause the computer to defuel, lowering the power output. Now how the heck did I get to here from a 1st gen turbo thread? :

 

The air is more dense when it is colder -- there is &quot;more of everything&quot; in it, e.g.: more molecules per volume. The turbo works more efficiently when the air is more dense, and allows for higher manifold pressure. Also, when it is colder, there is more BTU's per volume in the fuel, making for higher egt's and higher manifold pressures . . . I will try to post more on this if there is an interest, but I don't have time right now.<br><br>

 

I have to disagree with this. psi and density ratio are 2 different things. This is easy to see if you think about a non intercooled motor vs. an intercooled one. Both are fully capable of making 15psi for instance. But, all other things being equal, the intercooled truck is going to have a higher density ratio.

At 15psi (14.7) your theorical denisty ratio is 2.0. Our trucks don't work at the theoritical max of course tho, and ones with good aftermarket intercoolers probably run at best 1.7? without an intercooler 1.4-1.5 is probably more like it. So, you're building twice atmospheric pressure at the turbo, but due to the heating, you're only transmitting 1.5 atmos. to the intake.

I would argue that the truck will always be able to make the 15psi (again, for a given load and rpm), until some boundry point in elevation and heat where as StakeMan said, the turbo just can't work. I'm guessing that's a pretty far extreme however.

Now what elevation and temp do effect, is the density ratio, which has a direct effect on power, moreso than psi, as you hopefully now see.
We shouldn't be so concerned about what psi we can see on our gauge, we should be concerned with the density ratio at the intake. This mindset would give us a much better wholelistic view of the induction system and its efficiencies.
Running 30psi and actually being somewhere near a 2.7 density ratio would be a kick in the pants I'm sure! It would be very interesting to see the density ratios for some of our members running high boost.

 

Go from 5,000 to 10,000 feet and you will quickly realize the effect of altitude. So will people behind you.

 

ALTITUDE has a DIRECT effect on compression. I was sent to Mexico years ago to do start up on a compressor capable of 130 psi at sea level, we could only get 100-105 psi, so dummy me adjusted the pressure switch so it would shut down at 130. Called the office and told them &quot;how good I did&quot; the engineer almost had a duck, started screaming and cussing and reminded me I was at about 5000 ft. elavation, or so he said (I was wondering why i was out of breath after smoking a cig.), but backed the switch down and adjusted limits. Industry standards are 60* air, 29.90 barometric pressure, and always at sea level, in other words, not real world conditions, but altitude will effect it BIG time. M2CW

 

Am I getting this right?????.........<br><br>Air at sea level contains very specific levels of the normal components such as oxygen, carbon dioxide, etc etc. <br>As you go higher in altitude the density of the atmosphere drops, but the components are decreasing in equal proportion, based on density at altitude &quot;A&quot; and altitude &quot;B&quot;. So in effect you don't have a higher level of any one component.<br>So, if you are at eg: 10,000 feet, and the air density has droped to 50% of its sea level values (only an example) a turbo would have to work much harder to supply boost air that would likely &quot;just&quot; make it to sea level values. <br>Yes? No?<br><br>Now if you had 20 psi at sea level the turbo would have to spin much faster to come close to generating 20 psi at 10,000 feet, and even at that boost level, the atmospheric components would still be at reduced levels, resulting in &quot;running richer&quot; as you climb.<br>In my case I found that to be my experience.<br>Next, if at sea level you had low humidity and high ambient temps, you could theoretically find some &quot;compensation&quot; if at 10,000 feet you had much colder air such as is the case in a low pressure system bringing rain or fog. Those &quot;factors&quot; would (at least I'm guessing they would) improve the air density sufficiently to at least partially compensate/offset the altitude losses. <br>Yes? No??<br><br>When I have had my truck in the interior of the province which has alot of sem-arid/desert like conditions, I need to turn back some the fueling so that in camp the truck, at idle, doesn't haze out the ajoining 10 acres so to speak.<br><br>Or is that all out in left field????<br><br>bob.