Here's a curve for ya. Not completely off target (pun intended). When I was an F.O. in the service, the Barometric pressure would really make a difference in the artillary explosions at the range. When it was really humid and muggy, it would be much louder and you could feel more blast. The 8&quot; howitzers would really wake you up. The Fire Direction Center had to factor in the weather with the data we sent them, to tell the guns where to shoot.<br><br>Just wondering if its AIR density that affects the boost, or lack of oxygen in the air at higher elevations that makes less power, meaning same boost, less O2, which means less burned fuel, more smoke? Slow night, just thinkin out loud, heck maybe I'll even learn something.

 

One thing to understand, when your gauge says 20psi, it's not 20psi absolute, it's 20psi more than the atmos. pressure at the elevation you're at. I think that may help some understand how this works.

If I'm at sea level (14.7) and making 20psi, I'm making 34.7 psi absolute. If I'm at 10,000ft (I have no idea what the atmos is there, but we'll say 10) and my gauge says 20 I'm making 30psi absolute. So, you are making less boost, but not the way most people were thinking about it.

Here's some more to think about - the way I take how some of you are explaining the turbo having to 'work harder', I think to follow your logic you would have to say that the motor is not completely filling it's cylinders every stroke? Follow me here, no matter what the air density is, we all need to agree that the motor still displaces 5.9L of air. It still expells 5.9L of air. The volume of air moving thru the turbo will still be comparable, right?
So the way I understand it, the turbo still has plenty of food to chew on, it's just less nutrious food. But the turbo doesn't need the nutrition, the motor does. So, the turbo doesn't care what the O2 content of the air is, and is uneffected by it. It's only effected by the volume of air moving thru it.

Now, why do you fog out the campground, well, cuz up there, for a given volume of air, there's less of everything in it, including O2, so you just got a rich condition. That would happen if you had no turbo at all.

 

here's something I didn't realize: I thought boost gauges were referenced to the atmos, like a mech. fuel pump is. My brother says no, just calibrated for 14.7. So, that means, when you're in the mountains, and say atmo is 12, you have to make 2.7psi of boost b4 your gauge will even move. So in the mountains you're gauge will always read lower than at sea level.<br><br>I didn't know that, maybe y'all did.

 

If this were the case, you would be able to make full boost by holding the accelerator to the floor in neutral. :P This is not the case because there is very little fuel with no load. Little fuel = little temperature = little drive pressure = no boost. You need to take into account the effect of temperature and pressure on flow.<br> Not necessarily. If you have a wastegate and you are below the rated altitude, the turbo will still be able to make the same boost pressure as at sea level.<br><br>Are we having fun yet? ;D

 

good point at first.....what's the formula for drive pressure, if there is one?


Not so. The gauges are calibrated to read 0psi at 14.7 (sea level) . They are not atmos. referenced but the wastegate is atmos. referenced. So if you're up in the air at atmos 12, your gauge should actually be reading -2.7psi, that is, you need to make 2.7psi of boost to get back to 0 on the gauge. So at 0psi on the gauge, your motor is actually producing 2.7psi of boost, right?

 

I don't know if there's a way to calculate drive pressure.<br><br>Wastegate actuators have a diaphragm and a really strong spring (more than I can compress with my hands). One side of the diaphragm sees atmospheric pressure and has the spring, the other side sees boost. The area of the diaphragm is small enough and the spring is strong enough that I don't think atmospheric pressure has much influence on the wastegate function. <br><br>If zero on the gauge is 14.7 psi absolute, then yes, you would need to make 2.7 psi of boost if the atmospheric pressure is 12. However, I think you need to get up around 15,000 feet to get that low of pressure.

 

the wastegate (and lift pump) are 'referenced', so yes, they will work correctly at elevation, it is the gauge that will be inacurate.