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      04-11-2010, 10:31 AM   #1
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Supercharging Discussion: Boost Pressure vs. Altitude

Nice numbers LV!!
Different cars can be just stronger from factory. We hve seen as much as 40 HP differences.
Roman gave me a 95mm pulley for
my kit due to my mountain goat altitude of 5000ft!
I'm running DCT to so hopefully it will get me close to 5,5psi
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      04-11-2010, 12:47 PM   #2
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Quote:
Originally Posted by Bob MG View Post
Nice numbers LV!!
Different cars can be just stronger from factory. We hve seen as much as 40 HP differences.
Roman gave me a 95mm pulley for
my kit due to my mountain goat altitude of 5000ft!
I'm running DCT to so hopefully it will get me close to 5,5psi
A centrifugal type supercharger does not provide absolute boost like a turbocharger, it provides relative boost due to the fact that the speed of the compressor is directly related to the engines rpm. So at 5000 feet your stock car has -2.5 psi "boost pressure" compared to sea level ( 0 psi ). To get out 5.5 psi @ 5000 feet your SC kit needs to make 8 psi boost at sea level. How do you manage those 8 psi once you really leave your altitude ?
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      04-11-2010, 12:57 PM   #3
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Quote:
Originally Posted by e.n335 View Post
A supercharger does not provide absolute boost like a turbocharger, it provides relative boost due to the fact that the speed of the compressor is directly related to the engines rpm. So at 5000 feet your stock car has -2.5 psi "boost pressure" compared to sea level ( 0 psi ). To get out 5.5 psi @ 5000 feet your SC kit needs to make 8 psi boost at sea level. How do you manage those 8 psi once you really leave your altitude ?
I know altitude scrubs off boost pressure, but I don't believe it to be that much??

I will not go to sea level, I live in the moutains.
If I decide to, I will put a bigger 100mm pulley on.

Based on what you are saying, I would need a 7psi pulley?

Roman I would love a comment...
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      04-11-2010, 01:09 PM   #4
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Quote:
Originally Posted by Bob MG View Post
I know altitude scrubs off boost pressure, but I don't believe it to be that much??

I will not go to sea level, I live in the moutains.
If I decide to, I will put a bigger 100mm pulley on.

Based on what you are saying, I would need a 7psi pulley?

Roman I would love a comment...
PM sent
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      04-11-2010, 03:39 PM   #5
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Quote:
Originally Posted by e.n335 View Post
A centrifugal type supercharger does not provide absolute boost like a turbocharger, it provides relative boost due to the fact that the speed of the compressor is directly related to the engines rpm.
A super provides "absolute boost" just the same as a turbo does. A CF blower and turbo are essentially the same except one runs off exhaust gasses and has a wastegate and the other runs off the crank. I don't follow the second part of the sentence. Due to speed of the compressor being related to the rpm? The rpm range is the same at sea-level or at 5000ft. The car will rev from idle to 8500rpm at both elevations. So every revolution of the crank will produce the same amount of boost and spin the same amount of air into the engine at sea-level and at altitude.

The difference being that the air is less dense up here. 17% less dense. So you are increasing the power from a much lower base. And the extra air you are spinning in is also deprived of oxygen compared to sea-level due to the air being thinner and less oxygen rich. So I think Roman will run a smaller pulley due to the engine being nowhere near the limit of power one gets to at sea-level. You are starting with a 50hp deficit so one can run more boost safely up at altitude.
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      04-11-2010, 03:51 PM   #6
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Quote:
Originally Posted by Bob MG View Post
Nice numbers LV!!
Different cars can be just stronger from factory. We hve seen as much as 40 HP differences.
Roman gave me a 95mm pulley for
my kit due to my mountain goat altitude of 5000ft!
I'm running DCT to so hopefully it will get me close to 5,5psi
your comments are always funny when are you gonna get yours?
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      04-12-2010, 08:34 AM   #7
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Quote:
Originally Posted by LV-E92M3 View Post
your comments are always funny when are you gonna get yours?
Being fitted as we speak
Roman should have my ECU in his hands at exactly 10h30 this morning, FedEx says its delivering then.
Two days back to me, so Thursday or Friday I can be reporting on far back my scrotum stretches on take off, lol

I will report on the altitude, and what size pulley eventually gets me to 5,5PSI.
btw what is your altitude at your location, and have you dynoed your car there, and what was the registered boost?
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      04-12-2010, 08:47 AM   #8
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Quote:
Originally Posted by M&M View Post
A super provides "absolute boost" just the same as a turbo does. A CF blower and turbo are essentially the same except one runs off exhaust gasses and has a wastegate and the other runs off the crank. I don't follow the second part of the sentence. Due to speed of the compressor being related to the rpm? The rpm range is the same at sea-level or at 5000ft. The car will rev from idle to 8500rpm at both elevations. So every revolution of the crank will produce the same amount of boost and spin the same amount of air into the engine at sea-level and at altitude. Incorrect

The difference being that the air is less dense up here. 17% less dense. So you are increasing the power from a much lower base. And the extra air you are spinning in is also deprived of oxygen compared to sea-level due to the air being thinner and less oxygen rich.

Air has less available Oxygen as the ambient pressure is less. The minute the air is compressed to the same pressure, the oxygen levels are the same. Air is 20% Oxygen / 80 nitrogen.

So I think Roman will run a smaller pulley due to the engine being nowhere near the limit of power one gets to at sea-level. You are starting with a 50hp deficit so one can run more boost safely up at altitude.
Engine power is not the issue we are chasing here. Boost pressure is. Once we achieve 5,5psi at high altitude, the engine will be receiving the same volume of oxygen is was receiving at sea level when the boost pressure was 5,5psi


engine RPM is directly related to blower RPM as its belt driven.

At sea level the air is denser and therefore the blower spinning at a particular RPM produces X amount of boost pressure.

At higher altitude, the air is less dense, and therefore the blower spinning at the same rpm as it was at sea level, now produces less boost pressure.

To compensate for the loss boost pressure, one must run a smaller pulley on the blower, thereby spinning the blower at a higher RPM, to achieve the same boost as it was previously making at sea level.
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      04-12-2010, 01:36 PM   #9
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Quote:
Originally Posted by Bob MG View Post
Engine power is not the issue we are chasing here. Boost pressure is. Once we achieve 5,5psi at high altitude, the engine will be receiving the same volume of oxygen is was receiving at sea level when the boost pressure was 5,5psi
Technically speaking, that is not true. When density changes, the volume remains the same.

But you are also compressing air that is less dense. As air gets thinner the molecules are further apart. So there is less oxygen per square inch at 5000ft than there is at oxygen rich sea-level. Ever wonder why climbers use oxygen masks at the top of Mount Everest? Each breath you take, you ingest the same amount of air you ingest at sea-level. But the air is pretty much useless as the 02 molecules are so sparse it doesn't feed you.


Quote:
Originally Posted by Bob MG View Post

engine RPM is directly related to blower RPM as its belt driven.

At sea level the air is denser and therefore the blower spinning at a particular RPM produces X amount of boost pressure.

At higher altitude, the air is less dense, and therefore the blower spinning at the same rpm as it was at sea level, now produces less boost pressure.
Ok on that point I agree with you. When density changes, pressure varies by:

P2 = P1 (D2/D1)

So a 5.5psi charger at 5000ft will produce = 5.5psi(850mbar/1000mbar)
= 4.675psi or 15% less boost.

So you would need a 15% smaller pulley to make the same boost.

But even if you get the same boost, the quality of air is not the same. So actually you will need an EVEN smaller pulley to compensate quantity for quality.

But hey, that's just my opinion. It's a great discussion with lots of grey areas si I am open for correction.
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      04-12-2010, 01:52 PM   #10
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FINALLY!!! Some discussion and attention on those of us at 5000ft!!
I LOVE IT! Keep it coming. I'd love some definitive conclusion on this before I flip the switch to supercharge.
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      04-12-2010, 02:22 PM   #11
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Quote:
Originally Posted by M&M View Post
Technically speaking, that is not true. When density changes, the volume remains the same.

But you are also compressing air that is less dense. As air gets thinner the molecules are further apart. So there is less oxygen per square inch at 5000ft than there is at oxygen rich sea-level. Ever wonder why climbers use oxygen masks at the top of Mount Everest? Each breath you take, you ingest the same amount of air you ingest at sea-level. But the air is pretty much useless as the 02 molecules are so sparse it doesn't feed you.




Ok on that point I agree with you. When density changes, pressure varies by:

P2 = P1 (D2/D1)

So a 5.5psi charger at 5000ft will produce = 5.5psi(850mbar/1000mbar)
= 4.675psi or 15% less boost.

So you would need a 15% smaller pulley to make the same boost.

But even if you get the same boost, the quality of air is not the same. So actually you will need an EVEN smaller pulley to compensate quantity for quality.

But hey, that's just my opinion. It's a great discussion with lots of grey areas si I am open for correction.
Quality? Erm, no. If what you mean is O2 fraction of the air, it is the same at sea level or 40,000 feet, viz. about 20%. As you go higher than this the composition of the atmosphere changes but down here, whether sea level or in the mountains, the proportion of O2 in the air is the same. That is, the partial pressure of O2 is about 1/5 that of the ambient air pressure. Period.

Without going too much into physics, it's the amount of the O2 that's in the cylinder that decides how much fuel the engine can take since we need to control mixture (A/F ratio). More fuel, more power. Since the volume inside the engine's cylinders is constant (at TDC) the amount of oxygen in the cylinder is then proportional to the density. The density depends on the pressure (altitude) and temperature. Higher temperature or higher altitude means less oxygen in a normally aspirated engine. There's less pressure up there but also less temperature. Pressure of the air is not determining quantity; it is density that is the key factor. This, of course, excludes talk of detrimental effects of too high a pressure or too hot the air.

For example, the density of air (standard atmosphere) at sea level is 1.225 kg/m^3. The density at 5000 feet is 1.056 kg/m^3, i.e. you've lost about 14% of the O2 (normally aspirated). Pressure-wise, it's 14.696 psi (1.013 bar) at sea level and 12.228 (0.843 bar) at 5000 feet, or about 17% loss of pressure, but not O2. And that is a loss of 0.17 bar.

With forced induction (super charger or turbo charger) you are forcing more air into the cylinders, increasing the pressure, but hopefully not the temperature, so the density of air is higher and hence more oxygen gets into the cylinders so that you can burn more fuel per stroke. To make up for loss of density from 0 to 5000 feet you only need to produce 0.17 bar of boost, assuming temperature is constant.

You'll notice a bit more power at sea level than at 5000 feet in an FI engine, but you'll need a dyno or accelerometer rather than your butt dyno to realize it.

I hope I got the point across. Sorry for being long-winded.


Cheers.
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      04-12-2010, 02:54 PM   #12
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^^^ thanks for the technical explanation. I was going to quote pressure laws from scuba diving experience but I forgot the exact terminology and didn't want to sound like an uneducated mountain goat lol.

Reinforces my raised points re 02 being 20% of our air regardless of altitude.
So bottom line is, spin the blower faster to produce boost equal to sea level.
Only down side is noisier blower.

On your above point re needing to produce 0,17 bar to counteract the loss in density at 5000ft, it therefore means that you would need to use a pulley that would produce 2,46psi higher than what you want to achieve at 5000 ft.

So if you have a VT2 575 kit producing 5,5psi at sea level, you need a pully good for 8psi to achieve 5,5psi at 5000

Last edited by Bob MG; 04-12-2010 at 03:04 PM.
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      04-12-2010, 03:38 PM   #13
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Quote:
Originally Posted by ersin View Post
Quality? Erm, no. If what you mean is O2 fraction of the air, it is the same at sea level or 40,000 feet, viz. about 20%. As you go higher than this the composition of the atmosphere changes but down here, whether sea level or in the mountains, the proportion of O2 in the air is the same.
No I did not refer to the proportion. That will, of course, always be the same.

Maybe you misunderstood me.

In one cubic foot foot of air at sea-level. How much oxygen is there? Not what proportion of oxygen, what absolute amount of oxygen is there?

So now. Go to 5000ft. I know the proportion is the same. What absolute amount of oxygen is there in one cubic foot of air? Is it the same as at sea-level?

Ever wonder why an NA doesn't require as much octane at altitude as it does at sea-level? I think someone has done some dyno testing at altitude and proven it.

On each stroke the air that is ingested into the cylinder has less oxygen than the equivalent sea-level car.

Hence my point that yes you and Bob are right you need a smaller pulley to make the same boost. We are all in agreement with that. But that "quality" of the extra air is poorer, so you need to add even more boost to get the same oxygen content in.

Make sense? Maybe I should have googled it instead of shooting from the hip with my personal experience of forced induction cars at altitude. I must say it's a great discussion as no-one really knows the real truth up here as most research is done at sea-level. So if we can learn, it will be for the benefit of all us mile-highers on this forum.
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      04-12-2010, 03:44 PM   #14
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I just realised that we are all saying the same thing. We all agree we need a smaller pulley? And we are all saying the oxygen content is less? I don't follow what we are disagreeing on then? Ersin, I think you are saying the oxygen content is determined by the density not the pressure? Well I think I'm saying the same thing.
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      04-12-2010, 04:04 PM   #15
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Quote:
Originally Posted by M&M View Post

So now. Go to 5000ft. I know the proportion is the same. What absolute amount of oxygen is there in one cubic foot of air? Is it the same as at sea-level?

But that "quality" of the extra air is poorer, so you need to add even more boost to get the same oxygen content in.

Make sense? Maybe I should have googled it instead of shooting from the hip with my personal experience of forced induction cars at altitude. I must say it's a great discussion as no-one really knows the real truth up here as most research is done at sea-level. So if we can learn, it will be for the benefit of all us mile-highers on this forum.
We are all on the right track, only thing I disagree with you and so does Ersin, is that partial pressures are different at 0 and then 5000 ft, not quality of air.
Air is air, same composition at ALL altitudes. (20/80 Oxygen/Nitogen) The second you compress air to a certain pressure, provided temps are the same, then the amount of USABLE O2 is the same at both altitudes. Hence the denser the air, the higher partial pressure of O2 in it, and the more fuel you can burn.

But its academic, your technical knowledge of pressure laws are not up to par, but your understanding of tuning is

When you go scuba diving and breath compressed air at depth, the partial pressures of O2 and Nitrogen, increase by one atmosphere per 10 meters you go down. Nitrogen becomes toxic as you approach depths of greater than 40 meters, hence why commercial divers dive with air mixtures, that substitute Nitrogen and or reduce it. All relates to partial pressure.

Last edited by Bob MG; 04-12-2010 at 04:15 PM.
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      04-12-2010, 04:05 PM   #16
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Quote:
Originally Posted by M&M View Post
No I did not refer to the proportion. That will, of course, always be the same.

Maybe you misunderstood me.

In one cubic foot foot of air at sea-level. How much oxygen is there? Not what proportion of oxygen, what absolute amount of oxygen is there?

So now. Go to 5000ft. I know the proportion is the same. What absolute amount of oxygen is there in one cubic foot of air? Is it the same as at sea-level?

Ever wonder why an NA doesn't require as much octane at altitude as it does at sea-level? I think someone has done some dyno testing at altitude and proven it.

On each stroke the air that is ingested into the cylinder has less oxygen than the equivalent sea-level car.

Hence my point that yes you and Bob are right you need a smaller pulley to make the same boost. We are all in agreement with that. But that "quality" of the extra air is poorer, so you need to add even more boost to get the same oxygen content in.

Make sense? Maybe I should have googled it instead of shooting from the hip with my personal experience of forced induction cars at altitude. I must say it's a great discussion as no-one really knows the real truth up here as most research is done at sea-level. So if we can learn, it will be for the benefit of all us mile-highers on this forum.
I'm still not sure I understand what you mean by "quality". On each stroke the cylinders have less O2 at higher altitudes than at sea-level. But it also has less "other stuff", mainly nitrogen. If the air is 20% less pressure than at sea-level, then the O2 is 20% less than sea-level too. Also, there is 20% less O2 in the cylinders by mass (which is the important thing) and therefore 20% less fuel can be used (to keep the same A/F ratio).

I don't really follow why you need to add even more boost over that of overcoming the pressure loss due to higher altitude.


Cheers.
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      04-12-2010, 04:09 PM   #17
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Quote:
Originally Posted by M&M View Post
I just realised that we are all saying the same thing. We all agree we need a smaller pulley? And we are all saying the oxygen content is less? I don't follow what we are disagreeing on then? Ersin, I think you are saying the oxygen content is determined by the density not the pressure? Well I think I'm saying the same thing.
The quantity that is critical is the amount of oxygen in the cylinders, by mass. Since the volume of the engine is constant then talking about the density of the oxygen is the same thing as talking about quantity. Pressure affects performance indirectly as it affects density.

Yes, a smaller pulley will give you more boost, we all agree. I was just pointing out that some things that you said were confusing.


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      04-12-2010, 04:31 PM   #18
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These last few post have been a good read ... I'm just glad I'm at sea level
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      04-12-2010, 04:32 PM   #19
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Ok I feel ye'. The density vs pressure thing means we were saying the same thing in a roundabout way.

Bottom line, we need more O2. I think how much more is what we need to decide on.
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      04-12-2010, 05:37 PM   #20
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Quote:
Originally Posted by Bob MG View Post
The second you compress air to a certain pressure, provided temps are the same, then the amount of USABLE O2 is the same at both altitudes.
OK then we on the same track but that sentence I disagree with. I could be wrong and maybe I'm just plain stupid. But 5.5psi of air at 5000ft elevation does not have the same AMOUNT (not ratio) of O2 as 5.5psi of air at sea-level.

Sea-level 5.5 psi of compressed air has much more oxygen. Like Ersin so eloquently said (which is what I was trying to say), is that the density changes up here. So yeah 5.5 psi is 5.5 psi. But it's not as dense. You are compressing air which is less dense to 5.5 lbs of boost.

Quote:
Originally Posted by Bob MG View Post
But its academic, your technical knowledge of pressure laws are not up to par, but your understanding of tuning is
Granted it has been a while since I studied Boyle and his merry men and their laws, but I'm pretty sure I aced it at the time. Must be getting old.

But what is the discussion about again?
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      04-12-2010, 09:03 PM   #21
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We are all getting older but I can recall we discuss about supercharging at altitude .

Back to Bob's scuba-diving: If you fill a bottle @ sea level up to 220 bar you will get 220 bar of pressured air with 20% oxygen and 80% nitrogen. Filling the same bottle at 5000 feet you will get also 220 bar of pressured air with 20% oxygen and 80% nitrogen, you just have to apply a higher compressor pressure ( + 2.5 psi ) to achieve the 220 bar in the bottle.

Turbocharging:

The ECU calls for 5.5 psi. Thus spinning of the turbos is not directly related to the engine revs the turbos will provide absolute 5.5 psi at sea-level or absolute 5.5 psi at 5000 feet. In the latter the turbos will have to produce relative 8.0 psi due to less dense air. To be able to provide the same absolute boost pressure at different altitudes the wastegate is used. So 5.5 psi absolute boost pressure at sea-level and 5.5 psi absolute boost pressure at 5000 feet.

Supercharging:

The Supercharger is driven by the crankshaft and has no wastegate. So it provides always the same relative boost at a specific rpm. Using a 8.0 psi pulley will provide 8.0 psi absolute boost at sea-level or 5.5 psi absolute boost at 5000 feet. That's the difference compared to turbocharging when we look at boost only.

BOV's / DV's:

Blow off valves or diverter valves are used both on turbo- and supercharged systems. They don't control boost pressure, they just blow off the boost spike in the air intake manifold when you close the throttle ( 8 throttle bodies in case of the S65 ). The difference between BOV's and DV's is where they vent to.

Anything missing ?

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      04-12-2010, 10:30 PM   #22
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^^^Hi buddy, yes please comment on these two fundamentally flawed statements

Quote:
Originally Posted by M&M View Post
OK then we on the same track but that sentence I disagree with. I could be wrong and maybe I'm just plain stupid. But 5.5psi of air at 5000ft elevation does not have the same AMOUNT (not ratio) of O2 as 5.5psi of air at sea-level.

You are wrong..pressure of air directly relates to density. So 5,5 psi pressurised air at sea level is exactly as dense as 5,5psi pressurised air at 10,000 ft, with exactly the same amount of O2 in it. Same ratio AND same amount of actual molecules per specified volume of air

Sea-level 5.5 psi of compressed air has much more oxygen.

Definately wrong again. Think of a box of Inflated ballons.
At sea level there is 20 oxygen balloons and 80 nitrogen balloons.

Put the box in the boot of your m3, and drive up to my mountain retreat in Kazakhstan.

Open the boot. Inside there is still 100 balloons, only each one is bigger than it was at sea level. Why??? Because the atmospheric pressure is less therefore the force of pressure outside the balloon is less, making the balloon expand. So the actual number of molecules per volume of air take more space, hence air being thinner at altitude. (if volume stays constant, actual no of molecules reduces at altitude since they are bigger.

Ever wonder why your half full butt lube tube explodes in your bag when you fly? Lol if the other half is full of air, it expands and BANG. That's why you should remove the excess AIR in any container you pack before flying.


Like Ersin so eloquently said (which is what I was trying to say), is that the density changes up here. So yeah 5.5 psi is 5.5 psi. But it's not as dense. You are compressing air which is less dense to 5.5 lbs of boost.


covered above... I hope lol
density means concentration of molecules in the same 20/80 ratio per volume o air. At sea level there are more molecules packed into the specific volume of air compared to the same volume of air at 5000 ft. ...... Because each molecule is bigger at altitude. Please say you get it now

Granted it has been a while since I studied Boyle and his merry men and their laws, but I'm pretty sure I aced it at the time. Must be getting old.

But what is the discussion about again?

Last edited by Bob MG; 04-12-2010 at 10:48 PM.
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