BOOST vs. ELEVATION (for those who like math)
09-06-2002, 04:24 PM
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BOOST vs. ELEVATION (for those who like math)
hopfully i’m getting my whipples this winter, so i’ve been thinking about this and i believe i have came to a conclusion. now don’t get me wrong this is only my theory on what i think will happen with my system once i have it complete. i also realize the resolution on boost gauges isn’t this accurate and they bounce around a lot, so this is only a calculation but i believe it to be in the ballpark of what should happen. people who have experience with this please let me know if i’m off track.
i live at about 4300 ft elevation and boat any where from sea level to 7600 ft. for ease of illustration i’ll use an even 4000’ for this example.
if i have my sc geared for 6 psi boost (40.8% pressure increase) at sea level, my boost gauge would read 6 psi, that gives me 20.7 psi absolute manifold pressure (14.7 + 6 = 20.7 psia)
if i come back home to 4000’ the average barometric pressure is just under 12.7 psia. i’ve lost 2 psi but without my engines running my gauge will read 0 because the pressure is = on both sides of the bourdon-tube.
now when i’m under full boost at 4000’ with the same sc pullys i am getting 12.7psi + 40.8%boost increase = 17.9 psia. or 5.2 psi of boost.
so my boost gauge will read 5.2 psi saying i’ve only lost 0.8 psi even though i’ve actually lost a total of 2.8 psia manifold pressure (20.7psia – 17.9psia = 2.8 psi).
in order to get my engine back to the hp i had at sea level i need to increase actual boost to 8 psi (12.7 + 8 = 20.7).
the problem is if i increase the compressor gearing to give me 8 psi boost at sea level when i’m at 4000’ since the air is 86.4% as dense i’m only going to get 19.6psia ([8 + 14.7] x .864=19.6)
the way i see it is i would need to gear my sc for 9.3 psi boost at sea level in order to achieve 8 psi boost at 4000’ (20.7 / .864 = 24)
so at 4000’ even though i’m geared for 9.3 psi of boost, my boost gauge reads 8 psi which is actually just like 6 psi at sea level, there for i run no risk of detonation (due to boost anyway).
have i missed anything? any comments are welcome.
i live at about 4300 ft elevation and boat any where from sea level to 7600 ft. for ease of illustration i’ll use an even 4000’ for this example.
if i have my sc geared for 6 psi boost (40.8% pressure increase) at sea level, my boost gauge would read 6 psi, that gives me 20.7 psi absolute manifold pressure (14.7 + 6 = 20.7 psia)
if i come back home to 4000’ the average barometric pressure is just under 12.7 psia. i’ve lost 2 psi but without my engines running my gauge will read 0 because the pressure is = on both sides of the bourdon-tube.
now when i’m under full boost at 4000’ with the same sc pullys i am getting 12.7psi + 40.8%boost increase = 17.9 psia. or 5.2 psi of boost.
so my boost gauge will read 5.2 psi saying i’ve only lost 0.8 psi even though i’ve actually lost a total of 2.8 psia manifold pressure (20.7psia – 17.9psia = 2.8 psi).
in order to get my engine back to the hp i had at sea level i need to increase actual boost to 8 psi (12.7 + 8 = 20.7).
the problem is if i increase the compressor gearing to give me 8 psi boost at sea level when i’m at 4000’ since the air is 86.4% as dense i’m only going to get 19.6psia ([8 + 14.7] x .864=19.6)
the way i see it is i would need to gear my sc for 9.3 psi boost at sea level in order to achieve 8 psi boost at 4000’ (20.7 / .864 = 24)
so at 4000’ even though i’m geared for 9.3 psi of boost, my boost gauge reads 8 psi which is actually just like 6 psi at sea level, there for i run no risk of detonation (due to boost anyway).
have i missed anything? any comments are welcome.
Last edited by dean campbell; 09-06-2002 at 04:29 PM.
09-07-2002, 03:30 PM
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You basically have it correct, but there are several things to consider when you are attempting to overcome the effects of power loss due to altitude by adding boost to compensate for the loss of "air density".Even though your calculations are correct on paper, that doesn't necessarily mean you will make more, or achieve the optimum power available by adding boost.
What is Boost? is pressure that the supercharger forces into the engine. In normal engines, the air is pulled into the cylinder by vacuum created on the downward stroke of the piston. The air that is pulled into the cylinder is at atmospheric pressure. Boost is the compaction of the air that exceeds the atmospheric pressure. So, if your supercharger provides 6 pounds of boost, you're getting 6 extra pounds (per square inch) of air pressure in your combustion chamber. The more "compact" the mixture is, the more forceful it is when it detonates.
Basically atmospheric pressure reduces with altitude by about 1” Hg per 1000 Ft.: Altitude will affect both pressure and volume. A reduction in both the performance levels of pressure and volume by 4% for every 1,000 feet of elevation. As most superchargers produce somewhere in the range of 6 to 9 additional pounds of pressure over the atmospheric pressure at that elevation (at sea level atmospheric pressure is 14.7 psi)- (14.7+6=20.7 psi as you said) but there is a bit more to it than that. The higher in altitude you are, the less the ambient air pressure will be. Henceforth, running at a higher altitude will reduce the performance of any engine in general, forced induction or not. However, you are still producing boost; you are just starting with a lower ambient air pressure.
CFM vs. Density?
CFM is Cubic Feet per Minute is a measurement of volume over time. It has no considerations as to what gas it pertains to nor the density of the gas. CFM is strictly a rate of volume measurement. But within that volume of gas, and in our case – air, we can evaluate the quality of the air and its ability to transfer heat. Every molecule of air has a mass, and this mass has the ability to absorb or emit energy; also known as transferring heat. If we count the number molecules for a given volume we obtain the density of the air (mass/volume).
If we pack more molecules of air into a given volume, increasing the density, we have more mass per volume and the ability to transfer heat increases. Consequently, the reverse applies also.
At sea level, the density remains fairly constant and you can calculate the CFM using a heat transfer equation:
CFM = Q/(Cp * r * DT)
Where:
CFM = Cubic Feet per Minute
Q = Heat Transferred (kW)
Cp = Specific Heat of Air
r = Density
DT = Change in Temperature
If the specific heat of air and the density are held constant (eg. sea level), the equation then becomes simplified:
CFM = 3160 * Q (kW)/DT (° F)
or
CFM = 1760 * Q (kW)/DT (° C)
Density Change at High Altitude
At sea level, the density of air is .075 lbs/ft3 (1.19 kg/m3). This value is created by all of the other molecules in the atmosphere weighing down on the molecules at sea level.As the elevation increases, there are fewer molecules weighing down and the density of the air decreases. For instance, at 5,000 feet, the density is .066 lbs/ft3 (1.056 kg/m3). At 25,000 feet, the density is .034 lbs/ft3 (0.549 kg/m3). For these altitudes, we need to recalculate our heat transfer equation using the appropriate density for the altitude the fan will operate in. But this time the constant changes in the equation because the density is now a variable.
CFM = 237 * Q (kW)/(r * DT (° F))
or
CFM = 2074 * Q (kW)/ (r *DT (° C))
Where the density units are lb/ft3 for ° F and kg/m3 for ° C.
Altitude (ft) Density (lb/ft3) Density (kg/m3)
Sea Level.......075..............1.19
5000.............066..............1.06
10000...........056...............904
A normally aspirated (NA) (non-supercharged) engine's maximum manifold pressure will never exceed the outside ambient barometric pressure for the altitude. For example, at sea level this will be 100 Kpa (Kilopascals) or zero psi of "boost" at wide open throttle (WOT).Thus, if you running at high altitude, you'll experience decreased performance since the air is thinner and less dense. This is reflected by your manifold absolute pressure (MAP), which will indicate less than 100 Kpa or "negative" boost (vacuum) at WOT. Additionally, if you run below sea level in Death Valley for instance, you'll experience increased performance because of the denser air. Of course, that's if you don't detonate your engine to death from the intense dry heat, but that's another subject! You'll actually see the MAP read above 100 Kpa, and show some "boost" due to the increased air density - a natural "supercharging" effect.
Roots type pumps were actually not originally intended for use as air compressors (our understanding is they were designed, of all things, as blowers for ventilation!) so some of this increased manifold pressure is recycled back into the blower case, reducing efficiency and creating additional heat. Anytime a fluid is compressed (yes air is a fluid), heat is produced - this is a fundamental law of physics called the ideal gas law - so even at 100% adiabatic efficiency, some heat will be produced when air is compressed simply due to this law. However, we all know that there must be heat created in compressing this air due to friction, so this lowers the adiabatic efficiency. Roots type blowers are actually quite inefficient since they create a great deal of heat under boost, meaning adiabatic efficiencies of typical roots blowers are only around 50%, A common misconception is that more boost is always better. This is true, but only to a certain extent, because superchargers operate most efficiently at a certain speed. To create more boost, blower speed must be increased, more heat is generated, and efficiency suffers. One must understand that high density is what's important, not necessarily high pressure (boost). Compressing air does increase both pressure and density, but the resulting heat generated simultaneously reduces density as well. To achieve optimum performance from a blower, we want to create the most pressure (boost) while adding the least amount of heat, thus achieving the highest air density possible in the intake manifold.As blower speed is increased, at some point the heat created decreases the air density to where addidtional boost is just offset by the increased heat, resulting in no density (and therefore power) gains; further increases in boost may actually even decrease power.
Altitude ft)....Pressure (in. Hg)....Temp (F.)....Density-slugs per cubic foot
0....................29.92....................59.0 ....................0.002378
1,000..............28.86....................55.4.. ..................0.002309
2,000..............27.82....................51.9.. ..................0.002242
3,000..............26.82....................48.3.. ..................0.002176
4,000..............25.84....................44.7.. ..................0.002112
5,000..............24.89....................41.2.. ..................0.002049
6,000..............23.98....................37.6.. ..................0.001988
Atmospheric Pressure: Normal pressure in the surrounding atmosphere, generated by the weight of the air above us pressing down. At sea level, in average weather conditions, atmospheric pressure is approximately 100 kPa (about 14.5 psi) above vacuum or zero absolute pressure.
Barometric Pressure: Another term for atmospheric pressure. Expressed in inches of Mercury (in.Hg.).How high atmospheric pressure (relative to zero absolute pressure) forces Mercury up a glass tube.14.5 psi= 29.92 in.Hg. pressure Absolute: The sum of gauge pressure and atmospheric pressure. One standard atmosphere = 29.92 in. of mercury (Hg) = 14.696 lb/in (PSI).
So there ya have it.... more to make it confusing>>>>> Good luck! If this doesn't work, don't try to "FindMe", I tried...lol
What is Boost? is pressure that the supercharger forces into the engine. In normal engines, the air is pulled into the cylinder by vacuum created on the downward stroke of the piston. The air that is pulled into the cylinder is at atmospheric pressure. Boost is the compaction of the air that exceeds the atmospheric pressure. So, if your supercharger provides 6 pounds of boost, you're getting 6 extra pounds (per square inch) of air pressure in your combustion chamber. The more "compact" the mixture is, the more forceful it is when it detonates.
Basically atmospheric pressure reduces with altitude by about 1” Hg per 1000 Ft.: Altitude will affect both pressure and volume. A reduction in both the performance levels of pressure and volume by 4% for every 1,000 feet of elevation. As most superchargers produce somewhere in the range of 6 to 9 additional pounds of pressure over the atmospheric pressure at that elevation (at sea level atmospheric pressure is 14.7 psi)- (14.7+6=20.7 psi as you said) but there is a bit more to it than that. The higher in altitude you are, the less the ambient air pressure will be. Henceforth, running at a higher altitude will reduce the performance of any engine in general, forced induction or not. However, you are still producing boost; you are just starting with a lower ambient air pressure.
CFM vs. Density?
CFM is Cubic Feet per Minute is a measurement of volume over time. It has no considerations as to what gas it pertains to nor the density of the gas. CFM is strictly a rate of volume measurement. But within that volume of gas, and in our case – air, we can evaluate the quality of the air and its ability to transfer heat. Every molecule of air has a mass, and this mass has the ability to absorb or emit energy; also known as transferring heat. If we count the number molecules for a given volume we obtain the density of the air (mass/volume).
If we pack more molecules of air into a given volume, increasing the density, we have more mass per volume and the ability to transfer heat increases. Consequently, the reverse applies also.
At sea level, the density remains fairly constant and you can calculate the CFM using a heat transfer equation:
CFM = Q/(Cp * r * DT)
Where:
CFM = Cubic Feet per Minute
Q = Heat Transferred (kW)
Cp = Specific Heat of Air
r = Density
DT = Change in Temperature
If the specific heat of air and the density are held constant (eg. sea level), the equation then becomes simplified:
CFM = 3160 * Q (kW)/DT (° F)
or
CFM = 1760 * Q (kW)/DT (° C)
Density Change at High Altitude
At sea level, the density of air is .075 lbs/ft3 (1.19 kg/m3). This value is created by all of the other molecules in the atmosphere weighing down on the molecules at sea level.As the elevation increases, there are fewer molecules weighing down and the density of the air decreases. For instance, at 5,000 feet, the density is .066 lbs/ft3 (1.056 kg/m3). At 25,000 feet, the density is .034 lbs/ft3 (0.549 kg/m3). For these altitudes, we need to recalculate our heat transfer equation using the appropriate density for the altitude the fan will operate in. But this time the constant changes in the equation because the density is now a variable.
CFM = 237 * Q (kW)/(r * DT (° F))
or
CFM = 2074 * Q (kW)/ (r *DT (° C))
Where the density units are lb/ft3 for ° F and kg/m3 for ° C.
Altitude (ft) Density (lb/ft3) Density (kg/m3)
Sea Level.......075..............1.19
5000.............066..............1.06
10000...........056...............904
A normally aspirated (NA) (non-supercharged) engine's maximum manifold pressure will never exceed the outside ambient barometric pressure for the altitude. For example, at sea level this will be 100 Kpa (Kilopascals) or zero psi of "boost" at wide open throttle (WOT).Thus, if you running at high altitude, you'll experience decreased performance since the air is thinner and less dense. This is reflected by your manifold absolute pressure (MAP), which will indicate less than 100 Kpa or "negative" boost (vacuum) at WOT. Additionally, if you run below sea level in Death Valley for instance, you'll experience increased performance because of the denser air. Of course, that's if you don't detonate your engine to death from the intense dry heat, but that's another subject! You'll actually see the MAP read above 100 Kpa, and show some "boost" due to the increased air density - a natural "supercharging" effect.
Roots type pumps were actually not originally intended for use as air compressors (our understanding is they were designed, of all things, as blowers for ventilation!) so some of this increased manifold pressure is recycled back into the blower case, reducing efficiency and creating additional heat. Anytime a fluid is compressed (yes air is a fluid), heat is produced - this is a fundamental law of physics called the ideal gas law - so even at 100% adiabatic efficiency, some heat will be produced when air is compressed simply due to this law. However, we all know that there must be heat created in compressing this air due to friction, so this lowers the adiabatic efficiency. Roots type blowers are actually quite inefficient since they create a great deal of heat under boost, meaning adiabatic efficiencies of typical roots blowers are only around 50%, A common misconception is that more boost is always better. This is true, but only to a certain extent, because superchargers operate most efficiently at a certain speed. To create more boost, blower speed must be increased, more heat is generated, and efficiency suffers. One must understand that high density is what's important, not necessarily high pressure (boost). Compressing air does increase both pressure and density, but the resulting heat generated simultaneously reduces density as well. To achieve optimum performance from a blower, we want to create the most pressure (boost) while adding the least amount of heat, thus achieving the highest air density possible in the intake manifold.As blower speed is increased, at some point the heat created decreases the air density to where addidtional boost is just offset by the increased heat, resulting in no density (and therefore power) gains; further increases in boost may actually even decrease power.
Altitude ft)....Pressure (in. Hg)....Temp (F.)....Density-slugs per cubic foot
0....................29.92....................59.0 ....................0.002378
1,000..............28.86....................55.4.. ..................0.002309
2,000..............27.82....................51.9.. ..................0.002242
3,000..............26.82....................48.3.. ..................0.002176
4,000..............25.84....................44.7.. ..................0.002112
5,000..............24.89....................41.2.. ..................0.002049
6,000..............23.98....................37.6.. ..................0.001988
Atmospheric Pressure: Normal pressure in the surrounding atmosphere, generated by the weight of the air above us pressing down. At sea level, in average weather conditions, atmospheric pressure is approximately 100 kPa (about 14.5 psi) above vacuum or zero absolute pressure.
Barometric Pressure: Another term for atmospheric pressure. Expressed in inches of Mercury (in.Hg.).How high atmospheric pressure (relative to zero absolute pressure) forces Mercury up a glass tube.14.5 psi= 29.92 in.Hg. pressure Absolute: The sum of gauge pressure and atmospheric pressure. One standard atmosphere = 29.92 in. of mercury (Hg) = 14.696 lb/in (PSI).
So there ya have it.... more to make it confusing>>>>> Good luck! If this doesn't work, don't try to "FindMe", I tried...lol
Last edited by FindMe; 09-07-2002 at 03:42 PM.
09-07-2002, 10:59 PM
#4
Guest
Posts: n/a
Well I don't do math but I do boat where you boat in the great offshore state of Utah.I tested my boat at sea leval 70mph 5000 r
pm, got it home 6000ft elavation (Jordenell) 2" lower on the prop
58mph @ 4900rpm. Added a blower (B&M 250) with a 4-5# pulley
boost gauge reads 2# couldn't pull the sea leval prop past 4600
went to a 6-7# pulley gained 1# boostand 200 rpm went to the 8-9# pulley and finally get the 5# I've been looking for and 70 mph @5000rpm.So my math says 6000 ft elev. + 5psi= sealeval
pm, got it home 6000ft elavation (Jordenell) 2" lower on the prop
58mph @ 4900rpm. Added a blower (B&M 250) with a 4-5# pulley
boost gauge reads 2# couldn't pull the sea leval prop past 4600
went to a 6-7# pulley gained 1# boostand 200 rpm went to the 8-9# pulley and finally get the 5# I've been looking for and 70 mph @5000rpm.So my math says 6000 ft elev. + 5psi= sealeval
09-10-2002, 01:37 PM
#5
Registered
I believe a good example of this would be the new Merc 1050SCi and it’s claim to develop the same power at any altitude (within reason).
This is only from observation and some very good resources…
This motor is boosted with large compressors that will give it boost well over what it would utilize at sea level (3-4 lbs over target) and the ECM regulates a bypass to maintain the targeted kPa at various altitudes.
In other words (and using 5 lbs as an example), even though the compressor is spinning fast enough to give 8-9 lbs the ECM is programmed to only maintain 5 lbs of boost. The ECM would read this from the MAP sensor as ~135.8 kPa and it will open the bypass to maintain this kPa level. As you increase altitude, which decreases ambient kPa, a larger amount of boost is required to maintain the same kPa. This is when the ECM is prompted from the MAP sensor to begin closing the bypass and maintain the target kPa. This is why I would think that even though this setup has some efficiency sacrifices at sea level; it does allow you to maintain approximately the same hp levels at all altitudes. I would also think that they have programmed in the deficiencies of the generated heat and it’s effects on CFM and density into the program. This is the nice thing about dialing in a single package and then duplicating it. They have a constant; we generally don’t.
Even though a motor at sea level targeted to run 5 lbs of boost (135.8 kPa) taken to 5000 ft should require a delta increase of ~2.47 lbs (~7.47 lbs total) of boost to maintain the same; this will be changed somewhat by all things mentioned in FindMe’s reply. He shows that there is indeed no perfect solution to this without real world testing, but we can get close.
As I see it, the many different intercooling capacities/efficiencies/configurations along with compressor capacity will play a vital role in this beings they both dramatically change the density of the air. All of these factors will need to be taken into account to accurately assess how much more boost can be effectively run at elevation without reversing the gains.
So, what do you all think, does this all mean that if you live and generally boat at high elevations you should buy the biggest blower and intercooler available?
Input??
I should have some good “real” data in the spring on this. I’ll be testing with an M3 ProCharger and 504 intercooler installed. I will be able to monitor and record my kPa and inlet air temps via a laptop through a F.A.S.T. ECM. We can then compare the on paper figures to the real world test!! Should be very interesting.
Beings I also live in the great “Offshore” state of Utah I should be able to get some good variable altitude readings from sea level to 6000+ feet. Guess I’d better order a few more pulleys ;o)
Later,
Dave
This is only from observation and some very good resources…
This motor is boosted with large compressors that will give it boost well over what it would utilize at sea level (3-4 lbs over target) and the ECM regulates a bypass to maintain the targeted kPa at various altitudes.
In other words (and using 5 lbs as an example), even though the compressor is spinning fast enough to give 8-9 lbs the ECM is programmed to only maintain 5 lbs of boost. The ECM would read this from the MAP sensor as ~135.8 kPa and it will open the bypass to maintain this kPa level. As you increase altitude, which decreases ambient kPa, a larger amount of boost is required to maintain the same kPa. This is when the ECM is prompted from the MAP sensor to begin closing the bypass and maintain the target kPa. This is why I would think that even though this setup has some efficiency sacrifices at sea level; it does allow you to maintain approximately the same hp levels at all altitudes. I would also think that they have programmed in the deficiencies of the generated heat and it’s effects on CFM and density into the program. This is the nice thing about dialing in a single package and then duplicating it. They have a constant; we generally don’t.
Even though a motor at sea level targeted to run 5 lbs of boost (135.8 kPa) taken to 5000 ft should require a delta increase of ~2.47 lbs (~7.47 lbs total) of boost to maintain the same; this will be changed somewhat by all things mentioned in FindMe’s reply. He shows that there is indeed no perfect solution to this without real world testing, but we can get close.
As I see it, the many different intercooling capacities/efficiencies/configurations along with compressor capacity will play a vital role in this beings they both dramatically change the density of the air. All of these factors will need to be taken into account to accurately assess how much more boost can be effectively run at elevation without reversing the gains.
So, what do you all think, does this all mean that if you live and generally boat at high elevations you should buy the biggest blower and intercooler available?
Input??
I should have some good “real” data in the spring on this. I’ll be testing with an M3 ProCharger and 504 intercooler installed. I will be able to monitor and record my kPa and inlet air temps via a laptop through a F.A.S.T. ECM. We can then compare the on paper figures to the real world test!! Should be very interesting.
Beings I also live in the great “Offshore” state of Utah I should be able to get some good variable altitude readings from sea level to 6000+ feet. Guess I’d better order a few more pulleys ;o)
Later,
Dave
09-10-2002, 04:03 PM
#6
Registered
Ouch!!! My head has started to hurt!! That's really good, deep info for us...... I guess now I will need to calculate all this stuff to find out how much I enjoy my PC's.....not!!
Just kiddin'....good stuff guy's!!
Just kiddin'....good stuff guy's!!
