[stressdoc]

Originally Posted by bilworth

I thought the computer looks at the absolute manifold pressure for reference.
In this case: 1.0 BAR (14.7 psi) + 0.6 BAR (8.8 psi) = 23.5 psi. So, the computer will try to maintain 23.5 psi. As the elevation increases, the 14.7 part of the equation decreases so the 8.8 part has to increase to maintain the 23.5 psi. The computer does this by regulating the wastegate up to a maximum of 0.8 BAR (11.8 psi). So, that means it should be able to maintain the 23.5 psi up to an elevation where the atmospheric pressure equals 11.7 psi (11.7 + 11.8 = 23.5). That elevation is approximately 6000' msl on a day when standard temperature and pressure prevail. Am I wrong?

+1, that's the way I understand it, back from when we were first trying to figure out (in early 2006) how much boost the turbos were providing. I initially thought that this might be what the aftermarket piggy-backs (Xede) were taking advantage of to get more boost. Of course it's more complex than that, but I would still be a little concerned about Xede or Turbo tuner if I lived at high altitude, just because the turbos would be working pretty hard (it seems that Shiv has thought this through for the PROcede map).

[Insider]

Originally Posted by bilworth

I thought the computer looks at the absolute manifold pressure for reference.
In this case: 1.0 BAR (14.7 psi) + 0.6 BAR (8.8 psi) = 23.5 psi. So, the computer will try to maintain 23.5 psi. As the elevation increases, the 14.7 part of the equation decreases so the 8.8 part has to increase to maintain the 23.5 psi. The computer does this by regulating the wastegate up to a maximum of 0.8 BAR (11.8 psi). So, that means it should be able to maintain the 23.5 psi up to an elevation where the atmospheric pressure equals 11.7 psi (11.7 + 11.8 = 23.5). That elevation is approximately 6000' msl on a day when standard temperature and pressure prevail. Am I wrong?

Yes, you wrote this as I was writing my response. This is correct.

[jworms]

Originally Posted by stressdoc

+1, that's the way I understand it, back from when we were first trying to figure out (in early 2006) how much boost the turbos were providing. I initially thought that this might be what the aftermarket piggy-backs (Xede) were taking advantage of to get more boost. Of course it's more complex than that, but I would still be a little concerned about Xede or Turbo tuner if I lived at high altitude, just because the turbos would be working pretty hard (it seems that Shiv has thought this through for the PROcede map).

Originally Posted by Insider

My understanding is that there is an extra 3psi of boost available in the manifold to compensate at higher elevations. So the ECU will measure absolute boost pressure, and it will try to reach the absolute number. Therefore, the manifold pressure will be higher at elevation because that's what it takes to reach the same absolute pressure that the car has at sea level.

Originally Posted by bilworth

I thought the computer looks at the absolute manifold pressure for reference.
In this case: 1.0 BAR (14.7 psi) + 0.6 BAR (8.8 psi) = 23.5 psi. So, the computer will try to maintain 23.5 psi. As the elevation increases, the 14.7 part of the equation decreases so the 8.8 part has to increase to maintain the 23.5 psi. The computer does this by regulating the wastegate up to a maximum of 0.8 BAR (11.8 psi). So, that means it should be able to maintain the 23.5 psi up to an elevation where the atmospheric pressure equals 11.7 psi (11.7 + 11.8 = 23.5). That elevation is approximately 6000' msl on a day when standard temperature and pressure prevail. Am I wrong?

we are talking strictly boost, right? boost is calculated after atmospheric pressure is reached, right?
8.8 psi boost at sea level is not equal to 11.8psi boost at higher elevation no matter how you slice it.

is the ECU telling the wastegate not to open at +8.8psi, but to instead open at +11.8psi at higher elevation? if that were the case turbo cars would make more power at higher elevation and like i said, we should build a drag strip at the top of a mountain.

i think i can see what you guys are saying and i agree that the turbos will have to make up that extra bit of pressure lost in the air density. but if you ONLY look at boost (the air being shoved in after atmospheric is reached) and not absolute pressure you will see that the boost gauge should still report stock boost at higher elevation...it will just take longer because it has to compensate for the lack of air density.

[serge]

FYI,

This is how altitude affects a regular 300whp Evo VIII. Measured with a G-Tech Pro Comp (sea level vs 6,000 ft.>>Colorado Springs):

Sea level vs 6,000 ft.

50-75mph in 3rd gear: 2.08 s. 2.55 s.
50-75mph in 4th gear: 2.98 s. 3.63 s.
50-75mph in 5th gear: 5.23 s. 7.31 s.
0-60 mph: 3.68 s. 4.35 s.
0-100 mph: 9.28 s. 10.49 s.
1/4 mile: 11.89 @ 116mph >>12.75 @ 111 mph

Enough said

[Josh49]

So, if you went 6,000 ft below see level you can go 50-75 in 1.61 seconds????

[jworms]

Originally Posted by serge

FYI,

This is how altitude affects a regular 300whp Evo VIII. Measured with a G-Tech Pro Comp (sea level vs 6,000 ft.>>Colorado Springs):

Sea level vs 6,000 ft.

50-75mph in 3rd gear: 2.08 s. 2.55 s.
50-75mph in 4th gear: 2.98 s. 3.63 s.
50-75mph in 5th gear: 5.23 s. 7.31 s.
0-60 mph: 3.68 s. 4.35 s.
0-100 mph: 9.28 s. 10.49 s.
1/4 mile: 11.89 @ 116mph >>12.75 @ 111 mph

Enough said

g-techs are hardly accurate enough to rely on at different locations. i only use one to see what my mods do to my times and i use the same strip of road every time.

besides that, i never said turbo cars aren't affected by elevation, they just aren't affected nearly as bad as N/A or S/C cars are. turbo cars have to worry about the turbo(s) taking longer to produce the same boost at elevation that they would at sea level and heat produced by the extra work the turbos are doing

[serge]

Originally Posted by jworms

g-techs are hardly accurate enough to rely on at different locations. i only use one to see what my mods do to my times and i use the same strip of road every time.

besides that, i never said turbo cars aren't affected by elevation, they just aren't affected nearly as bad as N/A or S/C cars are. turbo cars have to worry about the turbo(s) taking longer to produce the same boost at elevation that they would at sea level and heat produced by the extra work the turbos are doing

I'm glad you brought up the spool up issue. My car's max spool up at sea level is achieved @ 2500-2600rpm aprox. compared to 3000-3100rpm at 6,000ft.

I do now that G-Techs are not 100% accurate. They are .2 sec/2mph off (faster) when compared to time slips. However, I doubt they loose accuracy at different locations. Perhaps they are also affected by altitude

Anyways, since 335s compensate thiner air with extra boost, how come most of the 335 owners who race their cars at LACR use the 2500ft. elevation as the main reason for slower than average track times?

[chbrules]

As I said, you will see a noticable difference at a higher elevation with a forced induction engine. Even with electronically comphesating mechanics you're still going to have to deal with hudreds of other variables dealing with air pressure and temperature. Consider the intercooler; there's less actual mass of atmospheric atoms to absorb the energy from the air to air radiator. It's simple physics.

[Terry335]

Originally Posted by serge

I'm glad you brought up the spool up issue. My car's max spool up at sea level is achieved @ 2500-2600rpm aprox. compared to 3000-3100rpm at 6,000ft.

I do now that G-Techs are not 100% accurate. They are .2 sec/2mph off (faster) when compared to time slips. However, I doubt they loose accuracy at different locations. Perhaps they are also affected by altitude

Anyways, since 335s compensate thiner air with extra boost, how come most of the 335 owners who race their cars at LACR use the 2500ft. elevation as the main reason for slower than average track times?

I don't think it can compensate 100%, plus LACR has a DA of around 4500'-6500' on many days.

[jworms]

Originally Posted by serge

I'm glad you brought up the spool up issue. My car's max spool up at sea level is achieved @ 2500-2600rpm aprox. compared to 3000-3100rpm at 6,000ft.

I do now that G-Techs are not 100% accurate. They are .2 sec/2mph off (faster) when compared to time slips. However, I doubt they loose accuracy at different locations. Perhaps they are also affected by altitude

Anyways, since 335s compensate thiner air with extra boost, how come most of the 335 owners who race their cars at LACR use the 2500ft. elevation as the main reason for slower than average track times?

again, turbo cars are affected by elevation, just not nearly as bad as N/A or S/C cars. it's also for different reasons that turbo cars are affected at elevation compared to N/A or S/C ones (as chbrules mentioned below).

for example, i can easily beat a stock 335 with my car at sea level, but the fastest my car has ever run at LACR is 14.14@98.5mph with an almost 2.3 60ft (right lane):

now granted i think high 13s are definitely possible with my car if a better 60ft was achieved, but the trap is what i'm mostly referring to. compare that to what 335s typicaly get at LACR (high 13s to low 14s @ around 100-101) and you get what i'm saying.

also LACR is a horrible place to put any faith in the numbers that come from there. there is usually a headwind, poorly (at best) prepped track, and as Terry335 mentioned, the DA is typically well beyond the actual elevation (around 2700ft).

[bilworth]

jworms said: "we are talking strictly boost, right? boost is calculated after atmospheric pressure is reached, right?
8.8 psi boost at sea level is not equal to 11.8psi boost at higher elevation no matter how you slice it."

HUH??? Doesn't 23.5 equal 23.5???

[///AFRO]

I'm confused

[chbrules]

Originally Posted by Brisk

so do turbo cars peform better at sea level right... so the 335i will have max power at sea level?

Try it out in Death Valley, NV.

[bilworth]

I suspect most of the confusion results from the misconception that all turbo cars are the same regarding boost. To clarify the term "boost" in this discussion, it is the amount of manifold pressure (in psi) added to ambient barometric pressure. Ambient barometric pressure varies from day to day at any given elevation. Ambient barometric pressure is also effected by elevation. As elevation increases, ambient barometric pressure decreases. Most turbocharged cars add a set value (ie;12.5 psi) to whatever the ambient barometric pressure is regardless of any variations. So, such a vehicle will make less hp as the elevation increases or barometric pressure decreases. The BMW turbo system is more sophisticated as it works with a variable value of boost from 0.6 BAR (8.8 psi) to 0.8 BAR (11.7 psi) and thereby adds whatever is needed within that range to get to a pre-set boost of 23.5 psi. The BMW therefore compensates for variations in barometric pressure and elevation up to a point. On a standard day the BMW should be good for SL HP up to about 6000' msl. There are other variables regarding air density and turbo spool up that will likely effect all this, but they should normally be a minor effect. This is the way a BMW tech explained it to me. Hope it helps.

[bilworth]

Correction: The 23.5 psi is a pre-set total manifold pressure value, not boost.
Sorry.

[///AFRO]

good work gentlemen. You all made me proud.