high altitude question

[Endz0r]

for all the guys that are in high altitude places (denver, new mexico, etc), how are you dealing with the 90-91 octane (highest u can buy in some places) on turbo dsms?

 

[Defiant]

The ECU deals with it with the help of things like the manifold pressure sensor. Turbos are magic in thin air- less air resistance, so they can spin more freely, and they largely compensate automatically for altitude. The fuel tuning is in the ECU mapping.

 

[quest49s]

He kind of has it right the Ecu does adjust for the octane used, but dsm's have no Manifold pressure sensor or MAP sensor. It adjusts over time, based on feed back from the o2 sensor to change the high/mid/low fuel trims.

Also when the engine detotonates it will retard ignition timing to save the motor. It can only retard the timing so much before it can no longer help.

Turbo cars at higher elevation are much more easily tuned for power then NA. The atmospheric pressure in Ft. Collins, Co. where I used to live is in the neighborhood of 11.8psi while sea level is 14.7. You can just tun up the boost 3 psi to make up for this pressure loss, granted you have the proper setup to do so. It is almost as clear cut as this with few excptions.

The way I ran higher 91 octane was to get larger injectors, FMIC, Water Injection and SAFC. But this is for a heavily modified 91 oct. street car.

 

[Defiant]

dsm's have no Manifold pressure sensor or MAP sensor.

Hm? Non-turbo, "MAP sensor"; turbo, "Manifold differential pressure sensor". Not them?

 

[quest49s]

I don't know what you are saying are you asking a question or stating something?

No DSM's have MAP sensors. They all use karmen sensors, rpm, and o2's to define how much fuel to inject. I don't no if the barometric pressure sensors are used for fuel delievry or timing? The ecu has absolutly no idea the amount of boost that the car is pushing.

 

[yoshimitsuspeed]

Turbo cars at higher elevation are much more easily tuned for power then NA. The atmospheric pressure in Ft. Collins, Co. where I used to live is in the neighborhood of 11.8psi while sea level is 14.7. You can just tun up the boost 3 psi to make up for this pressure loss, granted you have the proper setup to do so. It is almost as clear cut as this with few excptions.

The way I ran higher 91 octane was to get larger injectors, FMIC, Water Injection and SAFC. But this is for a heavily modified 91 oct. street car.[/QUOTE]


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This makes no sense to me. If I am not mistaken your boost controller will controll pressure at it's set limit no matter what altitude. For example the stock setup is around 11 psi so at 14,000 feet the turbo will still be pushing 11 pounds it will just be working a little harder right?

To the origional poster. I lived in outside of Boulder Colorado. Boulder being at 5,000 ft, my house being at 9,000 ft a 15 mile drive that I did in close to 15 minutes regularly I don't think I even ran premium all the time, probably not a good idea but I never had a problem.
I guess a mile a minute doesn't sound like pushing very hard so I will say that included 110mph up a good grade on the straights to make up for the twisties.

 

[quest49s]

Yes but you forgot to take in account that if you are pushing 1 bar(14.7psi) of boost then you are running a total of 2 bar(29.4psi) in absolute pressure at sea level. The higher in elevation the lower the absolute pressure
ex:
~5000 FASL (feet above Sea level) absolute atmo. pressure ~11.8 psi
~0 FASL absolute atmo. pressure 14.7 psi

So say you are pushing 11psi for a drive from Florida to Colorado. The absolute pressure would be 25.7psi(14.7 + 11) in Florida. But as you rise in elevation the absolute pressure is getting lower. By the time you reach 5000 FASL your Absolute pressure would be 22.8 psi(11.8 +11)

So if you were at 14000 feet atmospheric pressure is est. 7psi and you were running 11 psi then your absolute pressure is 18psi, but if you had the supporting mods to raise the boost pressure 7.7 psi then it would be like driving at sea level. So at 14000 ft. you would need to run 18.7psi to compare to sea level.
BOOST ELEV. Atmospheric Pressure Totel PSI adjusted boost to = sealevel
11psi 0 FASL 14.7 25.7 0
11psi 5k FASL ~11.8 22.8 2.9psi
11psi 14k FASL ~7 18 7.7psi

This is in theory, for this discription I omited the fact that when air is compressed it becomes hotter

There maybe something I missed (3:30am) so if any can add or correct me go for it.

 

[Defiant]

I don't know what you are saying are you asking a question or stating something?

No DSM's have MAP sensors. They all use karmen sensors, rpm, and o2's to define how much fuel to inject. I don't no if the barometric pressure sensors are used for fuel delievry or timing? The ecu has absolutly no idea the amount of boost that the car is pushing.

Not a big deal, but I think you're mistaken. Sorry about the lousy pics.

 

[heavyD]

I live in Calgary which has an altitude of 3500 feet. :thumbdown N/A cars are alot slower here and while turbocharging helps, the air up here just doesn't contain enough O2 compared to lower altitudes. 12.3 sec is the fastest 1/4 time of the year for a SCC this year so far in Calgary while the same cars are running in 11's in Edmonton which has a lower altitude.

 

[quest49s]

What car is that diagram of??

 

[yoshimitsuspeed]

Yes but you forgot to take in account that if you are pushing 1 bar(14.7psi) of boost then you are running a total of 2 bar(29.4psi) in absolute pressure at sea level. The higher in elevation the lower the absolute pressure
ex:
~5000 FASL (feet above Sea level) absolute atmo. pressure ~11.8 psi
~0 FASL absolute atmo. pressure 14.7 psi

So say you are pushing 11psi for a drive from Florida to Colorado. The absolute pressure would be 25.7psi(14.7 + 11) in Florida. But as you rise in elevation the absolute pressure is getting lower. By the time you reach 5000 FASL your Absolute pressure would be 22.8 psi(11.8 +11)

So if you were at 14000 feet atmospheric pressure is est. 7psi and you were running 11 psi then your absolute pressure is 18psi, but if you had the supporting mods to raise the boost pressure 7.7 psi then it would be like driving at sea level. So at 14000 ft. you would need to run 18.7psi to compare to sea level.
BOOST ELEV. Atmospheric Pressure Totel PSI adjusted boost to = sealevel
11psi 0 FASL 14.7 25.7 0
11psi 5k FASL ~11.8 22.8 2.9psi
11psi 14k FASL ~7 18 7.7psi

This is in theory, for this discription I omited the fact that when air is compressed it becomes hotter

There maybe something I missed (3:30am) so if any can add or correct me go for it.

So now that I have a aftermarket boost guage, if I drive up to 5,000 feet it will read 8.1 psi

 

[Defiant]

What car is that diagram of??

First one's non-turbo, second one isn't.

 

[quest49s]

"So now that I have a aftermarket boost guage, if I drive up to 5,000 feet it will read 8.1 psi" quote]
No, the factory wastegate wont allow full boost to be that low and the atmospheric pressure will drop, not the pressured air out of the turbo. your vacuum in in/mercury*2 will defenitly drop however. In Colorado my vacuum was around 18in/m*2 but in arizona idle is around 21-22 in/m*2. This is do to heavier air pressure at lower levels.

First one's non-turbo, second one isn't." quote]

what year cars? because I have a 91 Tsi and I know for a fact it does not have a MAP sensor. There is nothing on my car that can relay boost pressure to ECU. What resource are you using, manual, etc...

 

[quest49s]

It looks like a 2g 95-96 because of the obd-2 and crank angle sensor on the block

 

[Defiant]

what year cars? because I have a 91 Tsi and I know for a fact it does not have a MAP sensor. There is nothing on my car that can relay boost pressure to ECU. What resource are you using, manual, etc...

Those are a 2G Talon, 95. Yours has a barometric pressure sensor embedded in the MAS.

 

[quest49s]

A barametric pressure sensor and Map sensor are two different things. I know it is in my mas barametric pressure sensor reads static atmosphere pressure not boost

 

[djpast]

2gs have a map sensor. its on the intake manifold. its used as sort of a back up in case the MAF fails or you blow off an i/c pipe. thats why when 2g guys switch to a 1g manifold they have to T in the map sensor on an existing vacuum line.



2.0 1gs (turbo and non turbo) do not have this. i believe the 1.8L 1g NT does tho. (not 100% sure on that tho)

 

[Defiant]

A barametric pressure sensor and Map sensor are two different things. I know it is in my mas barametric pressure sensor reads static atmosphere pressure not boost

Soooo... that'll compensate for altitude, right?

 

[kmartind]

In any case, to help answer the original poster's question...

(N/A cars don't need as high octane gas at altitude as they do at sea level, they ingest less oxygen and burn less fuel, have less chance of knocking, and as a result, will also produce less power and run slower. This is one reason they sell lower octane up here. Anyway, the altitude could be partially compensated for by raising compression a bit, perhaps advancing timing, and freeing up the intake as much as possible, etc. Note this would then make the car require a bit higher octane.

Turbo cars are different. The turbine is most efficient within a certain range of pressure ratios for a given airflow. If atmospheric pressure is 14.7 and you run 22 PSI of boost, your pressure ratio is 1.5:1. If you go up to a high altitude where atmospheric pressure is only, say 12 and run the same 22 PSI, your pressure ratio is now 1.83:1. The turbo will need to spin faster to maintain this pressure, and for some turbos this may push the compressor out of its efficency range, resulting in blowing hot air. Not only that, but the absolute pressure (atmospheric pressure plus boost) is still lower that at sea level, so you probably won't make as much power even at 22 PSI. You can partially compensate for this by raising the boost pressure even higher, say to 25 PSI to try and make your car as fast at altitude as it would be at sea level, but this makes the pressure ratio even higher (over 2:1 in this case). The turbo then has to spin even faster, and may be even farther from it's highest efficiency and blowing hotter air. Hence, the water injection and extra fuel to help cool things off and help prevent knock. A larger turbo may also be advantageous as it will have a lower shaft speed and not be as likely to fail prematurely as a result of running faster and hotter to maintain a higher boost pressure at altitude.
(of course anyone can correct me where I'm wrong).

In short, things like large FMICs, more fuel, larger turbos, and higher boost pressures and water injection are used to help combat the high altitude and low octane.

 

[yoshimitsuspeed]

In short, things like large FMICs, more fuel, larger turbos, and higher boost pressures and water injection are used to help combat the high altitude and low octane.[/QUOTE]
I agree with most of your post but I still don't understand why you would need to turn up your boost, in fact I am pretty confident you are incorect on this. Everything else makes sence but if your car is making eleven pounds boost it's eleven pounds boost. If I put 35 PSI in my tires at sea level then drove up to 10,000 feet I would understand a pressure change but if I had a regulator on my tires set at 35 psi then when I got to 10,000 feet they would still be at 35 psi. As far as I understand the same applies to the boost sustem. So if I am pushing 11 psi at sea level and I am pushing 11 psi at 10,000 feet what's the difference. The reason high altitude air is less efficiant is because it is less dense, because there is less pressure. Once inside the system if you pressurize it to the same psi it will have the same pressure, and the same density. I could see altitude making a slight difference in power but very minimal, but it would seem your turbo would have to work harder for it and would max out sooner. If I am wrong please explain this in a way in which I can understand.
Guest49s theory sounds well thought out but goes against so much of my knowlege of logic, psysics, and mechanical theory that it will not hold water untill I can get a clear picture and probably some real world evidence. Does anybody have Dyno records of a turbo car at altitude verses sea level with no changes made in between. I would bet the difference isn't more than a few horsepower.

 

[Defiant]

N/A cars don't need as high octane gas at altitude as they do at sea level, they ingest less oxygen and burn less fuel, have less chance of knocking, and as a result, will also produce less power and run slower.

Also, normally-aspirated, non-injected (or, mechanically-injected) engines tuned to run well near sea level will be pig-rich at 4000 feet and up, because of the thinner air. The liquid gasoline doesn't get any thinner.

 

[kmartind]

I agree with most of your post but I still don't understand why you would need to turn up your boost, in fact I am pretty confident you are incorect on this. Everything else makes sence but if your car is making eleven pounds boost it's eleven pounds boost. If I put 35 PSI in my tires at sea level then drove up to 10,000 feet I would understand a pressure change but if I had a regulator on my tires set at 35 psi then when I got to 10,000 feet they would still be at 35 psi. As far as I understand the same applies to the boost sustem. So if I am pushing 11 psi at sea level and I am pushing 11 psi at 10,000 feet what's the difference.

You are partially correct in that the difference between outside air pressure and the boost pressure in the manifold will remain the same no matter what the altitude, and the boost gauge will read the same since that pressure difference is what it measures, however there will be "less air" in the manifold at high altitude so the car will run slower at the same boost pressure. To get approx the same amount of air into the engine, you'd need to raise the boost pressure. When you do this, the boost gauge will (obviously) read higher and the car will gain power (assuming proper tuning in both cases).

The reason high altitude air is less efficiant is because it is less dense, because there is less pressure. Once inside the system if you pressurize it to the same psi it will have the same pressure, and the same density.

This would be true if you pressurized to the same absolute PSI, however, your boost gauge doesn't measure absolute PSI, it measures the difference between boost pressure (or vacuum) and outside air pressure. If your gauge measured absolute pressure adjusted for sea-level, it would read zero at sea level with the engine turned off, but at altitude it would read vacuum with the engine off. That could get confusing.

 

[yoshimitsuspeed]

I want to see some Dyno runs. That would convince me.

 

[kmartind]

I want to see some Dyno runs. That would convince me.

You have to be careful looking at dyno numbers. They may be corrected or uncorrected. See this page for more info:
http://wahiduddin.net/calc/cf.htm
In addition, the numbers also depend somewhat on the type of dyno being used. Uncorrected numbers from the same type of dyno under similar conditions would show lower horsepower at altitude unless the boost pressure were increased and the car re-tuned for the altitude (and even then it might still be lower due to reduced compressor efficiency).