Turbo Elevation Question

[ippielb]

So, i have a question about elevation when running a turbo.

Ambient pressure at 7,000 feet is 78kpa(11.34psi)
Ambient Pressure at 1,500 feet is 96kpa(13.91psi)

Turbo's pressure the cylinders with compressed air, so mine pressures to 7psi, does this add on top of the ambient pressure? So at 7k feet it would be 18psi, and at 1.5k feet it would be 20psi? Or does the turbo compress to a set pressure regardless of ambient pressure? I am confusing myself with this and would like someone to help me with understanding this.

I'm wondering what fuel i have to run in comparison between the two. I have my heads machined to run AV gas at 7,000 feet at 7psi. If i come down to 1,500 feet, what fuel will i need to run at home to get it on the dyno? I want to bring out my Land & Sea Dyno for the first time and put my sled on the dyno to break it in, and to give it a tune.

 

[Jamesh11]

Ooo,
This could turn into a very interesting discussion with a variety of different opinions.

My short answer is, I don't know.

While we wait for people who know to chime in I'll throw in a few things that I think.....not the same as what I know

1) Turbo will compensate atmospheric pressure by building boost to an absolute pressure of the waste gate meaning what the waste gate is set at......ultimately it has to reach the waste gate set pressure.

In effect this means using your numbers 13.91psi at 1500ft and 11.34psi at 7000ft that the turbo has to overcome the extra deficit of 2.57psi at 7000ft by building that extra boost.

Now.....extra heat of turbo rpms will "thin" the air (colder air is denser than hot air hence why people use intercoolers to help turbos and superchargers) Engines need air mass not volume....don't tear me apart for that

2) So, the turbo in effect works harder to deliver the air density the engine needs to make the same horsepower at higher elevation.

3) To contradict that, the air/fuel ratio should be easier reached at elevation with less detonation issues than at sea level. Sounds like a bunch of crap to me too, but you need better fuel or richer setups at sea level under boost to reduce the chances of lean conditions. That is a whole other discussion.

I can add more nonsense but I have to go to work.....Like I said I don't know, but I'm sure people will chime in now that I screwed up your thread which is in effect what you need )

I need to know as well )

 

[Jamesh11]

Some more thoughts which are not necessarily true.

Atmospheric pressure at sea level is about 14,7 pounds per square inch (psi)

If you run 6 or 7 pounds of boost a turbo compress that 14,7psi air by 6-7psi. So in effect you make the air denser.

At higher elevation where the air is thinner say 12psi, the turbo has to compress the air much more to achieve the same effect as at sea level. Hence you use higher boost to achieve similar horsepower.

Horsepower is made by the power of the explosion of the air/fuel mixture in the cylinder. Compress air and add fuel to achieve your air/fuel ratio and with this new denser air and more fuel in the cylinder you get a bigger explosion.

Take into consideration that at elevation a turbo works harder ie turbo rpms to achieve the desired effect as compared to sea level. This turbo workload causes heat and heat thins air which is counter productive.

Basically to explain it in another way in my head it comes back to air at sea level (14,7psi - 1 Bar) At 6-8psi you compress the air by about 1/2 Bar. If you use this same thought at higher elevation you need more boost to first reach that 14,7psi and then compress it by even more boost to reach the same effect as can be done at sea level with less boost(workload)

Not sure if this makes any sense, but I'm trying.


So what I'm getting at is that yes you add the 7psi to the ambient pressure at that particular elevation, BUT....
in order to get that same 7 psi at higher elevation the turbo will have to pump more air as the ambient air is less dense the higher you go in order to just reach the goal of 7psi compression. I'm trying to say that 7psi is 7psi on paper but really its not as higher workload of the turbo causes heat which thins out the already thin air at elevation. Not only this, you also have to make up the deficit in ambient psi at altitude.

 

[ippielb]

At least i'm not the only one not 100% sure on the answer. Can anyone else chime in and lend a hand to try and understand? So for the octane question, i just need to pump more fuel in at lower elevations or use a higher octane level fuel? I'm confusing myself here.

 

[Jamesh11]

In a car at least you can compensate somewhat to reduce lean conditions by increasing fuel delivery. You can only increase the fuel delivery so much before you screw up the A/F ratio and thus performance of the vehicle.

However, if you advance the timing enough you end up building too much heat and have to compensate for lean conditions by running higher octane fuel. Higher octane fuel reduces the burn rate of fuel allowing you to run higher compression due to reduced temperatures inside the cylinder.

Higher compression leads to more power.....

 

[ippielb]

In a car at least you can compensate somewhat to reduce lean conditions by increasing fuel delivery. You can only increase the fuel delivery so much before you screw up the A/F ratio and thus performance of the vehicle.

However, if you advance the timing enough you end up building too much heat and have to compensate for lean conditions by running higher octane fuel. Higher octane fuel reduces the burn rate of fuel allowing you to run higher compression due to reduced temperatures inside the cylinder.

Higher compression leads to more power.....

How much octane do you think i would require? I have a pail of 110 in my shop that i can use, but do you think i would need more?

 

[Arctic Thunder]

I would run 50% avgas with 50% 110. And watch the knock sensor.

Your never going to get a real straight answer.

But I was told I could only add about one lbs of boost when going from my standard 5000-8000 riding to 8000-12000. When running the same fuel.

So IMO not much of a change. You would need to back off on your boost by 2lbs or so to run 100ll at 1500'.

Just a stab in the dark, kind of like everyone else.

Thunder

 

[Jamesh11]

How much octane do you think i would require? I have a pail of 110 in my shop that i can use, but do you think i would need more?

What I've seen here..... 14lbs boost needs full race gas
10lbs boost needs AV gas
6lbs boost can run pump gas

Again that's just what I noticed people write. Do these people know what they're doing? I don't know.

What I can say is
1) The hotter your engine runs the higher number octane you need at the same compression ratio. So for example at a given compression ratio of 10:1 an engine running at 210F will need 5 points higher octane than an engine running at 170F. That works for any compression ratio.

2) At a compression ratio of 9:1 an engine running at 210F needs 100 octane. Use that and calculate from there. 8:1 running at 210F needs 95. Plot a curve by using this.

What you can try as well is.....
Start with your 110 octane at a ratio of 1:10 pump gas. That is add 1 gallon 110 to your tank of pump gas (which presumably is close to 10gal)
Put in a new spark plug and do a full throttle run. Brake hard to a stop and look at the spark plug with a magnifying glass.....if you see black spots you have detonation and need more 110. If not, back down on the 110 .

Just some thoughts you can play with....

 

[ippielb]

I would run 50% avgas with 50% 110. And watch the knock sensor.

Your never going to get a real straight answer.

But I was told I could only add about one lbs of boost when going from my standard 5000-8000 riding to 8000-12000. When running the same fuel.

So IMO not much of a change. You would need to back off on your boost by 2lbs or so to run 100ll at 1500'.

Just a stab in the dark, kind of like everyone else.

Thunder

Wish i had a knock sensor on my 700 laydown(800 big bore), but sadly they didn't come with one. I had the heads made up by rktek to run 100 av gas at 4-7k, maybe i should give them a call and see what they think.

What I've seen here..... 14lbs boost needs full race gas
10lbs boost needs AV gas
6lbs boost can run pump gas

Again that's just what I noticed people write. Do these people know what they're doing? I don't know.

What I can say is
1) The hotter your engine runs the higher number octane you need at the same compression ratio. So for example at a given compression ratio of 10:1 an engine running at 210F will need 5 points higher octane than an engine running at 170F. That works for any compression ratio.

2) At a compression ratio of 9:1 an engine running at 210F needs 100 octane. Use that and calculate from there. 8:1 running at 210F needs 95. Plot a curve by using this.

What you can try as well is.....
Start with your 110 octane at a ratio of 1:10 pump gas. That is add 1 gallon 110 to your tank of pump gas (which presumably is close to 10gal)
Put in a new spark plug and do a full throttle run. Brake hard to a stop and look at the spark plug with a magnifying glass.....if you see black spots you have detonation and need more 110. If not, back down on the 110 .

Just some thoughts you can play with....

I don't have the knock sensor so when i get my sled going on the dyno i guess after i warm her up then i will have to check for detonation that way. Thanks i didn't know you could do it that way.