Ok. Some common misunderstandings and myths resolved. Let's start with the basics.

The atmosphere we all breathe and drive our N/A beater cars in are breathing normal air, which is standard atmospheric pressure that is contingent upon your altitude above sea level. With that being said:

Relative pressure-Pressure deemed over and above the atmospheric pressure. Example, "I run 6 PSI of boost"

Absolute pressure- Pressure including atmospheric pressure. An example is, when we where in FL, we where about 20 ft about sea level, so we where almost exactly 1 Atmosphere, ( I will get into the measurements later), and running 6 PSI of boost would mean: Atmospheric + 6 PSI of turbo pressure

Now for the specifics on pressure:
1 Atmosphere ~ 14.5 PSI ~ 101kPA~ 1 BAR
^^These are all approx equal!

With this useful information, I can know explain absolute pressure of 14.5 PSI + 6 PSI of pressure, which would mean 20.5 PSI of ABSOLUTE PRESSURE

Now that we know the equivalents of one another, we can easily convert amongst each.
IE: 1.7 BAR in relative pressure is approx 23 PSI ~ 160kPa.




Now, that we understand that....Now the correlation of altitude comes into play. Recall as I stated before, that barometric pressure changes as altitude changes.....With that being said, at sea level or around there it is 101kpA. (Kilopascals, for those wondering ) Anyway, as the altitude increases the barometric pressure decreases because the air gets thinner. As a result, you will have a lesser base (atmospheric pressure) therefore will make less boost overall due to this fact. Understand?

At sea level, your pressure is 101kpa. So with 6 PSI of boost ~ 60 kpa that is ADDED to the 101 (speaking in terms of absolute pressure), is 160kpa. Great!

Now move to the 5000 ft above sea level (in the mountains or Denver), and it's approx 84kpa, which now means 8 PSI of boost ~ 60 kpa is ADDED to the 84, which means that it is 144kpa. This means you will see about 4 PSI on your boost gauge. This means LESS boost and LESS power on your boost gauge due to barometric pressure changes caused by altitude.

Questions?!??!

 

Duh I knew all of that already! That is sooo common sense!

 

DOH!

 

You'd be suprised on how many calls we get that people do not understand this.

 

(calling as we speak)

 

Another good right up. So our boost gauges are reading the absolute pressure not the relative pressure? Or its the relative pressure that our gauges reads but then actually depending on altitutde and temp, performance varies due to the diffrerence in absolute pressure or actual pressure our car is getting. But also denver is a lil colder than FL so, its also a factor..

 

Boost gauges read relative pressure.

 

HAHAHA in English... hes saying those of you who are calling from the mountains saying my turbo is only boosting 5 psi instead of 7 I want a refund.... QUIT CALLING!

 

^^ lol.. but no, boost reading should be the same unless he has some leak somewhere.. but absolute pressure is not, thus performace varies.

 

good stuff, however with respects to FI the pressure loss from seal level to higher altitude isn't going to be that signifigant. yea the turbo is going to work harder but the pressure drop is going to be as significant. now n/a on the other hand. that's a whole different story

 

Bring your turbo car to Colorado Springs or Denver and get back to me about that statement. This 100% true and 100% factual.

 

Wouldnt the ecu adjust to make the car make the same power regardless? Not sure just asking..

When i drove my old wrx up a mountain i had hit about 19 psi and my tune was for a max of 17.5 so the car adjusted timing/fuel/boost on its own or was that just a natural reaction to the air?

I just learned on top chef that altitude even affects cooking to an extreme lol lol lol i thought it was interesting

 

yea, altitude does affect cooking as well. the temp at which water boils changes with alitude.

paul i'm not trying to start a ____ing match with you, and i wasn't disagreeing with you. i stated that the change would be less with FI.

PSI is PSI, doesn't matter if it's in denver, new jersey or wherever. pressure is pressure. the turbo is going to have to work harder if higher up cause the air is thinner. if oyu were making say 10psi at sealevel, and you go to denver to make that same 10 psi the turbo is going to work harder and produce more heat. hell it might even be out of effiect. island. your car is probably going to be slower (unless you have a good as IC system)

if you have a good FI setup the loss is small cause your turbo will make up for it.

that is all i'm stating

 

Not bad Paul. A couple things though.

60kpa is 8.7psi, not 6psi

As far as power losses due to atmospheric pressure changes on a turbocharged car, i agree, the losses are small compared to an N/A car.

Our MAP sensors read Manifold Absolute Pressure. If you're running 200kpa MAP at sea level, and you're running 200kpa MAP at 5000ft., then the pressure in the intake manifold has not changed. Yes, it took more "boost" to make 200kpa, since atmospheric pressure was lower, but at the end of the day the manifold pressure is the same. What does change is the compressor outlet temperature (since you're technically making more "boost") and the exhaust backpressure. The amount of change you'll see in these two things is dependant upon where you are on the compressor map and turbine map of your turbo.

If you're maxing the turbo out at sea level, then you're going to see significantly less power at 5000'. If you're running 10psi at sea level (~165kpa), and you go to 5000' and run 165kpa MAP, then you are not likely to see much difference in power.

From personal experience racing all over the country, in Denver (about 85kpa atmospheric pressure), we were seeing about 5-8psi less boost compared to when the turbo was maxed out at sea level. We would also see as much as 15-20psi more exhaust backpressure on our race cars that were maxing the turbo out.

As far as Dynojet numbers at 5k ft., the SAE numbers are bull____ because the dyno adds 20% to make up for the loss in atmosphere, but the turbo is already making up for it. The difference in power at a given boost level is more in the area of 5%. On a 600whp turbo dyno, a graph from Denver or Utah will read almost 120hp higher than it should. Take high elevation dyno graphs with a grain of salt... lol

That's my $50.

 

thank you tony.,... i was running out of breath

 

DAMN, tony!
$50 for a 2 minutes wrieup? I don't even want to know you hourly tuning charges!

 

relative pressure is also known as gauge pressure. Good write up. i didn't know this was a huge problem?

 

haha, i didn't know it was a big problem either.

And yes, Absolute pressure and Gauge pressure are generally what it is called.

 

nice infos, lol it cleared up some thoughts that i didnt even realized was unclear lol

 

also equals 760torr haha just thought id try to raise my selfesteem by feeling smart.