Comparing boost from sea level to our altitude

[Dallas J]

I was on 20vTurbo.com and came across this thread.

Was just searching the web and found this.
Is this true or not?

""Most common pressure gauges that are not "absolute" pressure gauges, will have higher readings as you go up in altitude above sea level.

For example, at sea level if your boost gauge reads 18 psi above the atmospheric pressure, (14.7 psi). With the same amount of boost pressure in the intake manifold, this same pressure gauge will read 22.6 psi at 10,000 ft altitude, due to the atmospheric pressure only being 10.1 psi at this higher altitude.""


The only reply was:

Correct -
If you've every flown in a small plane, the first thing you do is adjust your gauges to the altitude of the airport relative to sea level. This keeps you from flying into the ground. Some even have adjustments for barometric pressure for the current weather system.
Rookie



Now I know that the turbo has to spin faster to make the same amount of pressure, something like 17%. I thought that 1psi of boost was 1psi. One turbo or car at a different elevation might have a more dence charge cause of the heat difference

Does this guys theory hail any revelence to why for instance paul was able to run 16psi on his 18g and a stock fuel system and most people at sea level can only run 16 with a 14b?

Also, what constitutes an Absolute pressure guage?

[Bedlam]

I can safely say I have NO damn clue. :)

-Bedlam

[Dallas J]

Originally posted by Bedlam
I can safely say I have NO damn clue. :)

-Bedlam

lol...Wheres John when you need someone to chime in and make us younguns feel stupid:Beer:

[newt2]

""Most common pressure gauges that are not "absolute" pressure gauges, will have higher readings as you go up in altitude above sea level.

AFAIK that's incorrect.

An absolute gauge reads absolute pressure, ie it accounts for the atmosphere. At sea level an absolute gauge would read 14.7psi with no boost applied, or 14.7 psi would be represented as a zero so as not to confuse people. Here in Denver that same gauge would read less than zero because there is literally less pressure.

A relative gauge measures pressure relative to the outside pressure. It always reads 0 at any altitude with zero boost applied.

1psi is 1psi everywhere. If anything an absolute gauge would read less boost at altitude than at sea level and a relative gauge would read identical.

[davidm_sh]

His statement is entirely TRUE. Look at the boosting picture in two chuncks.

first chunk: Lets say your turbo is NOT working at all = making NO boost. At sea level normal atmospheric pressure is ~14.7psi. So you would now have 14.7psi total pressure in your manifold with the car even turned off. When it's running you would have the full 14.7psi when the boost gauge reads 0psi. So whenever your boost gauge reads 0psi (no matter what altitude you are at) that is when you are getting full atmospheric pressure into your intake manifold. NOTE: atmospheric pressure is what changes with altitude.

Second chunck: Now lets say you fixed that POS turbo (stupid thing [heh]), and you are now pushing 10psi at sea level (10psi on boost gauge). So now your total pressure in your manifold (absolute manifold pressure = MAP) is about 14.7+10 = 24.7psi total.

Now lets say you get in your car and drive to lovely Colorado Springs [hah] at 6000ft above sea level the atmospheric pressure will drop by about 3psi so you now have 11.7+10 = 22.7psi.

This is EXACALLY why Paul is able to run 16psi with the stock injectors while sea level people can only run about 14 like you said.

ANother example since I have been tuning my LInk lately is I have been boosting about 1.3bar = 18.4psi(on my boost gauge) BUT my MAP sensor is only reporting about 202kPa = total pressure of about 29.7psi. So if you subract my boost gauge pressure from the total you will get the atmoshperic pressure = 29.7-18.4 = 11.3psi which is about 3psi less then sea level :).

I hope some of this made sense. Another thing to think about is this. If you want to truly try and compensate for altitude you would have to boost about 3psi more. So my setup could probably support 17psi at sea level = 14.7+17=31.7psi total pressure BUT if I wanted to get that same total pressure up here I would have to boost about 20psi.

BUT the other thing to think about is the turbo works about 17-18% harder up (like you already stated) here so that 20psi my turbo is making up here is equivalent to 20*1.18 = 23.5psi at sea level in terms of how fast it's spinning. So you better make sure you know the efficiency range of your turbo ;).

As such mine is getting VERY close to it's limits at even 1.3bar. Then you have to look at intercoolers and blah blah ... I will stop now [heh].

[Weston]

Yeah, I had always assumed that the gauge is relative to the outside atomospheric pressure, so 2 psi of boost here would similar to running N/A at sea level.

[davidm_sh]

Originally posted by Weston
Yeah, I had always assumed that the gauge is relative to the outside atomospheric pressure, so 2 psi of boost here would similar to running N/A at sea level.

Actually from what I have been seeing lately you would have to run about 3psi to make it equivalent to sea level. But in short yah :).

[Dallas J]

Outstanding knowlege guys...It all made sence one Newt said about the how the guage works...

So right now if my car can handle 13 psi at sea level with my B16g, then at 16psi my turbo should be singing. I do be l heard that the B16g runs really nice at 19psi which is about 18% more than 16psi.

Im gonna turn the boost up and log it to see how she likes it. Then do the rewire and see if 17psi is doable.

[V8SpankR]

So far with my re-wire with the stock fuel pump I was able to push 17psi on 104 octane with zero knock and the timing looked great all the way up to 7K rpms.
I hope to have another pump in by next week and try 20psi.

[thimages]

I have a MAP sensor and a boost gauge.

If I remove the altitude correction, it reads about 2.4 psi higher than the boost gauge.

So, you can use 2.4 has the difference in pressure.

Hal

[bikenski]

If you REALLY want a geeky technical explanation of changes in pressure at altitude, check out http://www.skyrunner.com/story/pikespeako2.htm

It's written for runners, but the concepts apply to our cars also. It has graphs and even calculators to tell you the air density at any altitude. Too bad the runners don't have turbos to "cheat" the lack of dense air like we do!

[B16crxTurbo]

So if you had your boost controller set at say 15psi at altitude, then drove to sea level, would it still boost to 15psi? or would that ~3psi difference make you run 18psi?

[thimages]

Boost controllers are a little different.

If it's a manual b/c (set to 20 psi), it will still take 20 psi to crack it... you'll just get there sooner because of the increased air density.

If it's an ebc like a profec, yes... you'll actually make less boost because the difference in manifold pressure vs absolute atmospheric pressure.

EBC's are different because they use a MAP sensor, which 0 represents sea level. At this altitude, the MAP has a value BELOW zero.

It's confusing (sometimes) and VERY dependent of the actual components in a specific car.

Mechanical gauges here read lower than they would at sea level.

Hal