FYI: Altitude Tuning Differences - Hyundai Genesis Forum
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post #1 of 15 Old 04-27-2009, 12:38 PM Thread Starter
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FYI: Altitude Tuning Differences

I did a little research into tuning for altitude and wanted to share some information with you guys. I know some people on here are new to turbo cars and the tuning theory that goes along with them so I thought it's better to start now to provide people with a resource for information.

First, we need to start out by understanding what atmospheric pressure (ATM) is/means. ATM is referred to in the USA in standard units of PSI (pounds per square inch). By definition, this means that ATM is the amount of pressure the molecules of air in the atmosphere above your head at a given altitude exert on one square inch of ____ (insert object here). At different altitudes, there is a different quantity of air molecules exerting pressure which means that ATM goes DOWN as you go UP in altitude. The table below shows the effect of altitude on ATM (expressed below or above mean sea level (MSL):

MSL (ft) ATM (psi)
-5,000 17.48
-4,500 17.19
-4,000 16.9
-3,500 16.62
-3,000 16.34
-2,500 16.06
-2,000 15.78
-1,500 15.51
-1,000 15.23
-500 14.96
0 (SL) 14.696
1,000 14.16
1,500 13.91
2,000 13.66
2,500 13.41
3,000 13.17
3,500 12.93
4,000 12.69
4,500 12.46
5,000 12.23
6,000 11.78
7,000 11.34
8,000 10.91
9,000 10.5
10,000 10.1

So, as you can see, there is quite a difference depending on where you are in altitude in the amount of ATM you would be working with. The most important aspect of all the information here is that the air at higher altitudes is less dense than air at lower altitudes. I think that explains the ATM portion well enough, so now on to what this means to us!

When a turbo car "makes boost", it is COMPLIMENTING (adding to) atmospheric pressure. When you look at your boost gauge in your car (if you have one) it only expresses the relative pressure that the turbo is generating. This means that the gauge reading does NOT express any correlation to changes in ATM! So, lets look at the absolute pressure, ATM+boost essentially, in relation to our cars:

MSL (ft) Abs Press(psi)
-5000 35.48
-4500 35.19
-4000 34.9
-3500 34.62
-3000 34.34
-2500 34.06
-2000 33.78
-1500 33.51
-1000 33.23
-500 32.96
0 32.696
1000 32.16
1500 31.91
2000 31.66
2500 31.41
3000 31.17
3500 30.93
4000 30.69
4500 30.46
5000 30.23
6000 29.78
7000 29.34
8000 28.91
9000 28.5
10000 28.1

*Note that this table expresses normal ATM plus 18psi boost*

So, what this means is that if you tune your car at say 3500' at 18psi and drive to a place where it is 1000' you will experience an approximately 2psi equivalent jump due to air density! Conversly, tuning at 1000' @ 18psi then ascending to 3500' would result in a 2psi equivalent drop! Granted, if your car is stock, the engine computer will compensate for this and make adjustments accordingly. BUT, if you use a MBC or EBC, you "set" your boost level and the computer does not compensate anymore! A 2psi equivalent drop/gain CAN make a car run rich/lean depending on other variables (not good)!

Now, on to what this means in terms of your turbo. The turbo in any vehicle acts as an air pump. It sucks ii, pressurizes, and expells air not giving any notice to changes in ATM. BUT, changes in ATM do effect the density of air the turbo sucks in and, therefore, the density of the air it puts out. What this means is that going up in altitude requires your turbo to work harder to obtain the same intake charge density, or psi equivalent, that it did at a lower elevation! Also keep in mind that both sets of values expressed in the charts above DO NOT take into consideration humidity or temperature (both of which can effect tuning as well)!

The stock turbo in our cars is only capable of pumping so much air (read as efficiency range). What this means as far as our discussion so far is that our stock turbo will have different limits just in reference to altitude. For example, it has been stated that the stock turbo runs out of steam after 18psi in places relatively close to sea level (500-800'), so what this means is that when you take that same turbo and ask it to pump less dense air, it will be able to generate LESS boost. Different locations have different altitudes and therefore this changes the options/viability of using certain turbo configurations as well as changing the way a car is tuned.


I could go into a lot more detail here but I wanted to mainly provide some conversation points and get some of the various tuners represented here involved so that people can be more directed on their pursuits/desires for their cars based on their locations. I will answer any questions to the best of my ability or find you answers to your questions if I cant but I'd like to see some input here from some tuning vendors as far as what they have experienced/know about this whole deal.

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post #2 of 15 Old 04-27-2009, 04:01 PM
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Great write-up. This should be a good intro to understanding boost for people newer to tuning.

Now if only a company would finish cracking the ECU!!
(I don't want to resort to a piggyback unit)
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post #3 of 15 Old 04-27-2009, 04:27 PM
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Quote:
Originally Posted by majred View Post
Great write-up. This should be a good intro to understanding boost for people newer to tuning.

Now if only a company would finish cracking the ECU!!
(I don't want to resort to a piggyback unit)
+1.....

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post #4 of 15 Old 04-27-2009, 05:16 PM
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now anybody wanna do a write up on what the temperature and humidity does to turbos??

...and i'm back
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post #5 of 15 Old 04-27-2009, 05:23 PM Thread Starter
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Patience grasshoppa, patience... haha. I'll do that another day if no one else beats me to it.

Also, I definitely agree about the ecu cracks deal. Preserving some of the stock functionality or being able to reprogram it to your liking is VERY appealing.

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post #6 of 15 Old 04-28-2009, 04:07 AM
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Hey MSUGeoman22! Long time racer short time turbo car drive/tuner. (wheres my s4 86 rx-7?!?!)

Could you let me know if this logic is correct?

Say I had a MBC and I set it to 18psi on my car @ 3500 feet. THen lets say I was hungy and descended to 1000' cause that is where the closest place to eat is. I am starving, anyway, now since I am at 1000' my turbo would be pushing out potentially 20psi, BUT due to the ecu cut off it would detect it is above 18 psi and go into limp mode. Thus I would need to turn off the car, disconnect the negative terminal, re-set the MBC to 18psi for that altitude and be on my merry way right?

I am Canadian
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post #7 of 15 Old 04-28-2009, 04:42 AM
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Quote:
Originally Posted by majred View Post
Now if only a company would finish cracking the ECU!!
(I don't want to resort to a piggyback unit)
I have a hammer at home so if you bring your ECU over I can crack it for you well at least the outer plastic housing...
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post #8 of 15 Old 04-28-2009, 05:36 AM
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Quote:
Originally Posted by jiyooniexp View Post
now anybody wanna do a write up on what the temperature and humidity does to turbos??
As for temperature:
I remember this equation from chemistry class: PV = nRT ( just looked it up, it's called the Ideal Gas Law )
P is Pressue
V is Volume - this will be constant so we can ignore it as we are only interested in relative numbers, really.
n is number of moles - WTF, ignore it it's a constant anyway
R is the gas constant based on what units the others are.
T is temperature

So... here are some numbers for 1 litre (V), 1 mole (n). PSi is so high because I tried to squeeze "so much" air into a litre of volume. Half the volume, double the pressure. Double the volume, half the pressure. But the relative delta will still be the same
HTML Code:
Temp. F(C)       PSi         delta
32 (0)            329.37	
35 (1.7)         331.38	0.61%
40 (4.4)         334.74	1.01%
45 (7.2)         338.08	1.00%
50 (10)          341.43	0.99%
55 (12.8)       344.78	0.98%
60 (15.6) 	348.13	0.97%
65 (18.3)       351.48	0.96%
70 (21.1) 	354.83	0.95%
75 (23.9)  	358.18	0.94%
80 (26.7) 	361.53	0.94%
85 (29.4) 	364.88	0.93%
90 (32.2) 	368.23	0.92%
95 (35)          371.58	0.91%
100 (37.8) 	374.93	0.90%
105 (40.6) 	378.27	0.89%
110 (43.3)	381.63	0.89%
115 (46.1) 	384.98	0.88%
120 (48.9) 	388.34	0.87%
125 (51.6) 	391.68	0.86%
130 (54.4) 	395.02	0.85%

So from what I see there isn't much of a difference, 1% per 5 degrees.
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post #9 of 15 Old 04-28-2009, 05:50 AM
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Thnaks mister for taking the time to cover that & a special thanks to geoman for bringing up this topic in the 1st place, good stuff.

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post #10 of 15 Old 04-28-2009, 06:02 AM Thread Starter
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Quote:
Originally Posted by Canuck View Post
Hey MSUGeoman22! Long time racer short time turbo car drive/tuner. (wheres my s4 86 rx-7?!?!)

Could you let me know if this logic is correct?

Say I had a MBC and I set it to 18psi on my car @ 3500 feet. THen lets say I was hungy and descended to 1000' cause that is where the closest place to eat is. I am starving, anyway, now since I am at 1000' my turbo would be pushing out potentially 20psi, BUT due to the ecu cut off it would detect it is above 18 psi and go into limp mode. Thus I would need to turn off the car, disconnect the negative terminal, re-set the MBC to 18psi for that altitude and be on my merry way right?
Canuck, that logic is essentially correct. With a MBC there is no flexable adjustment for changes in ATM like the factory solenoid and MAP sensor system does. Once you set your MBC, it is set at that bleed off pressure until you physically change the setting. The best way to avoid this seems to be to let the ECU run the show for now until some tuning options come out. Some conservative tuning can mitigate MOST of the effects of altitude/humidity/temp and still make good reliable power IMO.

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