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Old 04-03-2012, 11:06 PM   #1
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Default Moving from VT to CO - noobish questions

I'm moving from about 600 feet above sea level to 7500 feet.

What should I be thinking about with my tune?

With an NA car, I lose about 10% horsepower at altitude. What happens with my boosted car? Since the air is thinner, can I increase my boost without an intercooler? I'm wondering if the temp increases from higher boost are the same with lower density air.

The car is a '93 LE 1.6 with a BEGI S kit, no intercooler, no blow off valve, no MAF.

Thanks for your thoughts.
Jim
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Old 04-03-2012, 11:27 PM   #2
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What are you running for boost control and engine management?

If you are running closed loop boost control, zero to minimal change would be necessary.

If you are running a MBC or open loop boost control, you would need to crank the **** a little to raise the boost up to match what it was before you increased altitude. Your boost level will be lower the higher you go, unless you make up for it by making adjustments.

Hmmm, making your turbo work a bit harder to make that extra boost will make the air hotter because you are compressing it more... I'm not sure if this will be a problem or not.
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Old 04-03-2012, 11:34 PM   #3
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Originally Posted by Bryce View Post
What are you running for boost control and engine management?

If you are running closed loop boost control, zero to minimal change would be necessary.

If you are running a MBC or open loop boost control, you would need to crank the **** a little to raise the boost up to match what it was before you increased altitude. Your boost level will be lower the higher you go, unless you make up for it by making adjustments.

Hmmm, making your turbo work a bit harder to make that extra boost will make the air hotter because you are compressing it more... I'm not sure if this will be a problem or not.
I'm using a Brain built MS2, AEM Uego wideband. Boost is set at 5lbs, fixed. No mbc or ebc.
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Old 04-04-2012, 12:06 AM   #4
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Based on an atmospheric pressure chart, you would lose ~2.9 psi going from 600ft to 7500ft. The question is whether your setup can handle 8psi at sea level without an interfooler.

According to Begi's website, http://www.bellengineering.net/produ...roducts_id=244 , you're good to 8psi. Whether the 8psi they're talking about is at sea level (cooler IATs), or high altitude (hotter IATS), I don't know. Still, you won't actually be running 8psi on your boost gauge, just compressing the air by 8 psi.

I'd be interested to see exactly how much your boost level drops once you move.
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Old 04-04-2012, 12:22 PM   #5
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Sounds about right. I live at 7200ft and I tuned my car at this altitude. You will still see 5psi on your boost gauge, but if you check your MAP reading in megasquirt you will only be doing about 115kpa now. For instance I run 10psi on the reg, but it only registers as 150 or so KPA instead of the 170 or so KPA i'd see at sea level.

Altitude is fun, wait until you start messing around with v8's. They are gutless above 5k feet. Muahaha
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Old 04-04-2012, 12:36 PM   #6
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I'll post back with what I learn.
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Old 04-04-2012, 12:44 PM   #7
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Quote:
Originally Posted by Bryce View Post
see exactly how much your boost level drops once you move.
Since "PSI" is just a measurement of your manifold pressure its going to read the same. At sea level hes reaching 5PSI(35KPA) over atmospheric pressure(100kpa at sea level). So at sea level 5psi = 135KPA.

Now at 7500ft altitude hes going to make the same 5psi(35kpa), but hes starting off with less atmospheric pressure (about 75-78KPA instead of 100 at sea level). So hes going to be at around 110KPA with the same boost levels hes running at sea level.

My greddy kit on wastegate (4.5psi or so) barely made 110KPA when i first started tuning it. Man did that confuse me at first.

Oh also, you will lose a little spool time.
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Old 04-04-2012, 01:42 PM   #8
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Your fueling/timing/etc./tables should be unchanged. If you're running a MAP sensor, it will show a decrease of about 2.9psi because it references absolute vacuum. Your wastegate and boost gauge will work the same as before since they both reference ambient air pressure. As has been said, you'll need to turn the boost controller up about 2.9 psi. Your boost threshold will be a touch higher in the RPMs, but if you're getting on it, it won't really matter because you'll still be well above threshold between shifts. Once you crank your controller up 2.9 psi, your engine will operate identical to your current setup at your current altitude. Aside from a slightly delayed boost threshold, (and possibly a barely slower initial response up top when you gun it), the only difference you'll notice is that your boost gauge will show you about 3psi higher.

And to answer your next question, yes, your intake temps are going to be higher - for two reasons:

First off, being at a higher altitude means that air molecules are going to have more energy relative to air pressure than those at lower altitudes - this works on the basic principal that heat rises. In opposition to this, the air is going to be less dense, and so cooler. Generally, in the pressure realm of the troposphere, the temperature decrease due to the reduction in air pressure is going to be more significant than the temperature increase due to altitude. This would be the reason the air feels colder - in theory, then - if you pressurize that thinner air to the same pressure as sea level, the temperature of that newly pressurized air will be higher than the temperature of the sea level air. This is of course assuming that the atmospheric conditions are either stable or overstable. An understable atmospheric condition, depending on the severity of the instability, provides the potential that the newly compressed air might still end up colder than the air beneath it at sea level. Now in layman's terms: Once you compress the Colorado air to 1 Bar, it is going to start out warmer than the sea level air which is already at 1 Bar.

Secondly, as previously stated, your turbocharger is going to have to work harder to make that extra 2.9psi. For each additional PSI of pressure, the turbocharger is going to add more heat than the last PSI. In layman's terms: your turbo used to increase pressure 5 psi. Your turbo will now have to increase pressure 8 psi. To get from 4-5 psi, your turbocharger might add (temperatures for example use only, I don't know what the real numbers will be) 15*f. To get from 5-6 psi might add 17*, 6-7 psi might add 24*, and 7-8 might add 33*. In my example of purely fictitious deltas, your turbocharger will add 74*f IAT in order to achieve the same absolute pressure as you did at sea level. This is in addition to the natural temperature increase of the air from the paragraph above simply to start out at 1 Bar.

Last edited by fooger03; 04-04-2012 at 02:04 PM.
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