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  #1  
Old 03-01-2005, 12:33 AM
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Default hp loss for altitude

hi, i need help on how much hp you would loose in altitude gaining.If my car has 200hp at 5500 altitude how much would it have sea-level?
Dont the manufactures rate the hp rating from sea-level? so if they come up to 5000-5500 alt. then they would loose some hp? Help me get this straight.
thanks
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  #2  
Old 03-01-2005, 02:33 AM
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I don't know the ratio for power loss at this altitude man. But I can tell you that for every 2000 feet gained from elevation above sea level you add 1 tenth of a second onto your elapsed time in a 1/4 mile.

if you are at 200hp at this altitude, you can expect 2-3 tenths gain in speed out of sea level.

You also have to account for humidity (barometer) increase at lower altitudes due to the moisture from having denser air and being closer to a body of water.

You gain 2.5 tenths in LasVegas coming down from Denver and you only gain about 1.8-2 in LosAngeles coming down from Denver. This is due to the increase in moisture in the air in LosAngeles.

For what it's worth, I have a '73 Plymouth Scamp with a 318 that is at about the same horse power (little higher) as your car and I run a 17.4 at Bandimere Speedway and a best of 16.88 in LosAngeles when I was there (this includes advancing the timing for their pump gas and rejetting my carb for sea level.)

I bet that if the car was left alone, I could account for 2 tenths due to lack of rejetting and advancing.

The minimum octane in southern california is 87 (here is 85 due to the thinner air)... Their premium octane is 95 (ours is 91)... This plays a factor as well.

I don't know that there is a substantial increase and decrease in Horse Power and Torque between altitude differences consistantly. Every vehicle acts differently in altitude changes and perform differently. Smaller displacement engines have less of a reaction to altitude changes because they don't use as many CFM naturally and the volume of air and quantity of burnable oxygen stays the same, no matter how small and how big your displacement is at the same altitude. A larger engine uses more air and fuel because it can, but the ambient air is more efficiently burned in smaller percentages.

Think about the use of air and it's mechanical properties... An airplane with a large wing surface would fly easyer at a low speed than an airplane that had a smaller wing surface area. This is due to the volumetric pressure of the atmosphere. The same principal applies to an engines displacement, only in an opposite manner. The smaller displacement of combustion chamber, the less air it needs in order to run correctly. A four wheeler at 12,000 feet will do better with a four cylinder than a v8 with lots of power. The v8 chokes where the smaller displacement engine can keep up with the lesser amount of oxygen.

In turn, I know that there is a correct jetting ratio for altitudes in the 1000th of an inch increase and decrease increments, but I'm afraid that horse power gains and losses due to altitude change always depends on the displacement and CFM flow of the specific engine in question. There is no ultimate rule of horsepower to altitude ratio for all naturally aspirated engines. Forced induction engines (tubocharged and supercharged) engines are also affected little to none compared to a naturally aspirated engine, so there cannot be a general rule of HP for altitude.

I have some experience in LA and here in Denver with my '73 Plymouth Scamp, 318/904/ 3.23:1 gears... If you'd like some numbers and things that I did it can give you an idea of what your car's numbers and performance may look like in lower altitudes.

Just let me know!
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Old 03-01-2005, 07:34 PM
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Could u give me some specs on the Scamp? does it have a posi? wat do u think my car will run? its a '74 swinger w/ 200hp 318 with a 904, 7 1/4 w/ 2.76. I might get more hp cuase im getting a new distributor and a new ignition kit from FBO. And we might get it jetted down alittle. So yea it might have 215hp. The torque is 300.
Later on ill add 4.10 gears and that will help so much, right?
and i might have some traction probs with the skinny 195/75/14 tires.
My firend has a '96 mustang gt w/ intake and exahust and says he'll kiss my A$$. What do u think botu that? he sais he'll run 15's, and when he get alum heads he run 12's...LOL!
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Old 03-01-2005, 09:43 PM
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Figure 5% per 1,000 feet above sea level as a rough guestimate. Actually, there's more to it, you need to figure in temperature and humidity to get the density altitude, but that's more than you probably want here. On a hot summer day here in the Mile High City, the density altitude can approach, occasionally even exceed 10,000 feet.

Sea level air pressure on a standard day is 29.92" of mercury. Here in the Denver area, it's around 24.5" to 25" or so. When the weatherman gives the barometer reading, it's corrected to sea level - subtract about 5" or so from it and you'll be close. You need an absolute pressure gauge (or an altimeter with a baro correction window) to know for sure.

I figure I'm down about 20-25% in this area (actually, my house is at 6,600 feet, I'm probably down 30% from sea level.

You'd probably be looking at 250 or so at sea level.
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Old 03-01-2005, 10:01 PM
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Power loss due to altitude is related to the change in barometric pressure. A simplified view would be to use the ideal gas law PV=nRT. Where P is Pressure, V is Volume, n is the amount of gas (expressed in moles), R is the ideal gas constant, and T is the temperature. The point here is that the pressure has a direct relationship to the number of molecules of oxygen in a given volume. Since the power you can make is related to how much oxygen you can get in the cylinder, reduced pressure will reduce the amount of oxygen. So using this simplified view :

Power@sea level = Power @ test barometric pressure X (29.92 inches mercury / test condition barometric pressure measured in inches of mercury)

So for your case Power@sea level = 200 x 29.92 / 25 = 239.36.

Humidity is a factor in making power, but for the e.t. the humidity contribution is due to the density of the air you have to move out of the way. Barometric pressure also has a significant impact in this area.

For an introduction to internal combustion engine theory you may want to pick up a copy of "Internal Combustion Engines and Air Pollution" by Edward F. Obert. Don't be scared away by the "and Air Pollution" part.

DB
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  #6  
Old 03-02-2005, 07:15 AM
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There is a lot of math envolved in calculating the atmospherical affects in an internal combustion engine. Like Dan, Bill and myself have stated, it can only project what your car will do. I think 5% is close, but there are so many variables that have to be calculated into the equation that it becomes within a margin of error to expect solid numbers.

In short, the only way to know is to experement.

A GT huh? Well, I'm not a Ford buff and the last thing I'll set foot in would be a Mustake, but I'm assuming it's a 4.8L V8? 15.0 flat is probably a bit of a stretch in the truth and if we're talking a V6 it's a straight lie.

Stock, you'll lose. That's a promise. I have 3.23:1 gears and ALL of the ignition components in my car are new. I haven't bought a recurve kit for my distributor yet though and I'm sure that's worth a tenth or so at least. But that 17.2 E.T. is through single exhaust and a new 2bbl. You're right about lower gears, but 4.10 is overkill. Those 2.73 gears are gonna hurt in 60 foot times. With your current setup on corrected timing, jetting, new ignition and 91 octane you should be running about an 18.2-18.4 E.T.

Our race club at Bandimere is made up of a lot of relatively stock or mild 318 and 360 A bodies that run anywhere from 18.7 to 13.2 1/4 mile passes. I know what these cars like to do with specific modifications because I have seen a LOT of it on these cars.

If you want to eat your friends Mustang, you'll need a few things:

New double roller timing chain for cheap insurance should be first on your list, your car has one of those "silenced" cam gears that has plastic teeth and can go out QUICK with a lot of HP thrown at it.

Get a 340 camshaft from http://www.hughesengines.com and matched lifters and springs. This is easy to do with the timing cover and chain off. I suggest their hydraulic lifter cam grind number HEH1019AL... it's a step up from a 340 and even an RV and will really wake up your 318!

Hooker 1 5/8" primary to 3" collector A-Body headers into 2 1/2" full exhaust and a set of turbo mufflers would be a good idea as well. a 1" of front suspension lift and rebuild (so you don't rip them off in a dip) with an alignment would be in good order as well. Call Automotive Alignment on Vasquez and I-70 (in the qwest phone book) if you want THE best alignment services for your Mopar in Colorado. They know our cars and are honest and have been doing it for 50 years.

Buy a dual plane aluminum intake manifold (mopar M1 for an A body or an Edelbrock Performer) and an edelbrock 550 CFM AVS or AFB four barrel and rejet it for 5500 feet.

Get an 8 1/4" sure grip for that car and ditch the 7 1/4" I have a 7 1/4" with an open carrier and 3.23 gears. The 8 1/4" sure-grip with 3.55:1 gears is all that you should go with in that car unless you hate money. The 8 1/4 is cheap and they make strong parts and you can get one from another A body that will bolt up with no modifications to your brakes or axles (remember that you get big bolt pattern from '73 up)

The 8 3/4 is the best for your A body, but the A body housings cost upwards of $250. A used 3.23:1 8 3/4 suregrip center section goes for about $350 as well. You will have to buy custom axles from Mark Williams up in Lafayette (conviniently) that have been cut for an A body and have the big bolt pattern. late model ('73 up) B-Body drums/rear brakes would also have to be used.

This is because there was never an A body that had an 8 3/4" from the factory with the big bolt pattern. By the time they went to the big bolt pattern for '73, they went away from the 8 3/4" in all A bodies unless you got a '73 Duster 340 (super rare as they ditched the 340 in early '73 production) and it still used the small bolt pattern.

I would just build the 8 1/4" for less than half the price. It should hold against 350hp. Mopar performance makes a good super stock leaf spring set for around $200

You may consider building a performance 904 with A1 planetaries and a decent TCI performance rebuild kit. The 727 is stronger, but you lose hp with them and your car is light enough to not worry about it. I would at least put new clutches and bands in that thing if you want it to last.

Buy a jacobs electronics pro street ignition kit instead of an MSD 6AL. They are actually computerized and not analog as well as 10-15,000 Volts higher in peak power, even when compared to the MSD 7AL. They also adapt onto your existing system and no wires are cut. They also have a built in alarm system, tune up mode and a lot of other stuff including a programmable rev limit that doesn't use chips like MSD.

If you want to make a fast Dart, I would suggest throwing a 360 in your car. It takes little to no effort to get a 360 A body to run high 13's with a junkyard 360 and a decent cam and carb with headers. I know someone that runs a 13.8 with a 140,000 mile 360 with 3.55 gears, a four barrel, headers and a stock 340 camshaft with a double roller chain and a mopar performance electronic ignition conversion (no capacitive box like Jacobs or MSD). You can get a 13.8 second 360 for under $1000 if you do it yourself and play your cards right. I would go to a 727 if you go the 360 route though or you'll hurt something.

Kind of off subject, but if you ever need anything for that car I have acess to about 8 cars worth of parts and I know a LOT of Mopar people in the Denver area. I also restore cars for people, do metal fabrication and I can give you a better deal on a cosmetic update than anyone in town. I use a $60,000 downdraft climate controlled paint booth and a $500 paint gun, so there is no dissapointment in the finished product. (also known as Maaco)

If you have any questions you can email me anytime you like

karatekid@worldnet.att.net
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  #7  
Old 03-02-2005, 10:39 AM
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Quote:
Originally Posted by TrashedCharger
The minimum octane in southern california is 87 (here is 85 due to the thinner air)... Their premium octane is 95 (ours is 91)... This plays a factor as well.
I have to disagree with you there. To prevent detonation, fuel octane has to rise with cylinder pressure. The lower octane pump gas we have around here is dictated by our higher altitude (thinner air means less cylinder pressure), so the 91 octane premium we have here is the highest octane needed for 99% of the cars on the road (here). On a higher compression engine that needs a higher octane fuel, your power potential goes up with better octane ratings until you reach the point that you are preventing detonation. After that, any higher octane fuel you use will not make any more power. The reason most people SEEM to get more power with higher octane is that they probably had too much timing advance and didn't know it, and the fuel helped knock out some detonation they had.

torch
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Old 03-02-2005, 12:15 PM
Kevin Garceau Kevin Garceau is offline
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Too much octane can hurt your ETs. At least seen in some of our vehicles going from 112 to 116 octane.

Trashed Charger sounds like a good source for someone looking to upgrade a stock 318. Sounds like he has played with a few.

I cant find anything he says to change. I ran my Dart the first time with 318, 4BBL and alum intake and it ran 16.5s or so with stock gears, and stock everything. It was soon capable of high 10 second runs!
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Old 03-02-2005, 04:37 PM
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I think there's some miscommunication here, Torch. I agree with you 100%.

Octane plays a factor with altitude changes because of it's limitations on advance in different atmospherical pressure. You couldn't run on the same initial timing on 85 octane that you get at the pumps in Denver out in southern California without knocking under load, right? I know this to be a fact because we towed the '73 Scamp down to LA and it ran horrible on 85 octane to the gas station. I had to pull over and retime the initial advance back to 3 degrees initial advance to get it to run decent and not ping for a few miles to the gas station without detonation. We filled up with 89 to bring what 85 was left in the tank up and retimed it back to 5-6 degrees initial and it ran great.

Of course higher octane does not = horsepower. When going to a higher octane fuel it will not bring anything to your horsepower by itself alone unless your engine is set up to accept it correctly. All a higher octane will allow is the engine to run more initial advance and higher compression ratios as well as running lower altitudes without adjusting timing depending. Your car needs to be set up for 110 octane if that's what you want to use as well as setup for 85 octane. Don't expect a 14:1 motor run on 85.

A good example of atmospheric pressure change to octane level ratio would be my brothers 1988 Toyota Supra 3.0L inline 6 Turbo. That engine has 10 degrees of initial spark advance at idle from the factory! It can do this and still run on 85 octane because the computer will pull more advance automaticly if it senses anything with the knock/ping sensor on low octane fuels.

I went through that entire engine and put a new head gasket and turbo in as well as a new engine harness and sensors everywhere. When we replaced the intercooler lines and fuel filter, we filled it up with 91 octane and the car ran INSANE! I'm willing to bet that it runs a full second faster on 91 octane versus 85.

This is, of course, is all credited to the computer not having to pull advance for the turbo with the lower octane, but this shows that the octane level DOES play a factor in combustion HP increase after all. It's factor is indirect, but controls other combustion factors in the equation. Sort of like an all over governing and limiting factor. When you change the octane level of your fuel, you simply change the limits of your combustion and nothing more.

You're absolutely right Torch. It does NOT give you more HP by itself and if it did, your car would be adjusting timing automaticly (like my brothers Supra) or it should have been running a higer octane level to begin with, but those are the only exceptions.
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Old 03-02-2005, 10:41 PM
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OK, I think we're on the same page now. Speaking of people thinking they will get more power with more octane, I once saw a guy with a brand new (temp tags) Mercedes with the V12 in it fill it up with 108 LEADED! The pump had a notice on it that "this fuel is not for on-road use", and it sits next to the carwash entrance, making you block the carwash to fill a vehicle's tank. Apparently this was not enough of a hint for this guy! Some of the best lessons ARE the expensive ones! Could you imagine buying all the O2 sensors and cats for THAT car!!!!??

torch
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Old 03-11-2005, 01:55 AM
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Quote:
Originally Posted by 74 DART SWINGER
hi, i need help on how much hp you would loose in altitude gaining.If my car has 200hp at 5500 altitude how much would it have sea-level?
Dont the manufactures rate the hp rating from sea-level? so if they come up to 5000-5500 alt. then they would loose some hp? Help me get this straight.
thanks
It's always good to get a fourth and fifth opinion.What's the matter,you don't trust the guys at moparts to give you a correct answer?

Take your car to Dyno Pro on Tennyson.The correction factor will be 20-21% on any given day.Trust me.
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Old 03-11-2005, 01:59 AM
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I saw this thread over there too....LOL

Hey man, what's with the name!
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Old 03-13-2005, 02:49 PM
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Quote:
Originally Posted by dave571
I saw this thread over there too....LOL

Hey man, what's with the name!
You regged first, you should've taken it.Come to think of it,I think I had it before you regged,when it was on the old server.Work it out with the admin,and you can have it.I'll use my moparts s/n.
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Old 03-13-2005, 11:37 PM
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No man I don't want dave 572. I'm just wondering how you derived it.

What's your moparts login?
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Old 03-14-2005, 08:25 AM
George G. Leverette George G. Leverette is offline
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Depends on the year of the car you are driving, The wife's 04 PT Cruiser will compensate for any change in altitude. In fact the T. Q. carbs had altitude compensation devices installed to prevent driveability problems. I live in Southern California about 15 minutes from LACR drag strip, which is about 2000 ft elevation, Los Angeles is about 60 miles south at sea level, the 360 Duster with the 91 octane our maximun octane rating with different mixes than other parts of the country because of pollution, operates better at the higher altitude maybe its because of the tuning. Humidity on the Desert floor is 10-30% most of the time with slight breezes, would this make a difference?
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Old 03-14-2005, 09:08 PM
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You may be confusing the lack of O2, with the function of an O2 sensor.Your PT cruiser's O2 sensor will keep the A/F ratio correct no matter the altitude.But when there's 20% less O2,there's 20% less power.

Contrary to popular belief,turbo cars suffer the same power loss.

Dave571, My login name at moparts is the same as it is here,but my username is 64postcarnut. I arrived at 572 by using 4.5 bore x 4.5 stroke.
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Old 03-14-2005, 11:37 PM
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64postcarnut, I remember.

Cool.

Dave 571 came about rather oddly. I was trying to join this board. All the good "Dave" combo's were coming up as taken.

I had just seen the movie "U571" so I added the number to my name and it stuck.

I use it on all the boards so I don't confuse myself. LOL.. I haven't gotten banned from any yet, so I haven't had to create a new name.
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Old 03-15-2005, 08:27 PM
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Yep, the PT (or any engine) can't compensate for altitude unless it's supercharged or turbosupercharged. It adjusts the amount of fuel for the amount of atmospheric pressure available. This means that as your altitude increases, the mixture stays correct and the it doesn't go rich like a carbureted engine will. But you still lose the HP unless you can jam that air back in.

Actually, a turbocharger CAN compensate for altitude, if designed to do so. If the wastegate opens at sea level, it's bleeding off excess boost. That same car's turbocharger might be able to produce the same pressure at higher altitude before the wastegate opens. Essentially, you have a turbocharger that's bigger than necessary, dumping excess boost when necessary, and it can compensate UP TO A POINT for altitude increases. Above a given altitude, you would still lose HP.

Many general aviation airplanes have a turbo that provides a function called "normalizing". Essentially, this means that at sea-level, the turbo doesn't add any boost. As the altitude increases, the wastegate starts to close and the turbo is allowed to provide air at sea-level pressure. What this does is allow an engine to produce sea-level HP, up to a certain altitude, without overstressing it at sea-level. Really is a pretty nice setup. Of course, above that critical altitude, HP again starts to decrease.

Humidity certainly does play a part in performance. All else being equal, higher humidity means less HP. Every time.
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Old 03-15-2005, 08:39 PM
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Quote:
Originally Posted by DanL
Actually, a turbocharger CAN compensate for altitude, if designed to do so. If the wastegate opens at sea level, it's bleeding off excess boost. That same car's turbocharger might be able to produce the same pressure at higher altitude before the wastegate opens. Essentially, you have a turbocharger that's bigger than necessary, dumping excess boost when necessary, and it can compensate UP TO A POINT for altitude increases. Above a given altitude, you would still lose HP.
You can actually make a turbo sytem GAIN horsepower as you go up in altitude. OR any boosted vehicle.

You can set the boost pressure based on intake manifold pressure using a MAP sensor. Once you set the manifold pressure (kPa), when you go up in altitude the turbo will just do more work (spin harder) and keep the pressure in the manifold the same. You gain power in this setup since the back pressure will be reduced on the exhaust and you gain power by there being a bigger pressure difference between the intake charge pressure and the lower exhaust back pressure.
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Old 03-15-2005, 09:50 PM
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Agreed. That's a good point and it does make sense that with the same manifold pressure at a higher altitude, you might gain a bit of HP. But, my point was that there has to be an altitude where the returns diminish.

In the aviation world, we call it the engine's critical altitude. Obviously, you can't continue to climb and build horsepower indefinitely.
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Old 03-16-2005, 10:23 PM
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Far be it for me to arguee with an engineer,although I understand your logic.I don't believe a SC can provide more boost at altitude because it's RPM driven and the atmosphere becomes less dense.Unless it's designed to make 10 psi and gated down to 5 or 6 at lower alt.At altitude the gate could be adjusted on the fly or electronically.I believe turbos suffer from the same density related problem although it manifests itself as exteme lag due to the engine's pumping loss of a less dense atmospere.

I had a TT Supra from '97 to '99.Perhaps it wasn't set up correctly and my memory is failing, but the car was a dog at "the Tunnel",11,000 ft.Corrected numbers were 385hp/390tq at the wheels.Had a remapped ECU and Boost controller.I never saw more than 18 psi,even though the factory turbos were supposed to be good for 23 psi at sea level.Ran all my mods through Rhys Millen who works closely with TRD.

A very lively topic.

P.S. Turbos cannot increase their rpm indefinately.They have an efficiency (compressor maps) range that can be exceeded,but without additional power increases.
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Old 03-17-2005, 01:43 PM
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Well, yes, with a boosted application, you have to make sure the turbo or SC will stay within its performance range.

Most boosted applications are never setup correctly to adjust for altitude. I know my turbo civic isn't setup that way. It can be though. A supercharged system will work the same way if the boost level is set by a MAP sensor and the amount of bypass is controlled by the manifold pressure and MAP sensor reading. It will only work if it reads absolute pressure vs. gauge pressure that's referenced from atmospheric pressure.
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