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      09-18-2012, 03:10 PM   #1
shiv@vishnu
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Exclamation Vishnu Tech: Effects of Altitude on NA, SC and Turbo

There has been some discussion on the effects of atmospheric pressure on engines with different forms of aspiration. Here is what I've learned over the years. The follow are simplified explanations meant for easy understanding. There are other issues involved (VE, parasitic loss, octane constraints, DME induced tuning changes) that will also have minor effects on power. But for this sake of discussion, we are going to keep things simple.

Naturally Aspirated
All other things equal, for every X% of atmospheric pressure gained or loss, a naturally aspirated engine will gain or loose X% of power. So if you are testing in Denver where atmospheric pressure is 11psi (instead of the 14.5psi we see at sea level), the engine will make 11/14.5 or 76% of what it would make at sea level. That stinks. Big time.

Supercharged
The effects of altitude on a conventional supercharged engine (single belt, fixed pulley diameter) isn't that different from a naturally aspirated engine. A supercharger basically acts like an atmospheric pressure multiplier. That is, it increases intake manifold pressure by a fixed percentage over stock. In the case of a positive displacement blower (Eaton, Whipple, etc,.) this increase in pressure is relatively constant (although influenced by the engine's VE) from low RPM to high RPM. In the case of a centrifugal supercharger, which is geared to spin as a multiple of engine PRM, boost ramps up linearly from 0psi to a max boost pressure at max engine speed.

Either type of supercharger is subject to the same nominal losses. And these losses are the same (in terms of %) as a naturally aspirated engine. For example, if you have a supercharger that runs 8psi at sea level (14.5psi), it is actually geared to provide a 55% increase in manifold pressure (8/14.5=0.55). So total manifold pressure is 14.5+8psi or 22.5psi. This absolute manifold pressure is the magic number that dictates power output.

Now let's go to Denver where atmospheric pressure is 11psi. With the same supercharger, absolute manifold pressure is 11*1.55 or 17psi. Compared to when it was run at sea level, the supercharged car went from 22.5psi to 17psi of abs manifold pressure. So power drops to the same 76% level that we saw in the naturally aspirated car. This is also sucks. What makes it suck even more is that the supercharged car is presumably making a lot more power than the naturally aspirate car. So nominal power losses will be greater with the supercharged car (although % losses will the be same). This is one of the many reasons that very few manufacturers use supercharging in any of their cars. It is primarily used in the aftermarket due to packaging/cost reasons.

Turbocharged
This is where things get a little tricky because different turbocharged engines will feel different effects of altitude. In many OEM turbo engines, the DME will adjust for the effects of lower atmospheric pressure by increasing the boost target (ie, target an absolute manifold pressure target). This, of course, will only work up to a point as running a turbo harder and harder will result in reduced inefficiency and power limiting exhaust back-pressure. But typically, most modern OEM turbo engines are mapped to make no less than 90% of their sea level power at atmospheric pressures down to ~12psi. It's only at higher altitudes/lower baro pressures will the power losses really begin to become noticeable. Above this point, those little turbos won't have it in them to make up for the thin air. As a result, you will begin to see power losses similar to that an NA or supercharged car. Fortunately, these power losses will only start to take effect at very high altitudes compared to SC and NA cars where they will start to take effect immediately.

However, if your stock turbo engine is tuned, things change a bit. Since it is tune, it's safe to say that it is running higher boost pressure targets already. Which means that it already has used most, if not all, of the extra "reserve" it had (in stock form) to make up for reductions in atmospheric pressure. In this case, these cars will see altitude-induced power losses similar to that of a SC or NA engine. However, even with an aggressive tuned turbo engine, one rarely uses all their turbo has to give at lower engine speeds. In this case, the turbo engine will be able to compensate for altitude induced power losses at lower RPM which is nice since it is where most of us drive, most of the time.

Not all is groovy though. While a turbo engine will be able to make up for some, much or even all of the pressure loss caused by altitude, the turbo will be working harder (at a higher pressure ratio) which means that the engine will be subject to more exhaust back pressure and higher intake temps which makes the engine more sensitive to knock. So if octane is a constraint, the turbo engine may have a tune-related power loss component. It will also feel laggier and less responsive since the turbo needs more time to spin up to the higher absolute pressure it is now targeting.

Now things start to look bright again when we are talking about big upgraded turbos. These turbos usually have so much more "reserve" than we ask for at sea level that taking them up to high altitude usually have very little effect on power since the turbo can easily operate at the high pressure ratio will very little effect on exhaust back pressure or intake temps. How high we can climb without noticeable power loss will depend on how much reserve capacity the big turbo has. If the turbo is able of supporting say 800whp and you only run it at 650whp at sea level, that is a lot of extra reserve to pull from when atmospheric pressure drops. Of course, the downsides of operating at a higher pressure ratio still exist (more lag). And that is more lag on a turbo engine that already has more lag than a small turbo.

However, one could argue that the NA and SC engine, when run at high altitude, are lagging all the time

Shiv

Last edited by shiv@vishnu; 09-18-2012 at 03:43 PM.
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      09-18-2012, 03:26 PM   #2
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Thanks Shiv, very interesting... I've always wondered this. I feel bad for you all up in Denver
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      09-18-2012, 03:33 PM   #3
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Nice write-up
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      09-18-2012, 03:53 PM   #4
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Thank you for making it simple enough to understand
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      09-18-2012, 05:26 PM   #5
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Aren't most modern SC'd cars from the manufacturer, such as the new Audi S4, designed for higher altitudes, and then have a built in wastegate/bypass to release boost pressure when the boost setpoint is reached. That way they are "oversized" for those of us at higher altitudes that would need it?
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      09-18-2012, 05:35 PM   #6
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Quote:
Originally Posted by raceyBMW View Post
Aren't most modern SC'd cars from the manufacturer, such as the new Audi S4, designed for higher altitudes, and then have a built in wastegate to release boost pressure when the boost setpoint is reached. That way they are "oversized" for those of us at higher altitudes that would need it?
I'm not familiar with the new SC'd Audi. But internal bypasses have been used in the past to reduce parasitic/pumping losses during cruise as well as to limit boost. Oversizing a blower for high altitude and then reducing boost (@ sea level) through the use of a bypass or pressure relief valve doesn't sound like a good idea in a performance-minded application.
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      09-18-2012, 05:51 PM   #7
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This is exactly why I was able to run neck and neck with two ESS VT1 Supercharged M3's...even though these cars made 480 wheel HP at sea level and my stock turbo 335 made 412. It was because the runs were at 5,000+ DA > warm temps @ Trona and Willow Springs which are both about 2,000 ft above sea level.

Glad the next Airstrip Event is at 600 ft above sea level...and hoping for cold temps otherwise my new supercharged VT2 600 M3 won't stand a chance against the singles.
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      09-18-2012, 05:55 PM   #8
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Quote:
Originally Posted by shiv@vishnu View Post
I'm not familiar with the new SC'd Audi. But internal bypasses have been used in the past to reduce parasitic/pumping losses during cruise as well as to limit boost. Oversizing a blower for high altitude and then reducing boost (@ sea level) through the use of a bypass or pressure relief valve doesn't sound like a good idea in a performance-minded application.
Yeah, didn't sound like the most efficient thing. In effect they are already using the work to do the compression, and taking that power away from the engine to do so, and then bleeding off some of the already compressed air. But I know a local guy who has a B8 S4, stock, and he is running very close at the local drag strip (Bandimere) to what he did with his stock 335xi....actually even a little faster maybe. That leads me to believe they do leave some headroom there.

BTW, most of the time in the summer months, we are actually running at about 9000+DA when we hit the drag strip...pretty much sucks.

Also, the thin dry air has less heat carrying capacity, so we can heat soak the FMIC easier than at sea level as well.
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      09-18-2012, 06:24 PM   #9
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Shiv, that is 100% correct & also been my findings from various logging procedures I have done at both locations.
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      09-18-2012, 06:31 PM   #10
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Quote:
Originally Posted by raceyBMW View Post
Aren't most modern SC'd cars from the manufacturer, such as the new Audi S4, designed for higher altitudes, and then have a built in wastegate/bypass to release boost pressure when the boost setpoint is reached. That way they are "oversized" for those of us at higher altitudes that would need it?
Interesting...

I know there are some clutch type superchargers. I know Mercedes have implemented them on some models. However, I thought the intention was just to reduce drag and not spin the blower when boost wasn't needed.

In turn reducing heat, and increasing efficiency of the engine as well as improving life expectancy of components.

I dont know if over-spinning superchargers for people at elevation is good for efficiency purposes for people at sea level though.
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      09-18-2012, 06:35 PM   #11
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Quote:
Originally Posted by raceyBMW View Post
Yeah, didn't sound like the most efficient thing. In effect they are already using the work to do the compression, and taking that power away from the engine to do so, and then bleeding off some of the already compressed air. But I know a local guy who has a B8 S4, stock, and he is running very close at the local drag strip (Bandimere) to what he did with his stock 335xi....actually even a little faster maybe. That leads me to believe they do leave some headroom there.

BTW, most of the time in the summer months, we are actually running at about 9000+DA when we hit the drag strip...pretty much sucks.

Also, the thin dry air has less heat carrying capacity, so we can heat soak the FMIC easier than at sea level as well.
In supercharges with internal bypasses, they don't use them to bleed off boost. But rather to circulate air between the charge pipe and pre-supercharger intake. Essentially eliminating compression and therefore reducing belt drive losses. Usually, the bypass is full open in moderate vacuum and then closes up as pressure approaches 3-4" of vacuum. At least this is how I've run them in Eaton roots style blower applications.
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      09-18-2012, 08:28 PM   #12
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What about timing advance at altitude? Would the cars suffer from decreased timing vs the same car at or close to sea level? Or is more timing going to be safe due to the decreased boost pressure?
Shiv, I know you used to used to visit the Denver area periodically, even if it was on different platforms. I'm not sure if you still do or have yet with the N54, which is why I ask. Maybe you can lend some expertise to us in the thin air, even if it is basic/general tuning knowledge. Speaking in FI applications of course.

TIA
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      09-18-2012, 08:33 PM   #13
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This is why I don't like to report corrected numbers. On maxed out turbos it may be somewhat accurate, but on less than maxed out turbos the N/A correction factors make it look like you are faster than you really are. Some people enjoy reporting ridiculously high dyno numbers at high elevation and it gets silly.

But don't feel too sorry for us. Even though we're kinda slow, we can beat up on N/A cars that would beat us at sea level. That means that turbo cars with some potential are higher on the food chain up here. That's why my last car was a DSM and my new car is a 335. Run 12s up here with AWD and you're pretty much kicking booty on the street.
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      09-18-2012, 09:09 PM   #14
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At bandimere the rule of thumb regarding et is:

N/a = 1.0-1.3 slower than sea level
S/c = .8-1.0 slower than sea level
Turbo = .5-.7 slower than sea level

The only good thing is smoking the one off vette or new 5.0 with zero issue.

The lag on my built / big turbo 1.8t a4 sucks though. I don't notice lag in my bimmer.
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      09-18-2012, 09:44 PM   #15
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This is extremely informative and slightly counter to how I originally thought turbocharged engines worked at altitude. I didn't take into account for what the computer would do!
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      09-19-2012, 08:30 AM   #16
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Nice write up Shiv. Yes my 36 years here in Colorado (20 of which have been driving) have sucked with N/A cars. I think I'm finally going to be making a move to California where I won't have to think about air density regularly hovering at 7000'+. Ah, I'll miss this place however...
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      09-19-2012, 08:45 AM   #17
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Nice write up. The other thing that doesn't work as well at alititude is a radiator, or any heat exchanger (to ambient air). Since the air is less dense it accepts less radiated heat. Which sucks even more because at high elevations usually you are under load climbing rapidly at least part of the time.

This is especially apparent on air cooled engines, going slowly up steep terrain, like my old R1150GS Adventure motorcycle.
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      09-19-2012, 09:01 AM   #18
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Quote:
Originally Posted by ajsalida View Post
The other thing that doesn't work as well at alititude is a radiator, or any heat exchanger (to ambient air). Since the air is less dense it accepts less radiated heat.
Huh, I didn't think of that. Good point.
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      09-19-2012, 09:31 AM   #19
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Yeah, so you are doubly screwed, turbos working harder to make boost targets and then you can't get rid of the excess heat generated as efficiently.
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      09-19-2012, 09:49 AM   #20
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Quote:
Originally Posted by ajsalida View Post
Yeah, so you are doubly screwed, turbos working harder to make boost targets and then you can't get rid of the excess heat generated as efficiently.
True - but I agree with Carl Morris and King Mode. Being able to dust some very fast NA cars more than makes up for it IMHO. Plus, when I head down to sea level, it is like driving a new car. Heck, even going to Denver from where I live feels faster

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      09-19-2012, 09:58 AM   #21
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Quote:
Originally Posted by richpike View Post
True - but I agree with Carl Morris and King Mode. Being able to dust some very fast NA cars more than makes up for it IMHO. Plus, when I head down to sea level, it is like driving a new car. Heck, even going to Denver from where I live feels faster

-Rich
No doubt, I just find it prudent to not get too carried away for long periods of time under boost @ 12k ft. The main reason I bought a twin turbo car was for altitude compensation. My house is @ 8k and most spirited driving above that.
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      09-20-2012, 02:03 AM   #22
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My 2.5 Cents

The back pressure should be relative to the ambient air pressure.
Which is actually 14.7 at sea level, you are confusing BAR which is about 1BAR=14.5 PSI.

As the absolute volume of air at he same pressure is still the same.

PV = nRT
http://en.wikipedia.org/wiki/Ideal_gas_law

So really, the only negative of spinning the turbos faster to create the same absolute pressure is the extra heat and wear and tear on the actual turbos.

The rest sounds right to me.
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