Thread: Elevation and the Turbocharger (very technical)

I thought this was common knowledge? Seriously though, it doesn't seem far fetched to comprehend that the air is thinner at higher elevation and a turbocharger will not be as efficient as operating at sea level. However, race up hill where the air gets thinner by the instant and the n/a car will lose to a forced car regardless of the n/a car (insert turbo 1.3L Geo Metro vs. hemi truck).

As long as the turbo doesn't exceed it's efficiency this is correct. The turbo is more dynamic and can be adjusted to lose less power as it moves to a higher elevation. BUT, if you have maxxed your turbo out at sea level, and start going up hill, you will probably lose to the NA car. Why? because the turbo will drop in efficiency, and no intercooler will be able to save it when it is blowing really hot. The turbo engine will be handicapped by the heat at this point and the turbo won't be able to flow enough air in order to make enough power.

This is mostly the case on engines tuned to the edge, pushing high PR at sea level, and having that number increase substantially as the air gets thinner.

Up here in Utah, there are very few people who make the power that others have made with these cars. A typical t25 car up here will not make more than 215-220whp, even when the numbers are corrected. A t28 car will max out at around 290whp corrected, and my turbo maxxed out at 450whp corrected.

When you take out the correction factor (let's say 14% average), then you get numbers that are way, way lower than the cars at sea level. There are tons of guys making way more power than any of us have been able to get up here. But the numbers get close enough. How mad would you be if you had a t25 setup here in Utah and made only 190whp on 12psi? Or a GT30 setup and made only 390whp on 24psi? These uncorrected numbers show a huge deviation from the sea-level guys that make as much as 15% more uncorrected power than we do.

In the case of the metro, I am not so sure the hemi would lose. If you did somehow get the metro to make the same power as the hemi at sea level, the turbo would experience a large efficiency drop with altitude (on top of the pressure drop), and would probably get walked on by the Hemi.

The power decrease with altitude is a pretty predictable curve on an NA engine, but on a turbo engine you notice a point of no return. A good rule of thumb for 4500ft is that your turbo will lose 10% of its airflow based on the altitude change. (again, just an estimate). Guys with 60-1 turbos see about 50-60whp less than guys at sea level when the turbo is maxxed out. GT4294R guys make no more than 800whp, and these are corrected numbers! Not only are you maxxing the turbo out at a lower air flow, but you still have the compounded issues: pressure loss, efficiency deficit, more heat, and less flow.

I would love to see a 600whp SR make within 10% of that number uncorrected at this elevation. If it did I would take notes of the setup and try to figure out how to make more power with every car I deal with up here.

I've actually tested the theory i proposed in Utah.lol. It was with a stock supercharged nissan pathfinder and a bolt on b14. The b14 was way faster on the highway, but going up in the mountains the n/a car was limited to about 45-50mph or 3krpm whereas the truck speeded past 75 mph.

This could be a number of things, gearing, tranny etc. The pathfinder most likely had an automatic, which can multiply torque and make more midrange power. But it probably had a more desireable powerband. I used to be NA VE and I hardly noticed a difference in 2000ft elevation, but since I have gone turbo I feel a big drop in power when I get up there (monte cristo 8000ft). Much bigger difference. The turbo just runs out of steam and the engine starts to see intake temps upwards of 215* after the intercooler.

Normally temps don't get over 120*, but you def see (and feel) the difference on my setup. When the boost is lowered the car seems to make about the same power. Surprised? I certainly was!

As far as the supercharged SUV, it is very possible that it didn't lose as much power as the NA engine, but (roots) superchargers generally run at lower pressure ratios (they move air rather than compress it, the compression comes from the intake manifold) and they don't need to worry about exhaust backpressure either.

There are a lot of things to consider in the end. My argument is not that FI cars will lose more power than an NA engine, but rather to explain that there are a lot of setups out there that are likely to be massively affected by elevation change.

Come down here sometime and I will show you my datascan data of everything going on. It is really interesting to see maf voltage dropping as the elevation goes up, until it gets to the point that lowering the boost actually brings voltage back up. Just more food for thought. My argument is only to address those who say that a turbo engine will always lose less power than an NA engine, or that the SAE correction shouldn't be used.

Again: the reasons I prefer to use the SAE is because the numbers get close to real sea level numbers. It also makes is easier to tune by seeing accurately power increases from changes to the tune. I have 6 people here who have dynod at sea level last season. 4 were right on the money (corrected #s vs corrected sea level), these guys saw within a 1% deviation from what the sae correction said they would make at sea level.

The other two, however, varied wildly. One turbo mustang made a lot less power from elevation (from vegas making 580 to here 530whp dynojet 248c), another SS cobalt lost very little to elevation. (made 238whp sea level iirc and 225whp here)

This information is just out there to give everyone a more clear look at the deeper workings of the turbo, sometimes you can lose a lot of power (or not as much). But 8/10 times the SAE number gets really close to estimated power levels.

Dude you dont realize that the problem on your car its not the elevation or anything related to elevation your problem its the same one I have its the friking log manifold those f@#ers dont flow well after 400whp and they create so much issues that the engine doesnt even react to advance the timin and the efficiency get lost after that point my car can make 429whp at 20psi and 446 at 24psi???? this in a prety big ass turbo that can flow up to 700 whp so the only restriction y see compare to other two cars i have tuned its the log manifold the other two cars make about 500+whp at 23psi and the make between 14-16 whp per psi of boost and almost 10 whp per degree of timing advanced... and that leaving afr's on 11.2-11.5 and the only difference its thy have tubular manifolds not even EQL so that's why I am changing mine ASAP......

I understand the log manifold is a big restriction in my setup, but that isn't what this thread is about. I will be changing manifolds soon, but even when I do I don't expect to see more than 450whp uncorrected. I would like to get around 475 uncorrected but I don't see it happening up here.

I have a all-trac celica here running a gt4294r that maxxed the turbo out at 700awhp. This turbo is rated to 850. It has an amazing setup on it.

The fact is that the turbo is limited by elevation. This is a physical limitation. The information I posted is true, and relevant.

I will be going eql tubular with a divided housing in about a month so I will have some really good dyno comparisons. One of my honda buddies made 550whp at sea level with this turbo, and I would like to see how far I can push it up here. It should be interesting!

Nice writeup Coheed. I'm afraid that most of this stuff is only going to be understood by a few people on this board who have taken and understand chemistry and physics adequately to see your logic. I've worked through some of this same stuff in my head before, but never organized it like this. Excellent work!

That's not the correct way to find a percentage. Try:
100% - (100 · (12.5 ∕ 14.7)) =
100% - (100 · (0.85)) =
100% - (85%) =
15%


Your 12% turns into 10.7% when the proper equation is used. No big deal.

(I've finally taken the time to read this, and I'm just responding as I read stuff. Nothing worth arguing over yet. I'll continue.)

The amount of absolute pressure the engine sees depends on how your boost gauge (or what ever you use to measure boost) is calibrated (assuming you use it to set your boost controller) or how your wastegate was calibrated (and if the wastegate uses a secondary reference signal from atmospheric). If it's calibrated at sea level, or for absolute pressure then 6 psig should also equal 20.7 psia in both cases.

You're right. In higher altitude, you start out with less initial pressure to work with. However, as long as you're working with components calibrated at sea level or operating in absolute pressure (never seen anything calibrated or operating otherwise) then if you hook up a boost gauge to an N/A car and drive it around at sea level you'll basically see 0 psi at WOT. Drive up the mountains and you'll notice the gauge reads less than 0 psi at WOT. It will be somewhere in the inches of mercury section. This corresponds to the 12.5 psi absolute that you get in the mountains.
Now say you slap a turbo on the car. You're going to have to "boost" 2.2 psi to even match the same output you saw at sea level, and to get the gauge to read 0 psi again. However, if you're using the boost gauge to adjust your boost controller and you want to boost 6 psig you're obviously going to keep going until the gauge reads 6 psi. This means you're asking the turbo to do the same amount of work as one at sea level boosting 8.2 psi. This means the engine is now going to see the same psia as an engine running 6 psig at sea level.

The only real difference between these two engines (one at sea level, and the other in the mountains) is that the turbo in the mountains is operating in a different efficiency area of the compressor map than the one at sea level.


I see you were setting up the prevailing argument for your own argument's sake. I'll spare you the correction of the rest of your math then. No big deal again.

An astute observation. It's essential to realize that just because the engine in the mountains runs the same (absolute) intake pressure, it doesn't mean it will make the same power.


All else being equal (compression ratio, etc.) running 1 bar usually does double the output of an N/A engine. Our SR20 engines are a perfect example of that. Not sure where you are going with this one...

Let me read the rest and let me respond.

Okay yah, I see your point. It's vastly over simplified to say that pressure equals power. For sure it's more important to note that air flow (and fuel to match obviously) equals power.


Oh snap! Is my head going to roll?
Time to read on.

Okay, okay. Take me there Coheed. I'm ready.

Even though this never really happens in real life (as I explained earlier) and thus I have a problem with your math, I like where you're going and I feel we're both on the same page. Let's see what happens.

Preach on brotha!

I feel a mention of compressor maps coming soon. =]

Yep. Yep.

Time to go read the next installment.
I'll be back.

Oh damn. I knew it was coming.


Ding ding ding ding ding ding!
And there you have it folks. The meat of it right there. Maybe I'll quote it again so people don't have to sift through to find it.



Feels good doesn't it?


I sure do man. Where did you get so smart?


Ehh... I feel like we've peaked. Do we really have to get deeper into it? Oh well...

Got to have one of these. Was hoping for it earlier. =D

I read the rest and it's all a bunch of QED. Not that it didn't need to be said. Just saying, no need for me to repeat it here.