How does altitude change wastegate action, power delivery, and safety margins?
 

Ive read a few different forum's threads on this topic, one in clubwrx...

http://www.clubwrx.net/forums/high-altitude-tuning/88576-boost-high-elevation-question.html#/topics/88576?page=1

And this one in a truck forum...

https://www.turbodieselregister.com/threads/124459-High-Altitude

And this one in a supra forum...

http://www.supraforums.com/forum/archive/index.php/t-269341.html

And im still not clear who is correct.

I want to know what my ebay 2871 clone with an 8psi wg actuator will do at 6000ft elevation. From what i have read, (and lets pretend the wg just snaps open at 8psi) wg is only mechanically activated when the source line pressure hits 8psi. This will occur higher in the rpm range at 6000ft, and also be at a different place in the efficiency map.

Since this turbo is not very small, it is safe to assume that while it is working harder to make that 8psi at 6000ft, it is not past an unfavorable pressure ratio that will be building excess heat?

If that is true, should 91 octane gas be enough to maintain the "220wtq safety margin" on stock rods?

I guess it is also unlikely that my setup would be efficient enough to reach that torque level at only 8psi.

I also guess this thread is pointless because there are too many variable to even talk about?

Sorry, i just woke up with some questions.

A few things:

- Pressures are measured as either "gauge", or "absolute". Gauge pressure is relative to ambient, while absolute pressure is absolute (or you can think of it as being relative to a complete vacuum). If you want to qualify a pressure value, you append either 'g' or 'a' to it. "kpa" is kilopascals (absolute), whereas "psi of boost" is usually gauge and should really be written "psig". There's nothing inherent about one unit being gauge and one absolute, you can have "kpg" and "psia" as well.

- mechanical wastegates are driven by gauge pressure, so an 8 psig WG will open when the boost source is 8 psi above ambient. At sea level that's 15-ish psia, so you get 23-ish psia in the intake manifold. At 5000 feet ambient is around 12 psia so you're only going to get 20-ish psia in the intake manifold.

- electronic boost control is typically set up to work off absolute manifold pressures. So if the car is EBCing to that same 23 psia at sea level, it's going to wind up with the same intake manifold pressure, but it'll take more speed out of the turbo to do it because you're asking it to make more boost.

- there are other factors as well. Turbos are driven off heat and pressure differences, as the gas in the exhaust manifold expands through the turbine, it cools and transfers some of that energy to the turbine wheel. The lower the backpressure the better, and since at 5000 feet you have less ambient pressure, it means that for a given speed/pressure ratio on the compressor, the turbo is actually more efficient.

- That's not enough to think about though, because the turbo isn't running at the same speed/pressure ratio. 23/15 is a pressure ratio of 1.5:1, whereas 23/12 is 1.9:1. That moves the working point to the right on the compressor map. The vertical point on the map stays roughly the same, because it's related to the volume of air, which is directly related to power, and since we're targeting the same absolute manifold pressure, we're targeting roughly the same power.

- Except it's not the same power, because since the turbo is working harder you need a higher pressure difference across the turbine to deliver that additional power. Part of this difference comes from the lower ambient backpressure, but it's usually not enough, so you wind up with a higher exhaust manifold pressure for the same intake manifold pressure. This reduces the volumetric efficiency of the engine (the ability to cycle air through it), so the same absolute manifold presusre will produce less power than it does at sea level.

- So, overall, if you take a MBC turbo up to alttude, it will make less power, although a smaller percentage drop than for a naturally aspirated car. If you take an EBC car up to altitude, it will make slightly less power -- more than the MBC, but still less than sea level. Both cars will require more revs to get the boost up to the target level.

In a naturally aspirated or positive displacement supercharger, power losses with altitude are linear with pressure. 20% less ambient pressure means 20% less power. A turbo car is a lot more complicated, and the precise amount of the losses depends heavily on the details on the turbo system like the amount of backpressure, the specific location on the compressor map before and after, etc. It is definitely possible to take a turbo that's running on the ragged edge at sea level and overspeed it with the same boost target at altitude. Running at a higher altitude will change the fuel map requirements, and can also change the MBT and detonation threshold ignition curves by a bit as well.

This is why dyno altitude correction basically doesn't work for turbo cars, and why FM's turbo dynos cannot be compared to sea level dynos in a meaningful way.

--Ian

I don't know specifics to your questions, but some thoughts: 1) Regarding the wastegaste reference, you need to know if uses an atmospheric reference to determine where (in absolute terms) 8 PSIG occurs. I'd be willing to bet that the wastegate actuator is open to atmosphere, so it's all relative to the elevation it is at. 2) 91 octane is a boost limiter, which is in turn a power limiter, yet it is a stretch to suggest that 91 octane would prevent you from exceeding the design strength of the rods. It's usually the speed at which they cycle and the stress they are under that determines when they break. 3) You would have to look at the map of your charger to determine if the elevation difference moves you way out of the efficiency zone. With elevation comes a loss in air density, and your charge to RPM ratio will dive. So, your answer is very specifically in the map of your turbo.

^ what he said, Ian beat me to it, and explained much better. Yet reads like the wastegate actuator is not open to atmospheric reference.

Very interesting. Thanks for the detailed post, Ian, i like how you addressed the different aspects brought up by each of the forums.

Hmm, tetra, ill have to contactthe ebay seller, and see if they have more specific info.

"I'll contact the eBay seller of my knockoff Chinese turbo ask them for an accurate compressor map" is the punch line of a joke that hasn't been written yet

Very true :)

We'll see what they say(if anything), but in the mean time...

Compressor
Trim 48
Inducer Diameter 49.2mm
Exducer Dia 71mm
A/R 0.6

Turbine
Trim 76
Inducer 53.8mm
Exducer 47mm
A/R 0.64

Since you aren't going to find a compressor map for many ebay turbos, I just look for a garret map that most closely matches the wheel specs in question, then subtract several percent from all the numbers in the map, more so on the higher RPM areas. That's about as close as you're going to guess IMO.

Also a ton of things change when altitude changes. If turbo reliability was your primary goal, you would run the turbo at a fixed pressure ratio instead of a fixed boost pressure. If maintaining power at elevation is your primary goal, you would want to raise the total boost level when increasing elevation as even at the same boost, you will make less power at elevation vs at sea level. A lower PR is going to be better for reliability on everything from the engine, to the turbo, to the safety of the tune.

My only real goal is having a snail fart of boost for more smiles on my face.

I agree with most all of what Ian has said, except what I emboldened. The higher pressure ratio moves you UP on the compressor map with EBC. That is why, in his last paragraph, he correctly says you can overspeed a turbo. It will happen when you go up beyond it's pressure ratio, which is highly correlated to shaft speed.

The one thing he mentions near the end is the change in fuel map requirements. This was spoken about at length early on in your build thread, when the MS3 altitude compensation was not working. What no-one would do in that thread, or so far in this one, is to put any numbers on what those changes might be. Likely it will be one of those things that will vary with individual set-ups. Like Ian points out, there are a lot of interactions taking place.

Yeah, for some reason I was thinking of a compressor map with pressure ratio on the X axis and CFM on the Y axis. Usually they're reversed, so you're correct, it moves you up.

--Ian

I thought that might be the case. It was a very good write-up, I thought.

Cats awarded. Thank you Ian, for answering a question I've had in a very understandable way and treating several pertinent factors. Seriously helpful and educational. :likecat:

Doesn't matter. It's an ebay turbo, and 8psi wga isn't enough to cause any meaningful negative effects on what's going to happen with it short of driving your car on the moon, assuming you actually get the advertised 8psi wga.

It'll either die a horrible death in the first 10 minutes of use like the last Ebay 2871 i saw in person, or it'll work just fine.

a china 2871 clone is a pretty bad match for a stock bp6d on pump gas at elevation
at elevation, it's that much more important to have an efficient setup because you're already fighting against a disadvantage

Also, what they said. How come you didn't get a small trubo? I feel like we might have talked about this before but don't remember for sure.

I was talking to patrick about it, and he ran the same turbo for 2 years at 350whp.

We both figured at lower psi it would be understressed, especially at altitude and last a long time.

I live at 6000, but in literally 10 minutes, i can be at 8000, and another road i like driving passes 9k, another 11k

If it ends up being terrible, i can just try a smaller turbo that had the same turbine housing bolt patterns.

I'll save you the trouble: At 8psi, it's going to be terrible.

Just get an MK setup and stop making this complicated. Or my Begi kit if you want more baller, less ebay turbo.

Patrick's experiences have nothing to do with your real world stock motor low boost scenario.

If you're running a laggy and inefficient chinacharger at low boost, and then you go up to 11k altitude, you're that much lower in the nonexistent compressor map, putting the already inefficient output down to barely above "wet fart".

Oh well. You should make a build thread and call it "I always do everything the hardest way possible" :giggle:

Wait!

Are we talking about THE Patrick? patsmx5?

:bowrofl::bowrofl::bowrofl:

He said he wanted 250-300whp reliably, I told him to run that turbo size ebay turbo since it won't be maxed out at 250-300. Ebay turbos don't like running maxed out, they break, ask me how I know.