toddk911

The higher the temps, the less space it takes to fit that same psi of boost in the motor.

i.e. 15psi at 50 degrees takes up less space then 15psi in 80 degrees (density of air) . So, if no changes are made to compensate (MBC/WG), you will have less psi getting into the motor at the same setings in higher temps.

toddk911

excuse me, The higher the temps, the MORE space it takes to fit that same psi of boost in the motor.

JustinL

Rage, you use EBC so there is a feedback system and it can compensate for changes.

A good boost controller should hit the same peak pressure regardless of other conditions as there is no temp or ATM pressure variables to set.

michael2e

So any guesses as to how much sooner I would see full boost at cold sea level instead of 85 degrees and 6000 ft? Doesnt hit until 4200 RPM with a Tial 38 here. Does the changing of density altitude necessitate altering fuel maps? Nice thing about the SMT6 is the ability to store 2 tune maps changed simply by flicking a switch. Interesting thread

phils87951

What boost are you running? and what turbo
If you are running high boost with the stock turbo, its simply not able to compress the hotter air.
These turbos were designed to give about 12 psi gauge under all operational conditions, not 18. at that high a pressure its simply getting winded if you will. it is simply running outside its designed parameters.

rage2

I don't use the feedback feature on my AVC-R .

turbite

No.
So at 14psi, the wastegate wouldnt be open, so it would still boost to 15psi. (Unless it can't)
A horse can run just as fast at 5000ft as it can at sea-level, it just has to breathe harder.
The assumption is that breathing (the turbo's efficiency) is not the limiting factor.

phils87951

A Horses lungs are variable in volume, an engine isnt. and the hotter the air, the less O2 per cubic foot, the horse may run just as fast , but he will tire out faster.

turbite

The horses lungs were an analogy to the turbo, not the engine.
The horse can breathe less oxygen because its less dense, so as a result the horse has to breath more times per minute (turbo RPM).

The end effect is that horses's lungs output (oxygen in this case, psi in the turbo's case) is the same. Assuming your turbo can spin faster.

Bottomline. You should boost the same, and the turbo should 'work harder' to make up for the losses because in ideal conditions the wastegate doesnt open till boost hits 15psi.

I'm interested in knowing what causes this, I'm just saying what its not.
The incorrect assumption was that the turbo was 'working' just as hard at full boost at sea level and 5000ft.

rage2

That's assuming the boost controller that's controlling the wastegate is working on ABSOLUTE pressure at altitude (opening up at 15psi boost, or 15 + 14.7 == 29.7psi ABSOLUTE).

If a boost controller is working on relative boost (pretty much all boost gauges reference relative pressure), then at 3700ft elevation where ambient pressure is 13psi, the wastegate would open up at 15 + 13 == 28psi absolute pressure. Your boost gauge would show 15psi of boost, your standalone (reads ABSOLUTE pressure) would show 28 - 14.7 == 13.3psi of boost.

BTW - most stock turbo cars' boost controllers control boost relative to ambient pressure, so they lose just as much power at altitude as NA cars. This is done so the turbo doesn't overspin (work harder) at higher altitudes.

Laust Pedersen

Here is how I see it. If the boost controller is working properly it is independent of temperature and only work on pressure. The metal (steel) springs in the boost controller (BC) and wastegate (WG) are unaffected by the temperatures we a talking about here.

The wastegate is not only actuated by the intake pressure (modulated by the BC), but also the exhaust pressure. When the ambient temperature increases the turbo has to work harder (higher rpm, more internal friction) to get the same boost pressure, but that can only be done if the exhaust pressure (between engine and turbo, i.e. to the WG) increases. So therefore the exhaust pressure pushes more on the WG valve, opening it slightly earlier.

Laust

tazman

One other thing is most boost controllers are connected to the pipe before the intercoller which will probably see more pressure in hotter temperatures.

TurboTommy

Laust,
that's exactly what it is; it's an exhaust back pressure issue.

It has nothing to do with gas laws or horse analogy.
It's some sort of turbo dynamics phenomenon that states that a turbo rotating assembly needs to turn faster to create the same boost out of initial warmer air.
Therefore the rest is what Laust said.

Now, some here with a different wastegate that's not affected by back pressure probably shouldn't experience this drop in boost. However power will be down not only because of the less dense air, but also because of the increase in back pressure.

Peckster

Don't make it so complicated. The denser the air, the more O2 in the combustion chamber. That's about it.

Rich Sandor

Did you guys know that in general aviation, most turbocharged engines are set up to maintain Sea Level Manifold pressure? What I mean by that is the engine at sea level is not boosted, and then as the aircraft climbs, the turbo kicks in and produces just enough boost to keep the manifold at sea level pressure. The whole point of this application is not to give you a more powerful engine, but strictly only to raise your service ceiling, (allow you to fly higher) by maintianing sea level horsepower to altitude.

I've heard that the reason they do it like this is because of reliability issues. They know you are not going to blow your headgasket at stock manifold pressure - but if you are running high boost at 20,000ft and then descend quickly, you could overboost the engine and blow a gasket, and turn into a flying brick.

When I was in college a few years back, one of my roomates (fellow pilot, but not a porsche-turbo-fanatic) arrogantly quoted the aviation textbook definition of a turbo-charged engine, and said: "I think it's totally pointless to buy a turbo-charged Porsche.. Theres no point if the car never drives much higher than sea level!"