I have to disagree, I think the ECU does know what boost your running because there is a factory MAP sensor. With the Power FC and Excel tuning interface we have been able to quite accurately log boost pressure (so far up to 22PSI) with the stock sensor. In fact there is a fuel correction map based off of the boost/load, measured from this sensor. This is with the L-jetro version, which is plug and play with no added sensors.
Why are we having to add a MAP sensor when converting to speed density? Good question

Also one of the first mods I attempted was removing the restrictor pill from the stock boost solenoid control system. Even with the car otherwise stock, my boost would occasionally spikes up to 22PSI at 3-4K. When this happened you would immediately see the boost drop 3-4 PSI due to the stock ECU abruptly lowering the solenoid duty cycle. So the stock ECU is obviously monitoring boost pressure.

Theoretically no, with the reduced air pumping efficiency at higher altitude you are in fact pulling in less air through the MAF sensor. Since the fuel maps are based off of this sensor, it should adjust accordingly. This is the main advantage of an MAF system, it can accurately maintain specific A/F ratios over a wide variety of environmental conditions.

The MAP sensor and at least my boost gauge sensor are sealed units. So I am not so sure that this is correct. I am however quite sure that compressor efficiency does drop significantly at higher altitudes though so....

 

After reading parts of the service manual I see the car is equipped with a barometric pressure sensor, so the car has enough information to adjust boost for altitude. I still couldn't figure out whether the ecu measures boost directly or infers boost from other inputs such as airflow, temp etc. I know that boost is not readable on the ODB port through devices like the SECS odb display unit from Quantum. On the EVO this unit requires a supplemental boost pressure sensor to display boost. That is what lead me to think boost is inferred rather than measured directly.

 

Apparently there are several sensors that cannot be read by these devices, another good example of this is the Knock sensor. The MAP sensor is that little black box mounted on top of the intake manifold.

 

Welcome to my world. Denver, "The Mile High City".

 

This a fun thread!

I do not understand how you can be sucking less air as the compressor turbine speeds up. A pound of boost is a pound of boost at the intake.


Speedlimit...

 

Well, without getting into quantum physics and whatnot like some of you guys , the way I have always looked at it (since I live at 5600ft) and deal with this dilema on a daily basis is... even though it is turbo charged, all it is doing is cramming MORE THIN AIR (less o2) into the engine. And the higher you go the less power you will make. This will also make the car run richer than normal and cause it to bog as well.

 

Sooo Rich!!! it's ridiculous. I had another post complaining about this.
WHen I reset my ECU my car is significantly faster on the first tank. Then ECU realizes that I am in denver and it enriches the mixture. Makes my car much slower.

 

I thought I understood this in my first reply on the thread but now I'm more confused than ever. I thought the boost was referenced against current (in this case high) altitude. Sense the reference pressure is lower, so is the boost. But looking deeper it looks like the EVO understands altitude - it has a barometric sensor and it undoubtedly is compensating as best it can. At the same time the turbo apparently has some excess capacity because that is what the waste gate is doing when the turbo would otherwise overboost. So we're back to the original question - why does the EVO drop off at moderate altitudes when you'd think it could just run the turbo a little harder by keeping the waste gate closed more. I don't know.

BTW cjb, does your boost gauge read exactly zero when you first turn on the key regardless of altitude? If so, I'd say the gauge is vented to the atmosphere. If it reads lower than zero with the motor off at higher altitudes it must be sealed and working like a barometer. Hey, if it's measuring BAR's it must be a barometer, right? This doesn't have anything to do with the power question, it only explains the gauge operation.

 

Silver surfer - I just reread my post and looks like I made a spelling error..yes I totally agree, the ecu DOES know the boost pressure.

Why do we need an aftermarket MAP sensor? Most likely becuase stock is only a 2 bar unit, thoughI have no tested one yet to know for sure.

As for his gauge, it depends if its electronic or not though.

But having run cars at altitude (WRX's the car is neer as quick and tends to run less boost. I believe the baromteric pressure sensor is in the MAF, so it can adjust fuel according to barometric pressure

 

Yes, the gauge reads 0 when I'm up in altitude (with the car off)...

I did notice that I was getting up to 1.4 bar of boost though, which leads me to think that the computer does know about the current atmospheric pressure, and hence giving me "more" boost.

I think I'm going to have to do more "testing" when I go to the snow next time.

Wildman, do you have a boost gauge connected to your intake manifold?

(Edit) - Adam, I'm running the stock SMK boost gauge, so it's a mechanical boost gauge...

-CJ

 

From what I understand it is tapped into a vac line coming off the fuel pressure regulator? I didn't install it myself.

 

Turbo cars loose power in high altitude conditions since the turbocharger is asked to work at a higher pressure ratio. As Silver Surfer mentioned, the compressor has to work in a less efficient part of its compressor map which results in high charge temps. This, however, is not the big component in the power less since the intercooler is capable of bring charge temps down to close-to-ambient levels. If one does the math, he'd see that a 10 deg differnce in charge temps doesn't have that much of an effect on engine output.

The real killer is exhaust backpressure since the turbo now has to operate in a nastier portion of its turbine map (yes, turbines have maps). More exhaust backpressure means poorer engine volumetric efficiency (VE) which is directly proportional to engine output.

My 2c,
shiv

PS. The ECU ECU does not know what boost pressure the car operates at. The MAP sensor only reads up to 1 bar absolute. Boost cut is triggered as a function of MAF and RPM.

 

Are you saying that more or less backpressure would help turbo cars in altitude?

Is this something that you can help tune with a ECU remap or EXDE map?

-CJ

 

I think some of you are not quite seeing the big picture here. I am hardly an expert regarding turbo engine systems, but the basic concepts seem pretty straightforward. Hopefully my simplified view of this system is correct and will be easier to understand?

Think of an engine as a big air pump, the more air you can flow through the engine the more power you can make. Provided of course that you have adequate fuel and ignition, but for this discussion lets just focus on the airflow.

There are two factors to keep in mind here:

1. You can have the same manifold pressure for a given configuration, but as the manifold air temp increases the air becomes less dense. So for a given pressure you have less air volume flow (mass) as the temperature increases.

2. Again with the same manifold pressure you can have more or less back pressure (air flow restriction). So even with the same boost pressure you will have less power with more back pressure/air flow restrictions.

At higher altitudes (or boost levels) the turbo has to work harder. This means the system has to force more exhaust gases through the exhaust turbine to generate the same boost that you would get at lower altitudes. This is what causes increased backpressure, reducing airflow through the engine, even though your running the same or even more boost.
Each turbo configuration has a specific air flow range based on its size and design, ("volumetric efficiency map"). As you begin to increase load (air flow/pressure) the airflow restriction begins to increase to the point of diminishing returns. Another way to put it is; you fall outside of the volumetric efficiency range of the turbo.
This is why a larger turbo can make more power even at the same boost and all other engine components being the same. I am not an expert regarding turbos, but it seems likely that a larger turbo with a higher VE range would suffer less power loss at higher altitude, compared to a smaller turbo already operating close to it's VE limits. Of course there are many variables to consider depending on the specific situation, and of course the larger turbo will also suffer more lag, so there is no free lunch.

The bottom line is you will always make more power at lower altitudes regardless of the configuration, but some configurations will be more effective than others at higher altitude. Does all this make sense?

I have no idea how much charge temperatures actually change at higher altitudes with the stock EVO turbo configuration. Shiv mentioned 10 degrees, I really don't know, could be more or less?
I tend to agree that the backpressure is most likely the larger factor. But also keep in mind that when running near the limit, a 10 degree change in charge temperature may cause a minor air density shift, but it could be the difference between experiencing engine knock or not. In which case it would have a greater impact on power output given the stock ECU's active knock correction.

Are you sure about this? If this is true, why is the MAP sensor there?

 

Yes. It exists for both barometric compensations (upon key on just before start-up) as well for misc emissions functions.

Shiv