Exhaust cut-out & more on the road tuning.

[knucklesplitter]

That's a nice feature. Perhaps since Cobb basically reflashes the ECU, the reprogrammed ECU has capabilities to adjust for elevation that I am not used to seeing.

Still, I'm skeptical, having seen the AccessPort vary boost dramatically with elevation no matter which map Cobb gave me to load - high WGC, low WGC, semi-custom. Also the stock ECU is prone to do this too.

[cody]

I thought you might find this interesting, this is Ed's latest email to me.

Sadly I cannot send these as real time maps because the tables I'm altering are only included in the base map format. In this latest one, I extended high altitude wastegate duty cycles a bit more as well as made the high altitude boost trim less aggressive. Let me know how this one does, and as always make sure to watch for spiking.

[Dean]

Boost will always decrase with altitude unless you had a variable pitch configuration somehow on the turbo which I can't imagine how you would begin to do. There is less air, period and efficiency goes down. The Pikes Peek Tuning
post was great on this stuff...

I guess you could size the turbo and design the two sides for higher altitude, but I think your sea level performance would suffer.

[cody]

Good info in that link. Do you guys think targeting boost at 17 PSI up here is dangerous?

[Kevin M]

Yes. Research the difference between pressure ratio and absolute/relative manifold pressure. Compressor efficiency maps don't read in "boost" they read in air flow vs. pressure ratio. Exceeding the most efficient pressure ratio at a given amount of power produces excess heat and puts tress on your turbo's internals.

Edit: 17 psi up here peak, tapering to the normal 12-13 psi at redline, is probably okay. Not as efficient as that curve is at sea leel, but probably not dangerous either.

[doubleurx]

Originally Posted by BAN SUVS

Yes. Research the difference between pressure ratio and absolute/relative manifold pressure. Compressor efficiency maps don't read in "boost" they read in air flow vs. pressure ratio. Exceeding the most efficient pressure ratio at a given amount of power produces excess heat and puts tress on your turbo's internals.

Edit: 17 psi up here peak, tapering to the normal 12-13 psi at redline, is probably okay. Not as efficient as that curve is at sea leel, but probably not dangerous either.

17PSI? I'm at a steady 20.

[knucklesplitter]

True that as altitude goes up, your turbo efficiency goes down, and the maximum potential power goes down. I agree with this totally and have argued till blue in the face with the "turbos make the same power at altitude" crowd.

What I'm talking about is something else I think. Take a turbo that has excess capacity - say the VF39 in stock configuration. The boost is not limited by the turbo at say 4000 rpm, it is limited by the wastegate opening and bleeding off the exhaust pressure as modulated by the ECU-controlled boost solenoid. If programmed to, the turbo is loafing along making 14 or 15psi at sea level and wastegating like crazy. This configuration will make 1-2psi less boost (guage) at 5000'. This doesn't change as the ECU "learns" either. That's not a turbo efficiency issue, it's a wastegate regulation issue. It's like the control system for the boost needs different parameters at elevation. This is a mystery to me and I wish somebody could explain it.

[Kevin M]

Originally Posted by doubleurx

17PSI? I'm at a steady 20.

You're not using a TD04 or an EJ20.

[Kevin M]

Originally Posted by knucklesplitter

True that as altitude goes up, your turbo efficiency goes down, and the maximum potential power goes down. I agree with this totally and have argued till blue in the face with the "turbos make the same power at altitude" crowd.

What I'm talking about is something else I think. Take a turbo that has excess capacity - say the VF39 in stock configuration. The boost is not limited by the turbo at say 4000 rpm, it is limited by the wastegate opening and bleeding off the exhaust pressure as modulated by the ECU-controlled boost solenoid. If programmed to, the turbo is loafing along making 14 or 15psi at sea level and wastegating like crazy. This configuration will make 1-2psi less boost (guage) at 5000'. This doesn't change as the ECU "learns" either. That's not a turbo efficiency issue, it's a wastegate regulation issue. It's like the control system for the boost needs different parameters at elevation. This is a mystery to me and I wish somebody could explain it.

Basically, Subaru got to 300/300 on the EJ257 and said "okay, that's plenty." And since boost is not part of the learning parameters of the ECU, it doesn't change. Which is by design- can you imagine the number of people going to the service department freaking out because their STi makes 1 or 2 pounds more or less boost today than it did yesterday?

[sonicsuby]

Originally Posted by BAN SUVS

Basically, Subaru got to 300/300 on the EJ257 and said "okay, that's plenty." And since boost is not part of the learning parameters of the ECU, it doesn't change. Which is by design- can you imagine the number of people going to the service department freaking out because their STi makes 1 or 2 pounds more or less boost today than it did yesterday?

If they didn't have the Subaru boost gauge SOA would just deny the claim

[cody]

Originally Posted by BAN SUVS

Yes. Research the difference between pressure ratio and absolute/relative manifold pressure. Compressor efficiency maps don't read in "boost" they read in air flow vs. pressure ratio. Exceeding the most efficient pressure ratio at a given amount of power produces excess heat and puts tress on your turbo's internals.

Edit: 17 psi up here peak, tapering to the normal 12-13 psi at redline, is probably okay. Not as efficient as that curve is at sea leel, but probably not dangerous either.

I'm now on my 2nd map update from Ed and I'm holding up to 17.5 PSI with the normal taper up here. But I think my gauge may read a touch higher than the ECU's boost sensor. Anyway, my boost is holding solid till it tapers above 6K RPM's. The car feels super fast and smooth. I just hope this isn't dangerously high boost. I love my car and would never want to hurt her.