High Altitude, Power, and Octane.
 

So I have been getting some conflicting ideas about living at high altitude and using high octane fuel. I live in Albuquerque NM which sits anywhere from 5500-6000 ft depending on where in the area you live. I am planning on going to the drags tomorrow to run the car for the fist time so I sent a message to Jerry at Eurocharged to ask about using a high octane fuel (I can get VP 101 at the pump) and if I would need a retune or not. He told me that I could expect to see a 20 hp increase with 101 and a 40 hp increase with 112. When I asked him about being at altitude he said that the ECU measures barometric and compensates accordingly. He also said that a retune would not be necessary.

When I was at the dealer the other day I was having a conversation with the service manager who told me to run 89 midgrade around town and that because of the altitude the car would run much better. He said that the 91 does not get a complete burn and causes carbon deposits and will cause the engine to hesitate. So I am getting conflicting answers to this question. I did some google searching and found that at higher altitudes that you can run a lower octane because the higher altitude has an effect on the engine like lowering the compression ratio because the air is thinner..

So I'm wondering if both can be correct .. in a nonperformance engine that normally runs on 91 you could run a lower grade fuel but on a performance engine seeking max power it may still be able to take advantage of the better fuel. Any experts want to way in ???

So your service manager is a chemist?

Why would 91 not burn properly just because the atmospheric pressure is lower?

Does that mean using a higher octane at sea level does the same thing?

Honestly, at 6000ft elevation, don't even worry about fuel. You're not going to break any records, and will probably be running in the high 13's anyway if you're lucky. I took my car to Bandimere outside of Denver a few weeks after I bought it, and it was comical how slow I was running. Just enjoy it.

As far as 89 running better than 91 at elevation, I don't think 2 octane points makes much of a difference either way. If that were the case you'd have an epidemic of cars with carbon deposits where you live. I call BS on that, never heard of it.

+1

I seriously hope its not that slow.. I'll be sure to bring the gopro and post up some video and time slips

It's going to be. I'm looking at my slips right now: 13.88, 13.77 and 13.59 (which DA adjust to 12.33, 12.23 and 12.19 respectively - the DA was 8796 that day). And I can drive the **** out of a car.

Like I said, just enjoy it.

Do not listen to the service manager. 89 octane is not recommended. These cars need and want at least 91 and the higher the octane, the slower the fuel burn the less issue you have with detonation.
Listen to Jerry at Eurocharger. Believe me when I say this (A stranger on the internet) You will have more issues trying to use 89 as a performance or daily fuel than the higher octane fuels.
Most cars don't need 91. This is true, but the 6.2 is 11.3.1 compression. You need and want the higher octane fuel. As far as carbon deposits. I have heard of it happening when you use cheaper fuel or California fuels but that's what a few bottles of intake cleaner will help with.

The octane number is related to resistance to detonation.
Detonation will damage the engine badly, relatively quickly.
The difference in tune for a turbocharged car between a 93 fuel tune and a 91 fuel tune is very noticeable, especially in terms of maximum torque.


The M156 runs a compression ratio of over 11:1, which means that it's very sensitive to detonation.
It very likely runs a suite of tables in the ECU where it detects detonation and reacts immediately by reducing timing advance for a duration of time, in stages, until there is no more detonation found, and then just runs at that timing level, while occasionally trying to raise it.
The torque will drop, the power will drop. However, with other cars, the very process of fiddling with the timing advance, waiting for detonation to happen, then pulling timing, causes damage.


In sum, carbon deposits with 89 fuel? I don't know.
Detonation with it? very likely. They may have a very good avoidance software suite, but I personally on my car would not risk finding out how good it is, if I could avoid it.
However, the higher altitude will cause a lower measured compression, but doesn't change the designed ratio. I can't say that I know what this will do to detonation, may well reduce it.

The other way around with extra high octane, unless there is a tune or a way for the software to compensate, you will have high EGT. This can be combined with high CHT (Cylinder head), which could burn off your carbon deposits, but could damage the converters, unless these are metal substrate. I did not read that the factory converters were metal substrate.


But I do recall reading in the XENTRY a live reading of the EGT, so somehow, although there is no probe (that I know of), they found a way to calculate it.
This means there's a table associated with the EGT and the ECU will take protection measures as soon as your high octane fuel raises the EGT and probably negate advantages you may have with it, unless there's a tune that accounts for everything.
Under my opinion it (the stock tune) will richen the mixture, to cool it down and you will burn through a tank of that stuff really quick.

Thanks for the info VLADDS. I am running the V6 tune.

So did the OP take the car to the track?

Probably doesn't want to post up the time slip :D...

Why yes he did... best time of the day 13.2 @ 107mph.. when you adjust for DA that comes out to 12.02 @117... so very close to an 11.99 but so far away. The car is definately capable but I could not get it to launch properly. Best 60 of the day was 2.10 and as the day got longer and the air temp heated up it got worse. I will post video soon

Sounds about right for a tuned car. You can get the exact DA here: http://dragtimes.com/da-density-altitude-calculator.php

Glad you had fun

So, I theory, higher altitude means lower oxygen density, which would mean less fuel and oxygen in the cylinder during the compression stroke, and therefore lower risk of predestination as less heat from compression is generated.

I would think that forced air from a turbo or supercharger would counteract this. We're at about 3500 feet in Calgary, and run 87 octane in our MDX without any noticeable degradation in fuel economy or performance (it requires 91).

I'd never try this in my E63 or any other forced induction engine.