Lower octane at higher altitude...

[RamFan]

Originally Posted by postjeeprcr
Originally Posted by RamFan
Yup, I brought this up several years ago on here and from what I remember was told that (essentially) the air being thinner due to the increased altitude allows for a lower octane to be run. I remember even seeing a few stations with 85.5 octane, threw me for a loop.


That is interesting, I have lived in Colorado almost my entire life and I've never seen 85.5, do you remember where the station was?


Pretty sure I was in Montana by that point of my trip.

 

[DGXR]

This topic makes me think about the computers compensating for less atmosphere (compression) at altitude. Ignoring the octane discussion, do modern engines inject less fuel when there is less oxygen in the cylinders? I mean with all the airflow sensors and oxygen sensors and whatnot, there must be some compensation. Yes I know less oxygen in the cylinders equals less compression. I'm taking this as a different perspective on the 14.7:1 ideal stoichiometric ratio for oxygen:fuel by weight. Maybe this should be a topic for a different thread, not meaning to jack this one. Thanks

 

[kschachn]

Yes, a mass air flow sensor measures the mass of the air entering in the engine and the oxygen concentration is assumed to be standard. The oxygen sensors do not measure at the inlet.

 

[92saturnsl2]

Indeed, modern fuel injection does vary the amount of fuel injected, compensating for altitude, temperature, density (moisture content), etc. MAF in combination with the intake air temperature sensor, and MAP sensors determine the "amount" of air entering the engine, then the pre-cat oxygen sensor reports back to the ECU how close the resulting burn is to the ideal ratio, allowing the ECU to fine-tune the mixture very precisely.

Most vehicles use MAF sensors, but there are a lot of vehicles that don't have MAF sensors (including most Chrysler vehicles). These systems do without the MAF sensor by using the MAP (manifold air pressure) sensor to determine air pressure, the IAT reports intake air temp, then the ECU calculates the density of the air. With the density of the air known, the ECU has a table of how much air should be flowing at any given engine speed and manifold pressure.

 

[ironman_gq]

Originally Posted by ammolab
Originally Posted by Kestas
Don't turbos make the effect of altitude a moot point?


Many turbos make a max boost (say + 13 psi) but this is 13 psi above ambient (atmospheric pressure at your location). Which might be 14.7 in Houston...but at my house only 10.4 psi (7000ft asl)


Not quite true, the MAP sensor (manifold absolute pressure) doesn't really care what the ambient air pressure it. If the system is calling for a certain manifold pressure, it'll make the turbo provide it. This should be how all electronically controlled turbo's are. ECU tables really only care about how much air gets to the motor and a turbo takes out the ambient pressure variables. As long as your setup isn't maxing out the turbo at low elevations there should be enough overhead in the capability of the turbo to compensate for the elevation difference. Almost all OEM turbo engines have more than adequate overhead to account for any elevation changes, a tuned up ecoboost making 5-600hp on the stock turbo probably won't be able to make that at high elevation but a stock one should make stock power at any elevation.

 

[92saturnsl2]

No turbo engine that I've ever driven has had the same amount of power at high elevation as low, and I've driven a more than a few back and forth from near sea level to 6000+ elevation. There is a loss in efficiency for sure, but certainly not to the level of power loss that a N/A engine experiences.

 

[edhackett]

Originally Posted by 92saturnsl2
No turbo engine that I've ever driven has had the same amount of power at high elevation as low, and I've driven a more than a few back and forth from near sea level to 6000+ elevation. There is a loss in efficiency for sure, but certainly not to the level of power loss that a N/A engine experiences.


Manufacturers cut back boost at altitude to maintain a set maximum turbine speed.

Ed

 

[Revving2Redline]

I seen 85,5 or 86 back in Casper Wyoming when I went home