85 Octane at high altitude in a V8

[RunUp]

Currently cruising around Colorado and Wyoming. Regular gas is 85 octane and mid-premimum is 87 due the altitude.
The owners manual says not to use anything lower than 87. Talked to a Toyota service guy in Casper and he says the same thing.
My question for anyone that drives at 5000 feet or higher, is it safe to use 85 octane?
(The one thing that sux out here is that they charge the same for 85 octane, as they do 87 octane in the lowlands.)

 

[Iowa4Runner]

Currently cruising around Colorado and Wyoming. Regular gas is 85 octane and mid-premimum is 87 due the altitude.
The owners manual says not to use anything lower than 87. Talked to a Toyota service guy in Casper and he says the same thing.
My question for anyone that drives at 5000 feet or higher, is it safe to use 85 octane?
(The one thing that sux out here is that they charge the same for 85 octane, as they do 87 octane in the lowlands.)

I'd feel safe if it were an '04 V8, because they don't seem to knock easily. Check out:

http://www.toyota-4runner.org/showthread.php?s=&threadid=12938&highlight=octane+test

Two things that are working in your favor-

1. High altitude means lower air density, and it's my understanding that lowering the air density makes knocking less likely.

2. The 4Runnner's engine management system can pull back the ignition timing if it detects knocking.

If you try 85 octane gas and you hear engine knock, then you know for sure that 85 won't cut it.

 

[RickyCRX]

My '03 V8 ran just fine on 85 octane at VERY high speeds in high elevations this past summer... And as hot at 102F on the outside temperature display. I was going anywhere from 80-95mph and had no problems.

 

[RunUp]

Thanks guys.
It's an '05 V8 but next fillup, I'm going to try the 85.

 

[greasefingers]

That is the nice thing about a MAF sensor (mass air flow) It will measure the mass of air in grams per second and it is independent of air density.

The knock sensor works like an accelerometer. It will measure abnormal vibrations from the engine. If it detects any then it will retard the timing in steps. Then it will advance it back to normal until another pre-detonation occurs.

 

[Iowa4Runner]

That is the nice thing about a MAF sensor (mass air flow) It will measure the mass of air in grams per second and it is independent of air density.

The knock sensor works like an accelerometer. It will measure abnormal vibrations from the engine. If it detects any then it will retard the timing in steps. Then it will advance it back to normal until another pre-detonation occurs.

I thought they were more like microphones. And if I remember correctly, the engine's computer(s) will look for a range of frequencies and send an error code if it doesn't hear normal engine noises. Pretty cool...

 

[OUTRNNR]

A few things:

A micropohone and an accelerometer are virtually the same thing, one is excited (or accelerated) by sound pressure waves and the other by physical motion, whether it be vibration (harmonic oscillation) or a single step input (bump, shock, etc.)

Either way, the computer can detect engine knock and retard the timing automatically, but this comes at the drastic reduction in performance and fuel economy.

I live in CO (Denver area ~5300-6000 ft.) and I run 87 octane just because I want the better performance and fuel economy of higher octane. Premium fuel of 90 octane or higher doesn't result in much of a performance difference or increase in fuel economy for me, so I know that the 87 is working just fine. I don't think would run 85 octane in my '05 V8 because I don't want the engine detonating at high rpm even if the computer can control it or not. If I lived at lower elevations, I would definitely run premium fuel.

I think living in CO and higher elevations >4000 ft make it easier to run a lower octane because the air is less dense up here. Atmospheric pressue is around 2 psi lower then at sea level which in turn results in less compression of the air/fuel mix (hence, less horsepower at altitude) Since the pressure during the compression cycle is less, the air/fuel mix heats up less and is less prone to detonating before the spark.

Remember- the only thing octane does is raise the combustion temperature of gasoline...NOTHING ELSE!!! It doesn't clean your engine, it doesn't add horsepower. Plain and simple if you have an engine with lower compression then run lower octane. If you have a high compression engine then run premium. The 4Runner is somewhere in the middle, which is why Toyota recommends 87 octane.

 

[Iowa4Runner]

A few things:

A micropohone and an accelerometer are virtually the same thing, one is excited (or accelerated) by sound pressure waves and the other by physical motion, whether it be vibration (harmonic oscillation) or a single step input (bump, shock, etc.)

Either way, the computer can detect engine knock and retard the timing automatically, but this comes at the drastic reduction in performance and fuel economy.

I live in CO (Denver area ~5300-6000 ft.) and I run 87 octane just because I want the better performance and fuel economy of higher octane. Premium fuel of 90 octane or higher doesn't result in much of a performance difference or increase in fuel economy for me, so I know that the 87 is working just fine. I don't think would run 85 octane in my '05 V8 because I don't want the engine detonating at high rpm even if the computer can control it or not. If I lived at lower elevations, I would definitely run premium fuel.

I think living in CO and higher elevations >4000 ft make it easier to run a lower octane because the air is less dense up here. Atmospheric pressue is around 2 psi lower then at sea level which in turn results in less compression of the air/fuel mix (hence, less horsepower at altitude) Since the pressure during the compression cycle is less, the air/fuel mix heats up less and is less prone to detonating before the spark.

Remember- the only thing octane does is raise the combustion temperature of gasoline...NOTHING ELSE!!! It doesn't clean your engine, it doesn't add horsepower. Plain and simple if you have an engine with lower compression then run lower octane. If you have a high compression engine then run premium. The 4Runner is somewhere in the middle, which is why Toyota recommends 87 octane.

Actually the more I think about it I think greasefingers was right, the knock sensor really is more like an accelerometer. There is a difference.