20’ TRD off-road have a timing belt or chain?

If your in Washington state your probably at a higher altitude so with the less oxygen your say 87 octane fuel acts more like a 90 octane at at high altitude due to less Oxygen. I'm not 100% exact on numbers but this is just an example.

 

How dare you recommend that someone RTFM!!

G.

 

Care to explain how that works? I'm not saying you're wrong, I just don't get it. If your air pressure is lower, yeah I guess your compression ratio is slightly lower but lower oxygen would usually mean you run leaner to get your stoich right, no? Hence why forced induction helps, you get more oxygen in the cylinder by way of cramming more of everything in there, which still only relates to how much fuel you can dump in with it, not so much the fuel's resistance to combustion...

I can't do the math right now but if someone can, relative psi at 5000ft is 12.23psia. Would that times the compression ratio lead to a low enough compression pressure to allow a lower octane rating?

I could be totally wrong, if I am, please teach me. There is zero snarkiness or sarcasm in this post, fr.

 

Ummm what?!?!?

 

"Research several years ago from the American Petroleum Institute showed that lower air pressure at higher altitudes allows vehicles to perform as well with 85 octane as they would with 87 at lower altitudes. ... Many car manufacturers recommend that motorists use at least 87 octane."
I live at sea level so I see up to 94 at the pump around here. It would make no sense in Washington state at a higher elevation. I had Oxygen mixed up with air pressure but theres obviously less oxygen at higher altitudes. Basically your getting a less dense amount of air coming in. Your engine is an air pump and can only take in so much air, if the air is less dense yes it will adjust for that on newer vehicles but your also losing power due to the less dense air. That's part of the reason for a lower Octane at higher altitudes.

 

This is true, lower atmospheric pressure lowers the volumetric efficiency and this lowers the potential for pre-ignition. The octane rating does not change as you stated in your previous statement.

 

That was a quote and I didn't mean that the octane changes it's just the lower grade octane ACTS like a higher grade octane at altitude.

 

The issue of octane and elevation has interested me for a while. I've never seen any definitive testing or reasoning about whether lower octane is ok at higher elevations. The manufacturers and the EPA simply say that sub 87 octane is not recommended in non carburated engines and leave it at that with no explanation.

I think you're on the right track though with your comments about stoichiometric AFRs. In carburated engines at altitude, they effectively run rich, possibly offsetting the lower octane. An electronically fuel injected engine should run the same AFR regardless of elevation.

All that being said, the lower air density does basically lower the compression ratio. It might be more correct to say it lowers the pressure in the cylinder at top dead center though. This lower pressure may or may not make up for the lower octane in a modern EFI engine. It depends on the specific engine. In my personal experience, I've never had issues running 85. I think the manufacturer recommendation of 87+ is more or less a CYA sort of deal. They just can't or won't go on record stating a cut off elevation for running 85. The testing isn't worth it and probably don't want to have any liabilities with people running a tank of 85 near sea level.

I'd be interested if anyone else has insights on this as well.

 

I'm at Sea level and I have been playing around with some of this in some high performance toys lately and it definitely improves power. The colder the day the better. It smells so good , like the gas from back in the day. It smells really good at the exhaust also.

 

This isn't really directed at you, but gave me another thought. There seems to a myth in some circles that lower octane is advantageous at high elevation. There is no advantage, it's just cheaper and if you can get away with it why not I guess?

Now I'll wait for somebody to tell me that premium ran like shit in their carbed engine and 85 solved it...

 

I totally agree with that why they would have it its cheaper and safe at that altitude.