Backstory: I recently returned from a very long road trip (5,000+ mi.) where I had the opportunity to observe at length the O2 and fuel trim readings via my UltraGauge. I generally run 93 octane "premium" gasoline as instructed. While we were traveling through Kansas and Nebraska, the fine people of those states think it's neat-o to only sell 87 octane gasoline at most of their filling stations. So, I of course had to oblige them—though I was sure to NOT run their corn-brew—thankfully they gave me a choice between corny 87 and your grandpa's 87. I prefer my corn 1) on the cob, 2) popped, or 3) in salsa form inside my Chipotle burrito, thankyouverymuch.

I noticed that when running 93 octane my long term fuel trim is +7%. When running the 87 octane my long term fuel trim dropped to +3%. Now, to me, this seems like a definite improvement: the truck is running more efficiently, requiring less fuel to hit its optimum mix. But this is just one number. Are there other numbers I should examine such as the timing?

I will also say anecdotally, on one occasion before this someone filled my tank for me with "cheap gas" and the truck seemed snappier overall.

How should I interpret this data? Should I switch to 87? I know that lower octane can lead to ping/knock. But if I'm not getting ping/knock...? I also know that higher-quality or premium-grade gasolines may have better additives which can prevent other issues. I'm not really concerned about price, I just want the best performance and longest life for my engine—which at 180,000 mi. seems like it might be one of the good ones.

Advice?

 

That's pretty cool data! Octane is just a flame retardant to keep the fuel from burning off from heat and pressure before the plug fires... maybe it doesn't need so much of it? Mine seems to run better on it too. I never thought to look at fuel trim data compared to premium.

 

Without further data it's hard to say. However, I would suggest that perhaps your difference lies in using straight gas as opposed to E10? Additionally, what does your normal driving look like? Highway driving in flat Kansas is very different than stop and go in a city with even small hills.

 

Let's say these numbers hold up—and initial indicators seem to be bear this out—despite terrain, driving environment, E10 blend, fuel brand—that is, apples-to-apples, 87 octane brings long term fuel trim closer to 0. What would be the next data point(s) to look at to judge whether running 87 would be an improvement?

Also, I'm interested in a technical explanation of what exactly is happening in there. It seems to me that the case with 93 octane is that the pre-cat 02 sensor is seeing higher oxygen content in the exhaust so the ECU dials up the fuel mix. From a combustion standpoint, you want the most oxygen burned up with the least amount of fuel needed, right? That seems to point in 87's direction. Is there a hidden benefit to having slightly more fuel in the mix such as a cleaner burn? The pea-sized mechanical part of my brain tells me less fuel injected means more efficiency and less deposit-generating material flowing through the system.

#curious

 

Good questions. It should be noted that lower octane fuels contain more BTUs per pound than higher octane fuels. However, the safety factor of preventing detonation in our high CR engines is usually more important than a small percentage increase in MPGs. Indeed, depending on the efficiency of the heads and the spark timing you might not see any increase in MPG by switching to 87. Generally speaking, the ECM tries to lean out the mixture and advance spark timing whenever it can, until it detects knock or the O2 sensors start complaining. Obviously, the higher octane fuels resist detonation and allow a leaner mixture.

I for one am inclined to go with the fuel that the Land Rover engineers specified.

 

Also curious. For most 'explosions' the word stoichiometric is relevant. That means that for every oxygen atom there is a corresponding amount of fuel atoms. No wasted space, no passengers. Literally, the biggest bang for your buck.

A rich mix indicates that incomplete combustion happens, there are spare fuel atoms after each 'bang'. Wasted fuel. Less travel per gallon burned.

A lean mix means the opposite, after each bang there are spare oxygen atoms, but all fuel is gone.

Lean burns hotter, so I was always told, and is bad for engine life. (Citation needed ) But would probably equal to the best mpg, in my mind. The fact there are a few spare oxygen atoms after the burn is going to be irrelevant considering how many spare nitrogen atoms are present. (Nitrous Disco anyone?)

Where was I going with this.....?

 

Lean burns much hotter, which can cause detonation; but it also means less emissions which is why the trend has been towards hotter running engines for the past 20 years.

 

Yeah,
Just because land rover says to put in premium.
Geez, why would Land Rover specify Premium?

Ah who cares..

You'll enjoy replacing the Head Gasket sooner.

Put in that Regular grade and ping those heads.

Hey but whatever.

 

Somebody want to test the difference between 87 and 93 from the same station. I can get some numbers between the difference of e10 and pure gas, but I don't really want to throw 87 in mine since I just did my partial rebuild 7k ago.

 

Ha NIMBY.