Fuel FAQ: What octane gas to run in my 5.0?

There is great myth and hyperbole surrounding octane ratings and cars. Such that you'd be hardpressed to find an online automotive community that doesn't face this question at least once a week. The case is no different with Fox Mustangs. In fact, perhaps octane questions are more common due to the Foxbody's performance nature, as who doesn't want to squeeze out as much performance as possible? The guide below is written to address the many common questions, myths and facts floating around the internet regarding automotive fuel and octane rating. For this article, the use of the word octane is synonymous with octane rating, unless otherwise noted.

The basics
What is octane?
Will higher octane fuel increase performance?
Does running higher octane increase fuel economy?
Will my stock 5.0 benefit from higher octane?
What fuel should I use?


What is octane?

When you pull up to the pump, there is usually a minimum of 3 different fuels you can select from. There is the low grade (usually 87), mid-grade (say 89-91) and 'super' or premium grade (93+). How do these fuels differ? They have different octane ratings, and thus different levels of octane! Duh! OK, but what does that actually mean?

Quite simply, octane rating is a quantitative measure of how a fuel will resist ignition. It is NOT a measure of energy density. Therfore, 93 octane does not offer more energy per gallon than a gallon of 87 would (more on this performance aspect later). In simpler terms, the purpose of octane in automotive fuels is to prevent engine knock. If there is more octane in the fuel (thus higher rating), that fuel is rated to ignite at higher temperatures. OK, so higher octane fuels effectively has a higher activation engery (tolerates higher temperatures before igniting), but what is so useful about this? It is important in regards to engine knock, which now begs the question, what is engine knock? (all these questions and few answers... patience, young padawan).

It doesn't take a rocket scientist to figure out an internal combustion engine produces heat, and a good amount at that. However, too much heat at the wrong time can spell disaster for an engine. During the compression stroke, the piston is moving upwards and compressing the air. As the air is compressed, the temperature also rises. This is where octane rating comes into play. Imagine during the compression stroke, the temperature becomes hot enough that the fuel self-ignites, without aid of the spark plugs. This event is called premature detonation, or engine knock. Consequences of this event can be drastic, potentially resulting in servere engine damage (melted piston, broken rods, cracked cylinder heads) as the piston will not be in the correct position for combustion. Thus it is very important that the fuel be ignited at the right time. Igniting the fuel at the correct time is managed by the ignition timing and ECU, which decides the best time to spark the spark plugs. However, the fuel also needs to make it to the 'best time' before igniting. If it gets to hot before the ECU decides to ignite, BOOM, premature detonation. Thus, controlling engine knock is done by using different octane ratings and varying ignition timing.

Engine compression, timing and octane level are all related. When considering one of those terms, always take the other two into account as well! (This will be better explained in the remaining sections)

How does octane affect my engine performance?

If you haven't already had this happen, at some point during your Mustang years, you will be conversing with a fellow Mustang enthusiast when all of a sudden said other enthusiast will say quite smugly: "I ONLY run 93 in my 'Stang". Oh no, alarm bells are suddenly ringing! You only put in 87! Sweat starts dripping down your face, you get the cold shivers. You need to get out of there fast. This guy has you beat!

Well actually, this probably isn't the case. As already mentioned, higher octane fuel does NOT mean it has more energy to burn or make more power, but rather requires a higher activation energy to spontaneously combust. Also recall, at the end of the last section, I mentioned that compresssion, octane level and ignition timing are all related. Let's look into this triangle affair a little further.

Octane level, engine compression, ignition timing: A tangled web of love

Compressing a gas causes the ambient temperature to rise. Greater compression creates greater heat. Greater heat increases the chance of fuel to self-ignite. Self-ignition creates a much greater risk of engine knock. A higher octane rating has an increased ignition temperature. See where I am going here?

A lower compression engine will receive no added benefit of running a higher octane than what is needed to prevent detonation. Because of the lesser temperatures seen in a lower compression cycle, it is entirely possible to have unburned gasoline left after the combustion cycle if using too high an octane (recall: higher octane has a higher temperature threshold). Conversely, a high compression motor running a low octane fuel will not fair so well, as their is a much greater chance the low octane fuel's temperature threshold will be surpassed during compression, causing the fuel to ignite prematurely. Ignition timing further ties into this cycle. If the spark is left too late, there is an increased chance of unwanted detonation. The same goes with advanced timing.

So, to finally answer the question. Running a higher octane fuel will not increase an engines performance unless the engine has the proper ability to take advantage of the higher activation threshold. What is the 'proper ability'? Increased compression and/or advanced ignition timing. A stock motor will not benefit from an octane increase than what was specified by the manufacturer, unless the compression has been upped or the timing advanced. The way I look at it, octane rating is the last piece of the puzzle. First figure out compression and ignition timing, as playing with these two aspects is where the extra power come from. THEN look at what octane is needed to satisfy those first two variables.

Will premium fuel increase MPG?

In the case of antiquated pushrod 5.0L Foxbody Mustangs, no, premium fuel will not increase MPG. That smug guy at the gas station smirking while you pump 87 and he pumps 93 into stock vehicles is just wasting money. You'll both go just as far on a tank, you'll just have spent less.

However, on a modern car, it just might. It also might also just increase performance a little bit too. How can I say this, you ask, after quite definitely saying higher octane will result in no gain in performance or MPG? I just finished explaining that the only way to necessitate higher octane is to increase compression or engine timing. Increasing compression isn't exactly something that can be done on the fly. However, in modern cars, the latter can. Due to ever more powerful ECU's and ever more many sensors, a modern car can monitor its own combustion cycle and increase or decrease timing based on it's findings. Thus, many modern cars today will recognize that the gas you've put in has higher octane, and will advance the timing accordingly.

Will my stock 5.0 benefit from higher octane fuel?

As previously mentioned, no, there will be no gain whatsoever in either the performance or fuel economy departments. Compression for a stock 5.0 is 9.0:1 and inital timing is usually at 10°. Ford recommended 87 will do just fine (as is indicated in the owners manual). However, if you bump either of these factors up, then you ought to consider using a greater octane fuel.

What fuel should I use then?

For street driven cars, use the minimum octane needed to prevent any knocking or pinging. Whether the minium is 87, 91 or 93 is entirely dependent on your motor setup. No need to spend any extra dough on something that is not needed.