# Cheap math on supposed longevity benefits of premium vs. regular

#### d00df00d

Was chatting with a shop owner who prefers premium gas even in cars that don't require it. This is not only because of knock resistance and fuel economy, but also because it has a lower level of impurities. Taking that for granted, and given the value of cleanliness in an engine, this makes some sense in the abstract. Though, whether this would make premium worth the money is another matter.

This all got me thinking. I can't assess the point about cleanliness, but I can do some back-of-the-envelope math to see if there's any chance a longevity benefit could pan out, regardless of how regular and premium differ.

If the only reason you clicked was to get mad at what you thought the answer would be, I'll save you the trouble: For engines that don't require premium, the most likely answer is no. At least, not according to what I found.

Caveats before you read further:

1. Nothing below says anything definitive whatsoever. This model is not another "index" or any of that junk. It's a crude thought experiment with some simple math. In some cases it might – might – yield inferences good enough for making practical decisions, which is all I wanted from it. In other cases it might yield some entertainingly weird results. Please, please don't pretend there's anything more to it than that.

2. This does not apply to engines that require premium gas.

The way I approached it was basically to compare per-mile costs of fuel and engine rebuilds for regular and premium gas. The math had several steps, but it's all basic:

1. [price per gallon of fuel] ÷ [mpg with that fuel] = [per-mile cost of that fuel]

For regular gas, I just filled in current numbers. For premium, I used the current price and generously assumed an mpg increase.

2. [cost of engine rebuild] ÷ [engine rebuild interval] = [per-mile cost of engine rebuild]

For regular gas, I made assumptions based on people's typical experiences. For premium, I kept the same number for engine rebuild cost but left the rebuild interval as an unknown.

3. [per-mile cost of regular] + [per-mile cost of engine rebuild w/ regular] = [per-mile cost of premium] + [per-mile cost of engine rebuild w/ premium]

4. Because each of the terms above is a fraction, the equation can be rearranged to isolate the unknown – i.e., the engine rebuild interval with premium:

Plugging in numbers on the left side of that equation and solving it should yield the minimum rebuild interval necessary to break even with premium gas. The engine would have to last at least that long for premium gas to be worth the money over regular.

I made a spreadsheet with all of this and threw in some numbers to play around. Here are some example outcomes:

Here, the model suggests the engine would have to last >2x as long on premium than on regular for the fuel price to be worth it. Not a chance in heck of that happening.

More:

Even more ludicrous numbers. I'm sure most of us are on the same page about this but the numbers really drive the point home IMO.

See that negative sign here? That means the model suggests premium gas wouldn't be worth using in that scenario even if it made the engine last literally forever. In other words, given these numbers, it'd be cheaper to run regular and rebuild the engine when it dies than to run premium and have the engine be immortal.

I also tried plugging in numbers like what I've seen at sketchy no-name stations vs. Top Tier stations around here:

Awkward.

Not actually worried by that last one. The model doesn't properly account for the elevated risk of a truly bad batch of gas from a sketchy station, which IMO is enough of a reason to write off sketchy gas and render the comparison moot.

Something else this doesn't account for: the fact that, the longer the engine runs, the higher the probability that it'll encounter a bad batch of gas or some other catastrophic event that'll cut its life short. That makes those long rebuild interval estimates even more hideously unlikely.

So, again, nothing definitive, but... seems safe to say any supposed benefits of premium gas in a car that doesn't require it are sketchy at best.

What do y'all think?

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• Sketchy vs Top Tier.jpg
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a5m
The falsehood that this starts off with collapses the whole theory.

"because it has a lower level of impurities."

Untrue.

Some brands offer elevated detergents in their top octane choice, but impurity levels are consistent 87 - 93.

I have a Nissan VQ40 that has 380,000 miles on it, owned since new - original fuel injectors even. Its always eaten 87 octane, and lots of it

I don't see how premium fuel can prolong anything. Modern closed loop control engines are not going to knock - the ECU will back off the timing if it does. Whatever you burn its going to burn it clean or you will throw a code.

Premium does have higher octane, you will be able to run more timing advance, and it will produce slightly more power and better efficiency, but its not enough to warrant the price premium - pun intended.

The falsehood that this starts off with collapses the whole theory.

"because it has a lower level of impurities."

Untrue.

Some brands offer elevated detergents in their top octane choice, but impurity levels are consistent 87 - 93.
Not always that case. There are some premium fuels out there that are "cleaner" "pure" or whatever. The issue is that it isn't the case region to region even for the same brand.

Most gas stations sell very little premium. It's more likely to be contaminated or stale.

87 octane fuels tend to be less refined and contain more unstable hydrocarbons. As the months pass during storage these unstable components react to form gums, varnishes and lower octane hydrocarbons. As a result the octane can decrease within months for 87 octane fuels, especially when stored under less than ideal conditions. 93 octane fuels are more refined and contain more stable hydrocarbons. These stable hydrocarbons can last 2-3 times longer than 87 octane fuel. Even in proper storage 87 octane gas can start to degrade in 3 months, 93 octane fuel should last closer to 9 months before degradation is noticeable.

Most gas stations sell very little premium. It's more likely to be contaminated or stale.
Bingo.

It may also include more ethanol. It won't exceed 10%, but refiners are allocated how much ethanol they must use in gallons per year, not in percentages - which is why the pump says it may have "up to 10% ethanol". Ethanol is an octane booster. Cheapest way to boost your octane is add the 10% to the premium, use up your leftover allotment to the regular, whatever that end up at.

Years ago I recall reading that high octane fuel burns dirtier than regular. The additives that reduce knock inhibit burning of the fuel thus leaving deposits behind. High octane fuels have extra cleaning agents to try to make up for this.

Years ago I recall reading that high octane fuel burns dirtier than regular. The additives that reduce knock inhibit burning of the fuel thus leaving deposits behind. High octane fuels have extra cleaning agents to try to make up for this.
AFAIK this can happen if the ECU doesn't have the flexibility to take advantage of / compensate for the higher octane. Wouldn't think this would be an issue in any relatively modern car but IDK.

Years ago I recall reading that high octane fuel burns dirtier than regular. The additives that reduce knock inhibit burning of the fuel thus leaving deposits behind. High octane fuels have extra cleaning agents to try to make up for this.
The refiners aren’t really using additives that reduce knock, refiners adjust the overall blend to achieve a target octane rating. 87 may have a higher concentration of say light hydrocrackate, light and heavy cat gas, or straight run naptha in it vs 91/93 which may contain more heavy hydrocrackate or alkylate.

The only difference at your local gas station is antiknock agent added to the gas. They don't refine 91 octane any special way.

All my local pumps are the same refined E10 with different levels of anti-knock added to it.

If your passenger car is tuned to have ZERO knock on 87, as they are, there's no benefit at all. 91 octane is just a worse burning fuel for you. They burn at the same rate, 91 octane is simply more averse to propagating a flame kernel.

I know it's heresy but I'm starting to use premium fuel in my Honda.

There may be a slight mileage improvement or slightly better performance but that's not why I'm doing it. We drive the Honda very little (1000 to 1500 km/year) and it often sits unused for months at a time. I've never had a problem with fuel going bad in it or really any fuel problem but I think there's less chance of that happening with premium. Our premium fuel is 91 octane and E0 (vs 87 octane and E10 for regular).

And I no longer fill the tank either. I could get maybe 500 km with a full tank in mixed driving. But 500 km might take us 6 months. So I let the tank get pretty low and then fill it to maybe half a tank. That's also in aid of having fresher fuel.

87 octane fuels tend to be less refined and contain more unstable hydrocarbons.

But that doesn't imply it is "dirty," which has not been defined.

This is yesterday's engine news, but the two new Nissan's I had in the 90's and years afterwards, did much higher fuel mileage on premium. One a Sentra SE-R, the other an Altima GLE. I repeated the comparison many times until I just stuck with premium. Both of these were distributor ignition, not coil packs. One tech told me that the Nissan's (back then) had wide parameter on their timing advance if the knock sensor didn't shut down the party. Of course, premium fuel ranged from \$.95 to \$1.70 a gal for many of those years. The mileage gains were on the order of 10-15%. That was compared with 89 pon, not 87.

You also have to consider the inconvenience of any downtime that could have been avoided by using premium fuels.

I have a company fuel card which I use to fill up my daily driver. I'm only allowed to buy diesel from supermarkets with said card. Now, our diesel is made to a higher standard I believe than diesel available in the US. However, I still treat the fuel with an additive of some kind. My go-to for a while has been Archoil 6900D-Max. I figure if it stops me having to replace an EGR or injector in the time I own the car then it's been worth it! Finances aside (the Archoil costs me £30 for enough to last 10,000miles), it would be worth it just to mitigate the downtime and hassle having a failed EGR or injector would cause me.

I'm having a problem with the whole, "Does not require Premium" thing. Just because today it's "not required" doesn't mean it's not beneficial to use it. Years ago before electronic fuel injection and computers, you could damage a high compression V-8 engine by running it on regular gas.

Back then it was even possible to knock holes in the pistons with severe detonation under certain conditions. That's why many of the owners manuals back then said, "Use ONLY Premium fuel".

Today's high performance V-8's can run on 87 octane regular, because the electronics detune the engine in order to make it possible without any damage. But that doesn't mean the car won't perform much better on 91 octane Premium. (There are still some exceptions with high performance turbocharged V-8's that MUST use Premium).

Many of the owners manuals specifically tell you that. So yeah, you can run it on 87 and not hurt anything. But that doesn't mean your engine is going to perform at its full potential. It won't.

And what's the point of spending thousands more on a larger, high performance V-8, if you're going to choke it's performance, by trying to pinch pennies, driving it around with a tankful of 87? If there ever was such a thing as false economy, that's all but a perfect example.

And if you just can't bear the thought of spending the few extra dollars on Premium at every fill up, then buy the standard V-6 and be done with it.

Premium being more refined makes no sense. There's nothing of the sort. Nor is there any difference in "purity" which makes no sense either. The octane rating, which is actually an anti-knock index in the US, is nothing more than a measure of the fuel's resistance to auto-ignition of the end gases. That's it. It doesn't represent purity, refinement, or other quality parameters. It doesn't change the burn rate, laminar flame speed, vaporization, air/fuel homogenization, air/fuel ratio, engine temp, piston/chamber deposits, etc... None of that.

Gasoline is a cocktail of many different feedstocks blended in different ratios to achieve a target octane rating or other property. Premium will tend to have more aromatic content as aromatics usually have a higher octane rating. Increasing that concentration 5-10% and adjusting some lower octane (but still high BP) alkenes, alkenes, and olefins to balance out the formula gives the higher final octane rating.

Those feedstocks change almost every batch as well. Pump gas is made with the cheapest stocks they can get at the time and batches can vary wildly to meet a price point.

Sadly, what's written in magazine articles and even Sunoco's "tech" page isn't written by engineers or blenders. It's written by marketing desk jockeys, phrasing stuff passed to them in their own over simplified interpretation of a very complex subject. They get stuff wrong more often than right when it comes to fuels. (and lubes)

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