In the United States at least there are 3 grades of unleaded gasoline at most pumps. Does it really matter what grade of gas you use? Can I use the lowest grade one week and the next week get premium if I can afford it? Does it help with milage or does it keep your engine clean? What is the difference?
In: Engineering
follow up question to this is: does the gas brand (arco, shell, 76, chevron, costco, etc) matter for you car?
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Premium gas denotes higher octane rating. The higher the octane – the higher compression (how much pressure it can take) it can handle.
Engines work by mixing gas and air, and igniting the mixture. The resulting explosion (fire) pushes a piston which then turns a crank… let’s just say moves the car.
**Allegory** – Imagine two springs: one is soft and one is stiff. The softer spring you can push down and release and it might jump off the ground a few inches. The stiffer spring will be harder to push down but once you release, would probably jump off the ground quite a bit higher.
The principle is the same for engines: lower octane gas can’t be compressed as hard and thus don’t create as much power. Higher octane gas can be compressed harder and the resultant explosion (release) is more powerful.
Higher octane gas can be compressed more and the resulting explosion produces more power (aka horsepower).
**Advanced explainer:** to take advantage of higher octane gas you generally need to do something called “Forced induction”. Forced induction is a fancy way of saying “force more air” into the cylinder (where gas and air mixes). Because the higher octane gas can be compressed more, it also needs more air in order to fuel the bigger explosion. The only way to get that much air and quickly enough into the cylinder is through forced induction (turbo/supercharger).
The grades of gas have a rating called an octane rating. In very VERY simple terms, this means how hard it is to make that specific gas combust. Higher octane rating means the harder (and subsequently, more controllable) it is to combust.
Premium gas (high octane gas) is harder to combust, which is good when you need gasoline to combust at a very specific moment (like for sports cars). Lower tier gas is cheaper but is easier to combust, which is fine if your car is a simple commute to work vehicle.
Ive seen it thrown around from time to time that lower tier (than the manufacturer recommends) gas, since it can combust earlier than needed (due to its lower octane rating), could damage a car’s engine due to combusting too early. I cant confirm that, but its food for thought.
Speaking SOLELY from my case, Its generally not a good idea to run a vehicle with a grade of gas lower than the manufacturer recommends. My vehicle (2003 dodge ram 1500) will die after running with lower than premium grade gas after about ten minutes. Hope this helps!
The quality of fuel has nothing to do with the energy contained in the fuel (it’s all essentially the same), but on your engine’s ability to extract that energy without unwanted and possibly destructive side effects. Higher-performance engines need fuel that can put up with higher compression ratios.
In terms of what you should use; use whatever fuel your manufacturer recommends. If the car is getting older and knocking more often, consider moving up to the next highest grade of fuel. In addition, if you throw an aftermarket turbocharger on, you’ll probably also have to move up a grade or two.
Use the gasoline recommended by the manufacturer. If you do not have a special high-performance car you would use the lowest grade.
There is no advantage to use a higher grade. You need to design the engine to have higher compression for it to be useful and then lower grad will work badly in the engine. The octane rating of the fuel is about how much you can compress it before is spontaneously detonate. So use the lowest grade the manufacturer recommend because it leaves more money in your pocket.
Follow up question. When would you ever use mid grade? Most cars use regular, some higher performance engines use premium. Is mid grade just a money maker for the oil companies from the people who think higher octane gas cleaned their engine or gets their non high performance engine better mpg?
Higher octane gas can withstand more compression in the cylinder than lower octane gas. The higher the compression ratio, the higher octane you need to avoid premature combustion of the fuel/air mixture, also known as spark knocking.
A compression ratio is the measure of how much air is squeezed to the top of your cylinder during the Squeeze stroke of a 4 stroke engine: Suck, Squeeze, Bang, Blow. The ratio is measured by comparing the amount of air in an unpressurized cylinder, 1, to the amount of air in a pressurized cylinder, usually 8-12. Engines designed during the days of leaded gas require the highest octane at the pumps nowadays because the tetraethyl lead in the gas greatly improved the gasoline’s stability, allowing for engines to run compression as high 16:1. As an aside, adding a turbo or supercharger to your engine, or building something like a [Mini-me](http://www.crx.honda-perf.org/articles/moreminime/moreminime.html) increases compression and can sometimes require premium gas, racing fuel with an octane rating of 100+, or an octane booster additive to avoid spark knocking.
Aside from being required in higher compression engines, premium fuel usual contains lots of additives like detergents to clean carbon build up in your engine, oxygenates to allow cleaner burning, antioxidants to add stability to the fuel, and anti-knock additives, previously including lead compounds. Racing fuel companies tend to treat their blend recipe like Coke and their secret formula, and for good reason. Different blends offer different performance, and racing blends run above $40 a gallon. It even smells different than normal gas.
To expand upon octane ratings, a common misconception is that higher octane gas, the premium stuff, will increase performance. To an extent this is true, due to additives in the gas, but not due to octane. Octane is a measurement of how much compression the fuel can take before it ignites, and compression is generally a determining factor on how much power an engine makes. Higher octane gas does not affect compression, and therefore does not affect power output. Putting jet fuel in a Miata doesn’t change the fact that it’s still a 1.8L Miata.
On the coal rolling side of things, diesel has a cetane rating that tells you how fast the fuel combusts, rather than how much compression it can take. Compression in a diesel engine can average from 14:1 to 23:1; the heat from the extremely compressed air ignites the diesel and produces the Bang, rather than a spark from a sparkplug. Higher cetane ratings mean the diesel combusts faster; a higher cetane is better for faster diesel engines, like those in road cars.
Huge shoutout to [Donut Media](https://youtube.com/c/DonutMediaTV), they make learning about cars and the ins and outs of engine much more entertaining than your dad yelling at you about your flashlight position. Would highly recommend to anyone that wants to learn something new or just want something entertaining to watch.
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