Go High or Stay Low? How Octane Ratings Affect Your Vehicle
There used to be a TV ad showing sexy sports car tear through traffic before pulling up at a gas station where the lovely lady driver rolls down her window to ask for…
You guessed it: some fancy gasoline with an octane rating of 100.
Unless you’re driving that same car, don’t be fooled by the Jedi mind trick and start sweet-talking yourself into getting a couple of liters thinking that by using the same fuel, your old jalopy will look and drive just as good as the car in the commercial.
Let me save you the suspense. It won’t.
Aside from the allure of speed and performance, oil companies have also long touted the engine-cleansing properties and ability to increase fuel mileage of high-octane gasoline.
Well, we dug up information about octane and cetane (for diesel engines) so that the next time you top-up, you’ll finally know how what these numbers on the rating mean.
What is Reasearch Octane Number (RON)?
The numbers that come after the brand name and before the word Octane is known as Research Octane Number (RON), although we’re more familiar with it under the plain old moniker “octane rating.”
By definition, RON (or octane rating) is gasoline’s resistance to pre-ignition or detonation, otherwise known as “knocking.” That might sound like mechanic-speak already, but stay with me because we’ll break it down a little later.
While we generally gravitate towards larger numbers, when it comes to RON, bigger doesn’t really mean better. Nor does it mean more power.
Let’s say for argument’s sake that the RON is 95—that means it contains 95 percent isooctane (an organic compound that’s particularly resistant to igniting when exposed to heat and pressure) and only five percent n-heptane (a hydrocarbon highly prone to self-igniting).
In simpler terms, 95 Octane gasoline will combust at a much higher compression ration than say a 93 Octane.
Why it’s important to know what RON your engine needs to prevent knocking we’ll discuss a bit later on but don’t forget that RON does not specify in any way the amount of power the fuel offers or its fuel efficiency.
What if Debbie does diesel?
Diesel fuel is rated between 8-15 depending on the brand, which is very low by the way, but remember that this type of fuel is classified very differently.
Instead of RON, Cetane Number (CN) is used to measure the compression needed for diesel fuel to ignite and its combustion speed, known more commonly as “cetane rating.”
When you see 55 Cetane at the pump, that means your fuel has 55 percent cetane (the easily ignitable element) and 45 percent 1-methylnaphthalene or heptamethylnonane (the least ignitable element).
Similar to RON, the higher the number, the more expensive the fuel, but for diesel, the reaction to compression and heat compared to gasoline is the total opposite. The higher CN, the quicker it ignites.
As an example, diesel with a 55 Cetane rating will ignite faster and burn much more cleanly than, say, one with 50 Cetane.
Read your vehicle’s owner’s manual
It’s not really rocket science and you don’t need a PhD to compute for the required RON or CN for your engine. Just read the owner’s manual.
It might not really be the sexiest thing to do—flipping through 120 pages of words half of which most of us probably don’t understand—but if it’s just the RON or CN that you’re after, it’s usually found under Fuels or Gasoline/Diesel Specifications.
It’s not Fifty Shades of Grey or whatever the kids read these days but this little non-fiction piece will save you lots of money because it will tell you specifically the correct RON or CN required by your engine.
Anything more and you might as well just pull out a wad of cash and burn it yourself.
Now if all that “pre-ignition, detonation, and knocking” sounds alien to you, here’s the full 411 on what happens to fuel inside your engine’s cylinders.
In a gasoline engine, the piston inside the cylinder drops to allow it to fill up with air and fuel. As the piston moves up, the fuel mixes with air, gets compressed and is ignited by the spark plug once the piston reaches the top.
The ignition doesn’t cause an instantaneous, bomb-like explosion of the fuel-air mix. From the spark plug, the flame consumes the fuel-air mixture, creates pressure to push the piston down, which then moves the crankshaft, the transmission and then the entire vehicle. It just looks like its exploding because it happens really fast.
A “knock” or “ping” happens if the fuel-air mixture combusts during the compression motion of the piston or before the spark plug ignites. The term actually comes from the loud sound made by the piston rapidly going down before it fully completes the upward stroke in the cycle.
That kind of stress on the engine can either break it or destroy connecting rods and even crankshafts.
Unlike its gasoline counterparts, diesel engines don’t have spark plugs. It uses compression to heat air and ignite diesel fuel molecules in the chamber.
During the compression stroke or while the piston is on its way up, fuel injectors atomize the diesel fuel just as it is sent into the combustion chamber.
The first diesel particles to reach the chamber absorb the heat, vaporize and then ignite to produce the downward “power” stroke. This process happens in quick succession and by batches during the diesel injection phase.
This is “pre-ignition” or “knocking” and in a car with a diesel engine, that’s good. In fact, rapid pre-ignition sequences are what makes the familiar and distinct diesel clatter/knock.
In the case of diesel engines, if you put in a lower rated fuel, all you’ll get is less power, poorer mileage and black emissions. This is caused by incomplete combustion because the fuel’s ability to ignite is very low.
Benefits of gasoline with higher RON
If you’re driving an older vehicle and depending on the maintenance and care it has received throughout the years, carbon deposits or a defective exhaust-gas recirculation system may cause the cylinder to have higher than normal temperatures, which will cause it to pre-ignite the fuel-air mixture.
As we’ve learned above, pre-ignition in gasoline engines is bad.
To counter that, use gasoline with higher octane rating because it will resist combustion despite your old engine’s hi-temp cylinders, which will also prevent any “knocking.”
Aside from heat-resistant properties and to justify the price, fuel companies have given fuel with high-octane ratings a lot of hi-tech cleaning detergent additives that will remove the carbon deposits in the cylinders and help keep it clean. This will keep your engine operating efficiently during the vehicle’s lifespan.
The only time it may be practical for you to go higher octane is, if and when you experience “knocking.” In most cases, this will eliminate the symptoms, but if it persists, consult your trusted mechanic right away.
Are there benefits for diesel fuel with higher CN?
When it comes to diesel, it may seem that going with higher CN is a no-brainer, but let me make a counter-argument.
As we’ve learned above, diesel with high CN numbers will have shorter ignition delay times. But that doesn’t mean that all of the diesel pumped inside the chamber will burn upon contact with the hot compressed air and achieve total and complete combustion.
It still all depends on the heat content and chemistry of the diesel fuel.
The CN is never indicative of the diesel fuel’s ability to achieve complete combustion, improve mileage, and cleaner emissions.
To get the most bang for your buck and power for your pickup, use diesel with the CN recommended by your vehicle manufacturer as this number is based on your engine size, design, and manner of operation.
More power, cleaner emissions and better fuel mileage, but only in the right engine
As a general rule of thumb, put in only fuel with the recommended octane or cetane rating as putting in anything higher won’t make a tiny bit of difference in terms of performance and mileage.
The truth is, anything more is just an awful excuse to rack up an expensive credit card bill.
All of the above may seem like a lot of auto mumbo-jumbo so to put it plainly, refer to your vehicle’s owner’s manual before making up your mind on what fuel to use. If you lost the manual, Google the info.
If your vehicle is running fine, sans any knocking, but you’re still itching to “treat” your vehicle to high-rated fuel because money grows on trees in your neck of the woods, resist the urge to overdose.
It’s a waste of money and since your vehicle isn’t designed to digest high-rated fuel, you will end up emitting toxins from poorly combusted fuel, adding greenhouse gases, and basically killing the planet one liter at a time.