Understanding Octane – When it matters and when it doesn’t

91 octane….or 87 octane.   Hmm…..91….or 87……  We’ve all been here, standing at the pump with a few crumpled American dollars in your pocket and a car that you know is a high performance beast (under the right conditions of course). The owners manual in your 22 year old Volvo recommends 87, but you decide to drop the extra 10 cents per gallon and go with the 89.  It is after all, 2 more than 87, a job well done.  But really, what does this all mean in a practical sense?

Putting in the more expensive liquid definitely FEELS faster, because more expensive always means better, but does it?  Surprisingly few people understand what octane means in the real world, but it is a critical number in defining the way your engine runs.  TL:DR use the fuel octane outlined in your owners manual.

What Does Octane Realistically Mean?

Octane, in a real world sense pertaining to our internal combustion engines, basically describes the volatility of the fuel we are putting in.  This is where a lot of people get kind of hung up, as HIGH OCTANE is usually used by event marketers and toy manufacturers to describe something exciting and explosive.  In reality, higher octane fuel has slightly LESS usable energy, but much more importantly lower volatility.  I am going to skip the science portion of this because it is a little drawn out, but it includes lots of chemistry and terms like 2,2,4-trimethylpentane.  Basically the higher the octane, the lower the susceptibility is to exploding unprovoked.

I do want to quickly mention that there are a few ways octane is measured, so numbers may slightly vary. US fuel pumps state octane numbers using the (R+M)/2 method, also known as AKI or Anti Knock Index.  This is an average of both the RON (primarily used in Europe) and MON octane ratings, which use the results of a very controlled test featuring variable compression engines.

When does octane matter?

To describe when octane matters, you must first understand the two basic conditions inside the cylinder of an internal combustion engine.

  • Compression and compression ratio – All engines are rated with a specific compression ratio as you have probably seen.  The compression ratio is a main influence to determine what kind of fuel octane an engine will run at it’s best, and describes how compressed the air-fuel mixture is when the piston is at top dead center.  For instance, a Volkswagen ABA engine has a compression ratio around 10:1, meaning at full compression, the air fuel mixture is squeezed down to around a tenth of what it was when the piston was bottom dead center.
  • Heat – If you paid attention in the thermodynamics portion of physics class, you would know that as you compress a gas it heats up, and as it expands it cools down.  As the air/fuel mixture is compressed it gets HOT.  The more compressed it gets, the hotter it gets.
  • Elevation – If you are in an area that is multiple thousands of feet above sea level, you may notice lower octane gasoline is available.  This is because at higher elevations, the air is less dense, effectively decreasing your compression and cylinder pressures because there is literally less air in there.  In the mountains you can often safely experiment with dropping to a lower octane rating.

Detonation and pre-ignition

The knock sensor is directly bolted to the engine to ‘feel’ for knocking and pinging. This Mercedes Benz unit has two.

With a high compression ratio or a lot of boost pressure, you add a significant amount of heat to the cylinder.  Heat ignites fuel, this is where octane comes in.  If you try and put 87 octane in that S54 engine, the mixture will be so volatile that it will explode without being set off by the spark plugs.  The result is a sudden, hot, and very uneven detonation of the mixture.  If this happens well before the piston gets to the top of the cylinder, it’s called pre-ignition and it’s very bad.  It can be from a glowing hot spark plug, or some burning carbon (sometimes as a result of detonation).

Also called knocking or pinging, if the mixture explodes substantially before the piston hits TDC, the explosion violently and suddenly pushes back on the piston as it still travels upwards, robbing horsepower and potentially damaging engine components. Often pre-ignition can occur in a single cylinder.  In the worst cases, I have seen pre-ignition bend connecting rods, crack pistons, and damage crankshafts.

When computers started being installed in cars, knock sensors were used to inform the brain box that detonation was occurring, and react by retarding the timing.  This allowed the spark plug to take back control of the ignition, but firing the spark later robs horsepower and fuel mileage as the piston has already started its way on the downstroke.  More advanced sequential systems can control the timing of spark and fuel to actively adapt to whatever octane you put in.

This bent connecting rod is the result of a severe turbocharger overboost and the incredible compression and detonation it causes.

Naturally Aspirated Engines – In a normally aspirated or naturally aspirated (non turbo) engine, power is a function of compression ratio and displacement.  Sport and racing engine builders often raise the compression of an engine at the sacrifice of displacement to make more usable power, which is exactly what BMW did with their M54 engine when preparing it for the E46 M3.  To get more power, the compression ratio was raised from around 10.5:1 to a fairly high 11.5:1 ratio.  Ways to raise the compression ratio would be to make a flat or slightly domed piston top, flat valves, use a thinner headgasket, and/or shave the cylinder head, lengthen connecting rods, and use a different crankshaft to reduce the volume of the combustion chamber as much as possible.

Forced Induction Engines – Because a turbocharger or supercharger crams air into the combustion at high pressure, it acts in basically the same fashion as a high compression ratio and introduces the same problem.  Consequently, if you want to turbocharge an engine, you want a lower compression ratio.  You can lower the compression ratio by having a deeply dished piston, dished valves, thicker head gaskets, shorter connecting rods, and a different crankshaft.  If not, when you add boost you are effectively adding onto the compression ratio, and can run straight into knocking and detonation.  Supercharged top fuel drag race cars have a ridiculously low compression ratio around 6.5:1 to cope with the amount of air being forced into the combustion chamber.

These performance forged pistons are designed for turbo applications. The dished look is to reduce the compression ratio and accommodate higher boost


The Fix: Higher Octane and Colder Plugs

By putting in a 91 or 93 octane fuel, the less volatile mixture stays calm all the way though the cycle, allowing the ignition computer to dial in the perfect spark timing.  Generally, that involves firing the spark plug right before the piston reaches TDC, so the slower moving and evenly formed flame envelope pushes the piston down just as it begins its downward stroke.

Often, a colder plug is also added to reduce the likelihood of a spicey hot electrode from setting the mixture off before the spark plug actually ignites.  This is why a stock Saab 9-5 uses an NGK plug with a heat range of 6, but specifies a spark plug with a heat range of 7 for the “hard driving option”, assuming that your cylinder temperatures would be higher because of “hard driving”.

Race gas is generally around 100 octane or more, which allows significantly higher turbo pressure and/or compression ratios.  Our Audi RS 3 LMS is able to reliably make around 350hp out of the 2.0l TFSI turbocharged engine without knocking, due to the use of 100 octane VP racing fuel.

When doesn’t octane matter?

Using a higher octane fuel does NOT make any difference in engines that are tuned to use regular fuel.  That means in general, normally aspirated engines with a low compression ratio or smaller cylinders/pistons.  My Honda VFR800 motorcycle has nearly a 12:1 compression ratio and makes over 100hp out of 782cc, but because the pistons and cylinders are so small it only requires 87 octane fuel.  My grandmother used to only put 93 octane fuel in her Mercury Gran Marquis, but in reality all benefit realized by spending the extra money on a modular 4.6l was strictly placebo.

In fact, an 87 octane fuel actually has a slightly higher usable energy content, so you may even be making less power on an engine that’s expecting 87 by putting in 93.  The only time you would raise the octane is if you happen to encounter the knocking and pinging, such as in higher altitudes or extreme temperatures.  After all is said and done, always use the lower octane fuel when you can, otherwise you are just flushing money down the drain (or in reality sending it to our friends in the world’s oil producing nations).


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20 thoughts on “Understanding Octane – When it matters and when it doesn’t
  1. John S. Brookes

    I’ve had a number of cars that call for “high test”; but, have run just fine on “regular”, for example, a 2005 Volvo non-turbo S-60; and currently, a 2001 Lexus GS 300, and, a 1999 BMW 328i. Now, while I’ve always been led to believe the mid-range, 89 octane gas is a waste, I’ve also been led to believe that the 93 octane has more of the beneficial cleaners, such as Techron, etc. Given such, I began to fill my tanks with 1/2 regular, and 1/2 high test. Both current cars take right at 16 gal. when it’s time to refill. I pump the high test first, stopping at 8 gal.; then, I do the regular, which gives me a little more of the high test remaining in the hose. This mixture, I believe, gives me the benefit of some of the touted cleansers, and, an overall octane rating of 91, for about the same price as the mid-range 89 octane. Makes sense to me, and seems to work just fine.

  2. J. Tucker

    Very well. However, if one wants to get whatever benefits there might be from the engine scrubbing additives that are advertised offered, one must buy the higher octane gasoline to get the additives. Right?

    • Adam Goral

      I haven’t done the research on that so I can’t say for sure, but as I recall most gas stations have the same additives in all of their fuels, with the exception of perhaps Shell with their V-power. However, all of the Shell gasoline’s are advertised as nitrogen enriched. I suppose take a look next time you’re at the pump. Gasoline itself is a solvent, so I believe myself that the cleaning properties advertised for the higher octane fuels are more marketing than anything. I’ve never seen an intake valve come out of an engine, even ones that have been severely abused, looking like the ‘before’ picture on a lot of those advertisements, ever. Even engines that have been running horribly. Sorry for lack of concrete data, but in general the octane rating is the most important thing to pay attention to.

  3. John S. Brookes

    an upper intake cleaning, via a reputable repair shop, $100 +/- is well worth the investment. You will, essentially, start w/a clean intake system.

  4. Paul

    The Saab 9000 CD I had a few years ago (2.0L non turbo – horrendously under powered…) would ALWAYS run noticeably better on higher octane – the higher the better.

    However the ‘99 93 I drive now seems happiest on mid-grade. I put high octane in not so long ago to see if I’d get any noticeable power increase and all I got was an “check engine” light! (can’t remember the exact code, but it was something to do with the fuel system). I went back to mid-grade and no more codes…

    Marketing can lead people to believe that higher octane is better. I agree that it completely depends on the car…

    • Adam Goral

      I can’t think of a reason that using high octane would trigger a check engine light, maybe you didn’t tighten the gas cap enough? It is my experience as well that Mid-Grade is good enough for the newer Saab trionic systems to work with, however as soon as you do any sort of tune, it’s time for premium.

  5. Arnie

    Hi I have a 1995 Volvo 960 that I only drive in the Summer. Manuel states that I should use Hi Octane only and in Canada that’s 91 Octane. I have used 87 and did not find any difference. Can I use the lower octane gas?

    • Adam Goral

      Is this a turbo car? Sometimes you can get away with using the lower octane fuel if you are driving 65 on the highway, or otherwise not loading the engine. That keeps temps down and keeps the ignition from retarding. I wouldn’t, however put in 87 all the time, especially if it’s hot out.

      • Jeffrey Oswald

        The 1995 960 used a naturally aspirated 2.8 liter inline six-cylinder engine. It has computer-controlled timing advance using a single knock sensor, as well as crank, cam and throttle position sensors. The engine was set up (optimized, programmed, pick your term) for 91 octane (based on the north american definition). Those electronics are able to compensate if lower-octane fuel is used, however the engine will not perform as well.
        I believe it also bears mentioning that fuel formulas and mixing change throughout the year, especially up north here. I’m in New York (state, not city), and we don’t have Shell or Chevron or any “top tier” gasoline available except the occasional Valero location. Particularly in winter, the detergent additive levels in lower grades of fuel are not sufficient to maintain a european-designed engine which was optimized for 90-91 octane. Forced inclusion of ethanol does not help.
        This article should emphasize:
        1. Use fuel of the same octane rating recommended by the engineers who made your engine. They know what they were thinking.
        2. If you modify your engine, know all the effects of what you are doing. Part of this involves knowing how your engine works. Your car has been designed on a long list of assumptions. In order to satisfy all of those, most cars made in the past 30 years have flexible, codependent systems onboard. These systems can adapt on their own, but usually that will impact multiple other systems trying to run at the same time. If you are going to move into the realm of aftermarket equipment, you need to understand how the aftermarket part or program operates on a different set of criteria.
        (Insert thanks to eEuroparts for distinguishing between OEM, OES and pure aftermarket parts!)
        3. Blame a poor-running engine on poor or incorrect fuel last, after all other options have been eliminated. Fuel usually takes the fall for other issues like weak spark, intake leaks, a less-than-productive fuel pump or relay, or loss of compression.

    • Patrick H

      Actually, in Canada, the highest Octane gasoline is 94, available at Petro Canada. It used to be Sunoco’s gasoline East of Manitoba. It is now Suncor’s gasoline throughout the country. I used it in my Saab 9000 Aero and it did make a difference in terms of power. Never had any knocking either. And Saab were recommending 93 Octane.

  6. Mike Riecken

    I operated an automotive repair shop for 25+ years and have a friend who works in the octane testing lab at BP in Whiting Indiana.
    We always used to recommend Premium Fuel to our customers NOT for the octane rating but the higher detergent package that it comes with. We would see many customers that used “regular” fuel for 60 to 80K miles come in with drivability issues due to dirty fuel injectors and/or gunk buildup on the throttle plate, valves, etc. Sometimes cleaning would resolve the problem and sometimes they would need injectors replaced ($$$). We had countless times a customer would call for an appointment and we would suggest putting in Amoco Premium (if their tank allowed) prior to their scheduled date & let us know if things changed. Many times they would call to cancel saying within that tank the car started running just fine. This happened quite a lot, not just a few random events. We had nothing to gain and would loose the opportunity for a repair but gained our customers trust. I am sure people will challenge my statements but I have proof, time and time again, and many loyal customers, that swear by this as they experienced the results first hand!

  7. Arnie Jantz

    I have a 1995 Volvo 960 ( Not with Turbo) and my owners manel states tat I should be using Hi Octane which in Canada means that I use 91 octane. I have used 89 Octane and did not find any difference in performance and mileage. Am I in danger of damaging the engine in any way if I use the lower octane.
    Thanks for the reply

  8. Sal Tagliamonte

    I ordered an ignition cylinder switch for a Volvo, but before I ordered the part I spoke to a gentleman from customer service I wanted to confirm if the part came coded or after purchase would I have to go to Volvo for additional service he took my number said Ill confirm and call you back, he called me within 15 minutes with an answer, I paid 290.00 with overnight shipping included, the dealer wanted 375.00 plus 160.00 to have the cylinder coded. Excellent customer service and company they saved me over 250.00 dollars for the same part from the dealer. I highly recommend this company

  9. KyleHobie16

    Can you speak to the affects of altitude on the octane equation? I live at 5,800 ft above MSL and all my cars call for 87 or 91 octane. I’ve never put anything higher than 85 in them and never had pinging in any of them. One of the cars that calls for 91 octane has well over 150,000 miles on it. I believe that ‘s because the air is thinner at this altitude and consequently, the compression of less air nullifies the need for higher octane. Would you agree?

    • Adam Goral

      Hi Kyle, absolutely. Because the air is less dense, the cylinder pressures are slightly lower. Low enough that you can step down in Octane and not have to deal with power robbing detonation. I will mention this in an edit, thanks for the comment!

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