Higher Octane Fuel – Is it worth it?

If you have an electric car, this post is not for you.   We will talk about how to effectively benefit from solar glare in another post.

For the rest of you, 0ne biggest questions faced on the road today, with the ever-growing rising fuel prices, is picking out the right fuel for your car.  We we decide in pulling up to any particular gas station, we are given with at least three choices of fuel grades.    Each significantly different, and of course with different prices.  The decision is that nowadays, you will find an almost a 2% percent difference in prices between REGULAR, MID GRADE, and HIGH TEST.

Differences between brand name gas (Shell, Mobil, Arco) and cheap gas (Alaska, Rotten Robbies, Costco) will also reflect the price of your full tank.  We also see differences of prices between stations is competing with other stations at a regular intersection, or if the station that might be outside of town forty miles of another station.

We all want to know if using higher octane improves miles per gallon, provide more power and better for your car?  In order to answer these questions you need to understand what and how fuel is defined.

All fuels have an Octane rating that measures proportions of iso-octane to heptane in a fuel.   That rated proportion relates to how much energy it takes to ignite that fuel.

It is important to recognize that the higher that octane number, the more energy it takes to ignite that measurement of fuel.

Chemical engineers constantly run tests on developing a proportion iso-octane to heptane levels to best obtain an octane rating.

High Octane does not give your car better gas mileage. This is because that octane levels is in proportion on how much energy it takes to ignite, NOT the energy the fuel produces.   Unless your car is explicitly designed to run on high octane gas, it will not give your car better gas mileage.   At the same time, using High Octane will not improve horsepower either.

It is important to recognize that it is fairly rare to find cars the require use of high octane fuel!  Note too that Higher Octane will not magically clean the insides of your engine!  It will not clean out deposits or fix “piston rings”.

If your car happens to requires high octane gas, such as a 2011 Porsche 911 Carrera Speedster (check your manual) then and only then you will want your car to run high octane fuel.

The main reason is that octane ratings has everything to do with the characteristics of compression and ignition timing of the engine.

You will notice some engines to some cars will have an 11:1 ratio vs other cars that have compression/ignition timing ratios of 6.5:1.   Knowing this compression/ignition timing ratio, will tell you which kind of fuel your car will need to run efficiently!

For example, the 6.5:1 compression/timing ratio found in a Porsche 356 Roadster will mean that it can ignite fuel very much differently than what might be found in a compression/timing ratio of 11.7:1 found in a Porsche 911 GT2/GT3.   This relationship between compression and ignition timing is your assurance that fuel will ignite with a specific needed amount.   You don’t want the wrong fuel octane level, because of course, ignite will be wrong and the car will run crappy.

What is interesting is that the compression/timing ration relationship has very little to do with the fact the car might be a track car such as the Porsche 935 Coupe that only has a compression/timing ratio of 6.5:1.  Does that mean the owners of a 935 will probably be looking for 86 octane and it will run happiest being leaded?   No, this is because he has additional adjustments in his Engine Control Unit (ECU) or Engine Control Module (ECM) that handles fuel, air and ignition timings electronically.   What about your street car?  If it had the same compression/timing ratio of 6.5:1 like the Porsche 935 Coupe, does that mean you will look for 86 octane.   Maybe.. you need to check your owners manual.   There is specifies what is the stock setting of your automobile.   If you happen to have your car chipped or that the ECU has been modified to meet a different compression / intake ration, then you will need to consult with those adjustments.   Some ECUs will know what your fuel might be igniting at and recalculate spark, air, timing, etc.    Knowing this key aspect of fuel is a great way to not spend more on gas than you need too for your tank.  You will not being doing your car any favors by purchasing better gasoline than it needs.

Back in the early 80’s there was this huge campaign regarding engine “pinging”.   Pinging, a more technical term than knocking was a sign that gas was not igniting when it should.   If you can imagine, a chamber (inside the engine) about to connect air-gas-ignite to push a piston, missing out on and not having that relationship of compression and ignition. That chamber fails to perform a duty efficiently and hurts the chamber, not giving you a better ride.

You will notice a reduction in power and your gas mileage will be horribly low.   Using a higher octane will then reduce that pinging noise.   If you are running, say that 356 and the manual says nothing about the car’s ability to use a higher octane, something needs to be adjusted such as timing and spark plugs.  Do not assume you can fix the problem with the simply using higher juice.    This goes back to the concept that relates energy needed to ignite the gas.   If the gas is igniting too easily, therefore igniting before it is suppose too, there will causes of engine knocking or pinging.   Of course then you will notice lower gas mileage and power zapping efficiency.

For your information… one might describe the pinging or knocking sounds a lot like shaking small rocks in a coke can.  Think of this knocking or pinging as detonation vs. ignition.  Think of knocking of an explosion and not a silent fizzle of a burn.   That detonation is rather harsh and you should well notice it.

You must look at your manual to know which gas is optimized for your car.  The car’s ECU/DMI or if carburetors; will completely dictate how the gas will interact with the listed spark plugs, cylinder wall chambers, air intake, exhaust, timing…all of that.  So back to the guy with the race car Porsche 935; if you remember his low compression/intake ratio (6.5:1), that he might have his car set up for a different ignite.   He is in a race car, which means he also has a 24/7 mechanic that tweeks everything related to maximizing power.  Air, electrical, fuel, spark.. all of that is focused on going fast (150mph fast); not sitting in traffic, a 20mph cruise around lake or barely getting above 70mph on the freeway.   Because he is in a race car, the car will need to have very different driving conditions that requires him possibly the highest in octane, as much as 117.     Even more on the extreme, the formula racers running alcohol or even funny cars that run on pure jet fuel would have settings that vary as well.

So take a look at your manual and determine what is the fuel you should be using, and don’t buy higher octane than what is required.  Doing so chances in damaging your engine, not making it better.

TURBO?

For turbos there is effective compression ratio (ECR) and static compression ratio (SCR).  Effective compression ratio changes with the air pressure from the turbo.  All  1976 – 1989 930 have low static compression (At least for 87-79 usa cars it is between 6.5:1 or 6.7:1, but effective compression is computed as: ECR= SCR * sq rt [1+ (boost/1)].  Given the primitive electronics were used in those cars, and not computerized its important to use high octane fuel.

Here are some compression ratios that might be helpful for you.  Look in your manual for exact octane needs.

1948 Porsche 356 – Compression Ratio = 6.5:1

1950 – 1954 Porsche 356 – Compression Ratio = 7.5:1

1956-1959 Porsche 356A 1600S – Compression Ratio = 8.5:1

1960-1963 Porsche 356B Super 90 Roadster – Compression Ratio = 9.0:1

1963-1964 Porsche 356C Carrera 2000 Coupe – Compression Ratio = 9.8:1

1964-1967 Porsche 911 Coupe / Targa – Compression Ratio = 9.0:1

1965-1968 Porsche 912 Coupe / Targa – Compression Ratio = 9.3:1

1972-1973 Porsche 911 Carrera RS 2.7 – Compression Ratio = 8.3:1

1976-1989 Porsche 930 – Compression Ration = 6.5:1 or 6.7:1

1979-1983 Porsche 928S – Compression Ratio = 10.0:1

1983-1989 Porsche 911 Carrera 3.2 = Compression Ratio – 10.3:1

1985-1991 Porsche 944 Turbo = Compression Ratio – 8.0:1

1987-1988 Porsche 959 = Compression Ratio 8.3:1

1989-1993 Porsche 911 Carrera 2 & 4 = Compression Ratio 11.3:1

1991 Porsche Turbo = Compression Ratio 7.0:1

1992-1995 Porsche 968 = Compression Ratio 11.0:1

1993-1997 Porsche 911 Carrera (993) = Compression Ratio 11.3:1

1996-current Porsche Boxster / Boxster S = Compression Ratio 11.0:1

1997-current Porsche 911 (996) = Compression Ratio 10.4:1

2001 Porsche Carrera 4S = Compression Ratio 11.3:1

1999-current Porsche 911 Turbo (996) = Compression Ratio 9.4:1

2003 – current Porsche 911   /GT3 = Compression Ratio 11.7:1

2002 – current Porsche Cayenne = Compression Ratio 9.5:1

2003 Porsche Carrera GT = Compression Ratio 12.0:1

________________________________________________________________________

More reading on this subject can be found on http://www.rennsportsystems.com/2a.html

~~~~~

All of the experts we spoke with say the most important thing for consumers is to choose one brand of gas and stick with it. Changing from brand to brand can lead to those deposit buildups that can cause problems for your car, including reduced gas mileage.

Paragon Laboratories

BP 87 Octane ... 17.2 milligrams per 100 milliliters
BP 93 Octane ... 26.4 milligrams per 100 milliliters

Citgo 87 Octane ... 6.0 milligrams per 100 milliliters
Citgo 93 Octane ... 9.4 milligrams per 100 milliliters 

Exxon 87 Octane ... 20.0 milligrams per 100 milliliters
Exxon 93 Octane ... 21.2 milligrams per 100 millilitersPilot 87 Octane ... 5.8 milligrams per 100 milliliters
Pilot 92 Octane ... 8.8 milligrams per 100 milliliters
Shell 87 Octane ... 16.2 milligrams per 100 milliliters
Shell 93 Octane... 31.0 milligrams per 100 milliliters
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11 responses to “Higher Octane Fuel – Is it worth it?

  1. Thanks a bunch for sharing this with all folks you really understand what you’re talking about! Bookmarked. Kindly additionally talk over with my website =). We will have a link trade contract between us!

    • As prices continue to soar, soar and all of us getting sour, I am only hoping the material I provide is useful so all of us an save a little bit more.

      Check those tires
      Know the fuel you need
      Avoid Idle standstills while your car burns gas. (turn it off if you think you are going to be more than a minute or so.
      Avoid traffic jams – use your smart phone to avoid traffic jams or go later
      Use gas when you have no other choices.

  2. You might want to mention that for turbos there is effective compression ratio (ECR) and static compression ratio (SCR). Effective compression ratio changes with the air pressure from the turbo. The chart leaves out the 76-89 930 which had low static compression (At least for 87-79 usa cars it is between 6.5:1 or 6.7:1, but effective compression is computed as: ECR= SCR * sq rt [1+ (boost/1)]. Given the primitive electronics (not computerized) you run a risk of detonation with lower octane gas. Can see in the chart that more modern cars can safely run high higher compression.

  3. Pingback: C250 Fuel Question? - Page 2 - FlyerTalk Forums

  4. I was unaware that the iso-octane / heptane ratios vary greatly from brand to brand. That may explain why the “check engine” indicator comes on for a couple days when 91 octane Shell (31 iso-octane ratio) is used in my Boxster S. This phenomenon does not happen with continued use of Shell, but does when changing brands.
    Has anyone experienced the same thing?

  5. I was unaware that the iso-octane / heptane ratios vary greatly from brand to brand. That may explain why the “check engine” indicator comes on for a couple days when 91 octane Shell (31 iso-octane ratio) is used in my Boxster S. This phenomenon does not happen with continued use of Shell, but does when changing brands.
    Has anyone experienced the same thing?

    • I would perform the following checks on your car… you need to know nothing serious is going on!!!!

      fuel is fuel.. Though I am suspicious of brands of gas vs other brands…your experience may also be coincidence…

      – check all grounds… if you have an older boxster its ALWAYS TIME to check all grounds.
      – check all connectors… get electronic cleaning material to assure good contact. (its a spray)
      – check wire harness and plug wires… they will at times trigger that check engine light. (easy way might be to turn off lights run engine, and make sure you are not getting any kinds of arcing or electrical zaps)
      – check all plugs.. when was the last time you had a tune? pull out plugs..compare plug burn rate.
      – check tank.. make sure no foreign materials are in your tank.. (ie.. no water..no oil… )
      – check oil.. i mean..check the content in your oil.. make sure its clean. free of fragments or shards.. search “ims failure”
      – check fuel filters… are they new..clogged..check them.

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