Octane - Really No Mystery.

Discussion in 'SRT Hellcat General Discussions' started by ChallengerDad, Feb 28, 2015.

  1. ChallengerDad

    ChallengerDad SRT Hellcat Moderator Staff Member Gold Supporting Member

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    How many times have you heard someone say "getting the good stuff this time", or "going to give my baby a treat"? Then they waste money on high-octane gasoline that's not the best for their car.
    Octane is a "combustibility" rating for gasoline. Nothing more. Using high octane gas in a car that doesn't require it only empties your pockets quicker.

    Diesels fire on combustion pressure. The only "spark" they need is to get the combustion cycle started. So to get a diesel running, you need to use a "glow plug" to start the firing sequence. Once the initial dump of fuel is burning, the engine keeps going via combustion - squeezing the air-fuel mixture within the cylinders by the motion of the piston inside. When the fuel is sufficiently compressed, the fuel combusts. Therein, diesel fuel is "low octane" - it combusts under relatively low pressures. There are no "spark plugs" in a diesel engine. The compression of the air/fuel mixture causes the detonation.
    Move to gasoline.....

    Gas engines require a more refined fuel that must be "ignited" at the proper time in the cycle. High performance, high horsepower engines usually operate under very high pressures (the higher the compression ratio, the greater the combustion / energy released). They require a higher octane fuel.
    Regular car engines run at a much lower compression ratio since durability is an issue under extreme pressures.
    If the fuel is too low an octane, the fuel will diesel - combust before it reaches maximum pressure (and subsequently, the top of the compression stroke). The fuel is expanding while the engine is still trying to compress the air/fuel mixture. What happens there is the burning fuel is trying to push the piston away before it's reached the top of the compression stroke. Essentially, it tries to turn the engine backwards. NOT a good thing for power or engine longevity.
    To keep the engine from "dieseling", a higher octane fuel (one that can withstand more pressure without combusting from compression) has to be used and ignited with a spark at the top of the compression stroke - beginning of the power stroke.

    If your car requires 87 octane, run 87 octane. There's no benefit to running 89 or 91 octane since the engine doesn't develop the extra pressure required to combust it before the spark plug fires and ignites the mixture.
    If you want "better gas" for your car, buy yours at a "brand name" station such as Mobil, Hess, Citgo, Shell, etc. The "ExtraMart" gas is going to be whatever is cheapest for them to get and resell.
    But all you're doing by burning 93 octane in a car that requires 87 octane is burning your cash faster than necessary.
     
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  2. Carnage

    Carnage SRT Hellcat Supercharged Moderator Staff Member Hellcat Car Club Gold Supporting Member HCC Charter Member

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    Thanks CD very informative! I know the manual for the hellcat says octane 91 or higher and the supercharger is involved it makes a difference somewhat? I'm not a guru haha
     
  3. ChallengerDad

    ChallengerDad SRT Hellcat Moderator Staff Member Gold Supporting Member

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    Yes. The supercharger (or a turbocharger) pre-pressurizes the air/fuel mixture before it's compressed again inside the cylinders. That's why higher octane than regular gas is necessary.
    I was very surprised the Hellcat didn't require 93 octane, but they've made it work on 91. Cheaper, so I won't complain!
     
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  4. Carnage

    Carnage SRT Hellcat Supercharged Moderator Staff Member Hellcat Car Club Gold Supporting Member HCC Charter Member

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    Haha yup 93 is what I use so I'm not complaining either. Thanks CD
     
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  5. RadCat

    RadCat Left Shark

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    Yep. I've said this same thing on here at least once. Octane isn't a power maker, its a detonation preventer. yea, yea I know 'preventer' isn't actually a word...but this is the interwebs, so now maybe it is!

    The only way it can be considered to give/add power is if you live somewhere that you can't buy adequate octane fuel for whatever you drive to stave off detonation, and its computer controlled, with a knock sensor...it will cut timing and therefore power. In this case its not actually giving/adding power, its just getting you back to where you should be. On the other hand, same engine, with increased cylinder pressure from higher compression pistons, boost from a supercharger or turbo(s) (which along with the extra air , requires more fuel...and that = more power), will require more octane to operate at peak performance without detonation. Otherwise just like above, if it detects detonation it will pull timing and cut power.
     
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  6. Blackdevil77

    Blackdevil77 Silver Member

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    ChallengerDad gets it! No need to use a higher octane than required as it will have no added benefit if the engine isn't tuned/set up for the higher octane. That's why I call BS on the Challenger Hellcat that ran 10.4 at 133mph that only had the addition of some high octane race fuel.

    It doesn't hurt to add a little 100 octane to your tank if your gonna be driving the car hard. Can't always trust what it says at the pump, mixing a little higher octane fuel will just ensure that your getting at least the minimum octane required for your engine which isn't a bad idea if you know your gonna be driving your car hard.
     
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  7. Tannehill

    Tannehill Active Hellcat Member

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    Manufacturers don't build to 93 because it's not readily available throughout the U.S. and territories their cars are sold in.
     
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  8. ChallengerDad

    ChallengerDad SRT Hellcat Moderator Staff Member Gold Supporting Member

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    93 octane is always available where I live and work. My '09 SRT8 required a minimum 91 octane, but had more power on 93, and since it's available, it's always been run in my '09. The knock sensors have never been tickled on my Challenger.
    Most drag strips will have a vendor with race fuel available. Don't be surprised by the price, though. Most of it runs in excess of $8.00/gallon. That's available for the builders running 14:1 compression ratios in their race motors.
     
  9. ChallengerDad

    ChallengerDad SRT Hellcat Moderator Staff Member Gold Supporting Member

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    I should add that most "mid-grade" gasoline here is 89 or 90 octane. A few stations offer 91, but as stated above, I've made sure it's getting the "minimum" by going up a notch when in doubt.
    I also buy "ethanol-free" whenever / wherever it's available. That crap will shorten the life of your engine - and reduce your fuel mileage.
     
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  10. Catless

    Catless Silver Member

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    If you datalog under various conditions you learn a lot about what fuel/octane is the best for your particular vehicle. Just saying that 87, 89, etc is what the manufacturer states the vehicle requires does not tell you what gives the best performance. Actual dyno testing would be even better. My 09 stock Challenger SRT pulls a lot of timing running 91 octane. Shell 93 octane was worse than other brands 91 octane.
     
  11. B5 Blue

    B5 Blue Platinum Member

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    And to prove it put 93 in your lawnmower. If it even runs it runs like shizzy as 93 burns slower
     
  12. Catless

    Catless Silver Member

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    I have not found that to be the case but what I did find is how much better the small engines run on pure gas. This is especially true of 2 cycles (weed whacker) and really noticeable in hot weather. The pure gas here is stated to be 90 octane.
     
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  13. B5 Blue

    B5 Blue Platinum Member

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    Never tried a 2 cycle just a 4 and it ran like crap
     
  14. hellcat1

    hellcat1 Gold Member

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    Not many places sell pure gas,, mostly only the marinas, so your saying shell gas sucks,, because they have v power,, I only put 93 in...I will go further to get better gas
     
  15. B5 Blue

    B5 Blue Platinum Member

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    I run Shell no pure gas stations by me at all.
     
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