Using Low-Octane Gas in High Compression Engine

Discussion in 'The Champagne Room' started by SRT-Tom, May 11, 2019.

  1. SRT-Tom

    SRT-Tom Full Access Member Article Writer

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    I Have read that a few owners of high compression Hemi engines may be tempted to save some dollars by buying 87 octane gasoline, instead of 91 octane. This is a big mistake!

    This is important. Emissions are lower and fuel economy is better. If you switch to 87 while the initial cost is lower you will find the engine burns more 87 than it would under the same operating conditions as with 91. The difference will have you buying more 87 and often enough to wipe out the initial lower price of the 87 octane gasoline.

    There's more bad news. With a lower grade of octane the engine controller will have to dial back the timing to avoid detonation. This lowers the pressure in the combustion chamber which eliminates detonation but raises exhaust gas temperature. The mixture starts burning later in the power stroke and stops burning later. Thus subjects all components exposed to the exhaust gas to higher operating temperatures. This includes the exhaust valves, O2 sensors, converters. All very expensive items to replace if they go bad.

    You think because if you drive the car "easy" you can avoid detonation and the ignition timing retard? Nope.

    Detonation is not a WOT only problem entirely. At part throttle at relatively low rpms and in a higher gear -which is with the automatic transmission equipped cars is how the transmission shift map works- cylinder filling is quite good. As a result combustion pressure can be quite high. So driving around town easy with 87 octane gasoline and thinking one is doing no harm is just the opposite of what is going on.

    High compression is a very good thing. With 91 octane gasoline the engine controller can run more ignition advance which starts the combustion process at the more optimum time and thus obtain more mechanical energy from the chemical energy of the burning gasoline/air mixture.

    The power it takes to compress the air/fuel mixture is almost all regained by the initial push back of this against the piston. If the spark can be triggered at the right time the mixture burns and develops pressure earlier in the power stroke which pushes down on the piston at the most optimum time. Running the correct octane grade allows the engine controller derive maximum benefit from the high compression.
     
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  2. IntimidatorRT

    IntimidatorRT Well-Known Member Staff Member Super Moderator

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    well in my 2009 RT I run the 91 Octane tune from diablosport trinity. but run 93Oct gas. Been on the Dyno and thats was the best power output for it, as far as my ram hemi is concerned, no tuner, just stock and run 87oct.
     
  3. ZEN357

    ZEN357 Full Access Member

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    I always run 93 in my R/T!
     
  4. SRT-Tom

    SRT-Tom Full Access Member Article Writer

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    In NJ, a top tier discount gasoline is Valero. Its premium grade is 93 and I use it in my SRT.

    [​IMG]
     
  5. ZEN357

    ZEN357 Full Access Member

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    I remember when we could get 94 from a local Sunoco here but that's long gone.
     
  6. SRT-Tom

    SRT-Tom Full Access Member Article Writer

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    Back in the 60s and early 70s, you could buy Sunoco 260. It had 106 octane, however, the octane rating formulas were different then.
     
  7. IntimidatorRT

    IntimidatorRT Well-Known Member Staff Member Super Moderator

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    we have Valero down here in Louisiana, but only one Sonoco that i know of and that one is in Lake Charles. 50 miles from me. I use the local Murphy's gas station. I believe that is USA from what I have been reading about it. And have not had any problems from running it.
     
  8. Wizard of Iz

    Wizard of Iz Full Access Member

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    I run 93 octane from either Mobil, Shell, or BP in my SRT and 89 octane in our Durango R/T.

    Pay the extra pennies and stick with a Top Tier brand. There really is a difference. Just ask anyone who has data logged.


    Sent from my iPad using Tapatalk
     
  9. SRT-Tom

    SRT-Tom Full Access Member Article Writer

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    Another advantage of using top tier gasoline are the fuel additives. They help to keep your engine clean.
     
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  10. ZEN357

    ZEN357 Full Access Member

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    Our Sunoco now only carries up to 93 Octane.