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Old 05-04-2014, 08:22 AM   #11
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Originally Posted by fast murray View Post
I've been told there's no benefit to running higher octane gas than what the compression ratio of your engine dictates.
I don't think that this is so true anymore. In the quest for more power out of smaller displacement engines and better fuel economy, most modern engines have very high compression ratios. Ratios this high would have absolutely required high octane fuel to prevent detonation in the old days.

In today's world of variable cam timing and variable timing advance, one can use 87 octane in high compression engines. The ECM manages all of this with detonation sensors to get maximum performance for the fuel and load conditions.

So, there CAN be some benefit to higher octane fuels in many engines. This allows more advance; giving higher performance and economy. This is especially true in turbocharged or supercharged engines.

I've monitored the timing advance on my F150 with a ScanGaugeII. I can tell you that the ECM is constantly changing the timing. And when I tow, I can definitely tell the difference between 87 and 91 octane on the truck's ability to hold 6th gear on uphills. It runs fine on 87 and doesn't detonate. But there is a performance difference when running 91.

My turbocharged VW Jetta is definitely faster with 91 and gets about 10% better fuel economy. Why? On 87 it pulls a lot of timing to prevent detonation.
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Old 05-04-2014, 03:49 PM   #12
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I have run higher grades in my Tundra, it made no difference in economy and power. Same for our CRV. Se we continue to run 87 in everything. I have not experienced any improvements in consecutive tanks of the premium stuff.
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Old 05-04-2014, 05:37 PM   #13
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With today's electronic systems, the octane is dictated by the computer's setup (tune). If it's factory stock and the owners manual says 87, that's what to use. I have also seen worse mpg running higher octane.
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Old 05-04-2014, 05:49 PM   #14
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With today's electronic systems, the octane is dictated by the computer's setup (tune). If it's factory stock and the owners manual says 87, that's what to use. I have also seen worse mpg running higher octane.
Exactly.

People who run high octane fuel when the vehicle or tune does not ask for it are just throwing away money.
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Old 05-04-2014, 08:00 PM   #15
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Exactly.



People who run high octane fuel when the vehicle or tune does not ask for it are just throwing away money.

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Old 05-04-2014, 08:17 PM   #16
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US Gov site on octane.

CBC Marketplace test on fuel.

However adding ethanol in regular gas muddies the water and the engine ... Not many retailers use a mix of ethanol in regular gas here.
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Old 05-05-2014, 12:40 AM   #17
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Modern (since maybe 2000) engine computers sense whether there's any knocking on lower octane fuels and adjust ignition and injector parameters appropriately. I get about 8 mpg on my MH and it doesn't seem to matter what grade I use, so I get the cheapest.

On less than 5000 miles in a typical season, the difference in cost vs. fuel burned wouldn't be visible. If I was full-timing and driving 20K a year, I'd do a more detailed study. Most of the time we fill up at Costco, Safeway or tribal gas stations, since they're usually 5 cents or more chaeaper than the big names.
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Old 05-05-2014, 07:28 AM   #18
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Originally Posted by beerjay View Post
A CBC consumer show did an independent test of gas types last year and televised the results for various vehicles.

They used test gear for mileage and emissions as well as mechanical inspections. The high octane produced worse gas mileage and created more emissions than the regular gas. This is because the computers in the vehicle are set for a specific octane that is specified in the user manual. In some cases they noted worse performance on the road with the high octane since the computer was not set for it.

Some performance cars require higher octane but that will be specified in the user manual.

I think in the days of coils and mechanical carbs higher octane could be usefull.

Higher octane myths are fueled <g> by gas stations and gas companies since it costs the consumer a lot more.
This is false.

The computer adjusts according to "engine knock" to give the proper air fuel ratio and timing.
There "not set" to a pre determined octane ratio.
This is just silly.

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Old 05-05-2014, 07:29 AM   #19
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Originally Posted by acadianbob View Post
I don't think that this is so true anymore. In the quest for more power out of smaller displacement engines and better fuel economy, most modern engines have very high compression ratios. Ratios this high would have absolutely required high octane fuel to prevent detonation in the old days.

In today's world of variable cam timing and variable timing advance, one can use 87 octane in high compression engines. The ECM manages all of this with detonation sensors to get maximum performance for the fuel and load conditions.

So, there CAN be some benefit to higher octane fuels in many engines. This allows more advance; giving higher performance and economy. This is especially true in turbocharged or supercharged engines.

I've monitored the timing advance on my F150 with a ScanGaugeII. I can tell you that the ECM is constantly changing the timing. And when I tow, I can definitely tell the difference between 87 and 91 octane on the truck's ability to hold 6th gear on uphills. It runs fine on 87 and doesn't detonate. But there is a performance difference when running 91.

My turbocharged VW Jetta is definitely faster with 91 and gets about 10% better fuel economy. Why? On 87 it pulls a lot of timing to prevent detonation.
You are correct!


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Old 05-05-2014, 07:58 AM   #20
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Always has been lots of misinformation on octane. People have always assumed higher octane means more power when in reality the higher the octane number the SLOWER the fuel burns so that pre-ignition does not occur before the spark plug fires. (Ala a diesel ignition) a car designed for 87 using higher octane cannot fully burn the fuel/air charge resulting in raw fuel going out the exhaust valve. using higher octane fuel than required is a waste of money and will eventually kill your catalytic converter due to unburnt fuel constantly being fed to it.

Don't believe me, look it up,all the info is out there.
All the anecdotal stories in the ether mean nothing next to the hard science.

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