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Octane question

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Old 06-14-2004, 01:45 PM
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Question Octane question

Will too high of an octane hurt an engine?? - I'm only talking gas station stuff, not race fuel or anything. I was told that higher octane fuel actually burns cooler than low octane, thus, better for engines.
I always put at least mid grade stuff in my boat if not "Super". Everbody says I'm waisting my money.

The engine in question is an older Merc. 40hp outboard. What grade fuel would you use??

Sharkey??

Thanks,
Casey
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Old 06-14-2004, 01:50 PM
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Default Re: Octane question

How old? If it's before 78 it should have an unleaded kit.

You'll be OK with 89 octain.

Funny, the new merc 525 can only be run on 87.
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Old 06-15-2004, 10:34 AM
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Default Re: Octane question

It is a '74 Merc.

Unleaded kit??? Is that a lead additive or a hardware add on?
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Old 06-15-2004, 12:00 PM
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Default Re: Octane question

higher octane is more stable, so it won't predetonate under higher compression. If you have a low comp. engine the higher octane is not giving you any benefits.
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Old 06-15-2004, 12:14 PM
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Default Re: Octane question

Higher octane gas burns slower and will cost you HP if the motor doesn't need it for detonation protection. Dyno shows a 2-3% loss running 103 -vs- 85, but we have to match the host fuel requirements. They do run cooler on higher octane.

-Greg
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Old 06-15-2004, 01:56 PM
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Default Re: Octane question

Good information.

Thanks guys!

Casey
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Old 06-15-2004, 07:27 PM
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Default Re: Octane question

Take it for what you want we have 1998? 90hp outboard on the Poontoon, it runs like chit on 87, cuts out and stuff, we run 89 or higher in it and runs great. As for 500efi runs the same on 87 as 93, I can't tell a change?

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Old 06-15-2004, 07:32 PM
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Arrow Re: Octane question

Origin of the 'Higher Octane is Better' Concept
Higher octane gasoline did reduce engine knock in older engines that used carburetors to regulate the air/gas mix. The older engines could not regulate the air/fuel mix going into the engine as efficiently as a computerized fuel injector. A carburetor in need of adjustment could cause too much fuel to be mixed with the air, which meant the gasoline would not burn completely. The excess gas soaked into carbon deposits and caused a premature ignition of the gasoline from the heat of the engine cylinder. The premature ignition made a sound that came to be known as 'engine knock.' When this happened, people would change to the higher octane/slower burning gasoline to resist the premature burn, thus minimizing the knock. Upping the octane was beneficial then, but engines and gasoline formulations changed.

Since the mid-1980s engines use fuel injectors with computers to accurately control the air/fuel mix over all temperature and environment ranges. The accuracy of the fuel injectors and computers is based on using the recommended gasoline for that engine. Most cars are designed to burn regular unleaded gas with an octane rating of 87. If the vehicle needs a higher octane rating this requirement is noted in the owner’s manual and usually under the fuel gauge and by the gas tank.

Some gas companies are better at quality control. The first thing to do if you suspect a knock at a specified octane is switch to a different brand.

Last edited by Hydrocruiser; 06-15-2004 at 07:45 PM.
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