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post #9 of (permalink) Old 05-16-2019, 06:24 AM
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Join Date: May 2014
Location: Illinois
Posts: 3,841
Polaris recommends 87 octane ethanol free - 89 octane if using ethanol blended fuel

My Honda race bike calls for 93 or higher - I use VP Racing SEF (Small Engine Fuel) 94 octane - I have tried other octanes as high as 100 - what I found out (through experimentation) is my race bike started to lose power with fuel above 95 octane and at 100 octane the power loss was intolerable. Not only did the engine lose power, the exhaust ran exceedingly hot - the exhaust was so hot, the muffler at the rear of the bike melted (deformed) the plastic shroud shielding it.

Octane is a measurement - it measures (among other things) how fast a fuel burns. The higher the octane; the slower the burn rate. High octane fuels were intended for long stroke, high compression engines that were not particularly high revving. Higher octane fuels resist igniting due to compression. Ideally, the mixture ignites and the flame (pressure wave) pushes the piston to the end of the stroke and then extinguishes. With low octane fuels, the mixture ignites, burns rapidly and the flame goes out before the piston reaches the end of its stroke - if it burns too quickly, the pressure wave strikes the top of the piston (like a hammer hitting an anvil) and an audible click (referred to as ping) is heard. Continued operation with a low octane fuel (even if the ping is not noticeable) will damage the engine sometimes causing a broken piston. Too high of an octane results in the mixture burning

My CRF is a high 12.5:1 compression ratio engine, but has a short stroke. The engine readily revs to about 12,000 RPM and is most efficient between about 5000 to 8000 RPM. When I ran 100 octane fuel, the flame had not extinguished at the bottom of the stroke and the piston was pushing still burning fuel into the exhaust. Thus, a loss of power and the burning fuel heated my exhaust excessively.

If you think octane is a measurement of power - get some Turbo-Blue (typically 110 octane airplane gas), put it in your lawn mower and then go mow the grass. Depending on the brand of mower, it may not even start on Turbo-Blue, but if it does, and if it accelerates to the proper engine speed for mowing, it probably will not have enough power to mow and you will probably see the muffler get red hot and may even notice a flame coming out of the muffler. Dump it and put in 87 to 91 octane non-ethanol gas and it will return to normal. With a little experimentation, you may find the engine runs better and uses less fuel depending on the brand and quality of fuel.

Years ago I was addicted to Standard Oil premium for my 65 Mustang. Standard became Amoco and Amoco became British Petroleum. I was loyal through all the name changes, but recently noticed a decline in fuel economy. Casey's General Store built a new facility in our town and they had one pump with 91 octane non-ethanol fuel. I always tried to seek out non-ethanol fuel, but it was inconvenient to drive 20 miles each time I needed fuel. So I used ethanol fuel 2 octane points higher than the vehicle manufacturer recommended when I had to and would fill with non-ethanol whenever I was in a neighborhood that had a non-ethanol outlet. I not only filled my vehicle, I would fill all the gas cans I own to tide me over until my next trip.

Nuff said - run minimum 87, but 89, 91 or 93 octane pure gas is allowable, 89 or higher 10% ethanol, 95 or higher 15% ethanol and nothing with more than 15% ethanol unless the vehicle is specifically designed to use +15% or greater concentrations of ethanol. To use fuel above 15% ethanol, the fuel tank must be steel or special plastic, the petcock and fuel pump must be constructed without rubber parts and the carburetor or fuel injectors have to be specially constructed to provide an acceptable life span before requiring routine replacement.

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