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[QUOTE="jaguar, post: 1469319, member: 19340"] [B]There are two ways to determine fuel/oil ratio:[/B] 1) for race engines that you want the best power from them you just use a lot of oil, close to 32:1 (of any type oil with a viscosity of at least 2 @ 200*C). You also want this ratio for air cooled cylinders to prevent piston noise. 2) for water cooled engines and you are not concerned with absolute best power and are on a budget and just want adequate protection, then you can use my oil ratio calculator to know how high you can go on fuel/oil ratio and still be protected. My calculator bases the ratio of the oils viscosity which is the primary way an oil protects engines parts. From 65cc to 250cc Honda recommends 32;1 of their full synthetic HP2 oil, and Yamaha recommends 30:1 of their mineral non-synthetic Yamalube 2R oil. My calculator says Yamalube at 30:1 is good for the tiddlers up to 125cc (reving to 12K RPM) and 34:1 is good for the 250cc reving to 9500. So Yamaha is doing both- recommending lots of oil for good ring sealing (good power) and recommending according to the oils viscosity. Honda is in over-kill with a high viscosity full synthetic at 32;1 and they could just as well recommend using about 4 other oils that according to their viscosity would better match that ratio. But the HP2 oil is cheaper than they are so the temptation is to stick with it. But consider this- when you use more than what is needed of a high viscosity oil for decent protection then at low and mid range RPM you have more power loss due to viscous friction. So for owners of water cooled engines the question presents itself if we want to spend more money for best power or use just enough oil for decent protection if we aren't racing. The method that my calculator uses is that for oils not already plugged into it the user goes to [URL='http://www.jiskoot.com/services/calculations/viscosity-temp']this site[/URL] and types in the engine oils 40C and 100C viscosities to find out the viscosities at 150C and 200C and then enters those into the spreadsheet. Then by the max RPM and cylinder cooling type the calculator determines the max upper cylinder temperature and the consequent viscosity of the engine oil at that temp (since the viscosity lessens with more heat). Then it determines the fuel/oil ratio needed to have a certain mixed viscosity (diluted by gasoline) on the upper cylinder for ring protection. The idea is that ring protection at the elevated temps there requires the most viscosity of all the engine parts. This method is not far off from what some manufacturers are recommending for the needed fuel/oil ratios. It's a bit "off" only because it's more specific than what they recommend. So in reality the manufactirer recommendations are a bit off because they are too generic. My calculator gives a baseline ratio that can be changed for slightly more oil but shouldn't be changed for less oil to be mixed with gas, especially for ashless and FD rated oils used in competition engines because these oils have little to no ash producing additive which is there as a dry lubricant backup when engine stress is extreme. [/QUOTE]
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MX, SX & Off-Road Discussions
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All About Engine Oils
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