Re: what grade gas to get?
Jack, et al. - I DON'T use higher octane gas in my engine. Despite what Almeja maintains, higher octane is not of any real benefit at higher altitudes - it's more a function of lower humidity that calls for higher octane - that's why you find gas ranging from 87-93 octane at low-alt gas stations and only 85-91 at high-alt stations. Besides, properly timed, higher octane gas only gives a performance advantage in motors equipped with knock sensors.<br /><br />Here's a blurb from a major petroleum company, appropriately edited for us boaters:<br /><br />Your boat's octane requirements are mainly determined by its basic design. In addition, variations in engines due to manufacturing tolerances can cause motors of the same model to require a different octane of several numbers. Also, as a new boat is driven, its octane requirement can increase because of the buildup of combustion chamber deposits. This continues until a stable level is reached. The stabilized octane requirement may be higher than when the boat was new. Premium or midgrade fuel may be advisable to prevent knock.<br /><br />Other factors also influence your motor's knocking characteristics: <br /><br />Temperature - Generally, the hotter the ambient air and engine coolant, the greater the octane requirement.<br /><br />Altitude - The higher the altitude above sea level, the lower the octane requirement. Modern computer-controlled engines adjust spark timing and air-fuel ratio to compensate for changes in barometric pressure, and thus the effect of altitude on octane requirement is smaller in these vehicles.<br /><br />Humidity - The drier the air, the greater the octane requirement. The recommendations that vehicle manufacturers give are for normal- to low-humidity levels.<br /><br />Your engine's spark timing - The octane requirement increases as the spark timing is advanced. Both the basic setting of the spark timing and the operation of the automatic spark advance mechanisms are important in controlling knock. In some computer controlled engines, the spark timing can only be changed by replacing modules in the computer. If they are equipped with knock sensors, these computer controlled engines have the ability to retard the ignition temporarily when a sensor detects knock. This temporarily reduces the octane requirement and may also temporarily reduce vehicle performance.<br /><br />Method of operation- Rapid acceleration and heavy loading, such as pulling a skier or running against a current may result in a greater octane requirement. Stop-and-go operation and excessive idling can increase octane requirements by causing the buildup of combustion chamber deposits.<br /><br />Hope this helps - keep your timing in check and your mixture and idle speed adjusted correctly and you should avoid trouble with combustion chamber deposits (carbon). I hydrodecarbonize my motor regularly to prevent such buildup - I'll tell you how if you're interested.
<br /><br />And, Bandit - it's more important to make sure your mixture and timing are set properly than to fill your tank with high-octane gas. Marine motors are based on automotive and industrial motors, which do NOT routinely use premium fuel. The reason that premium fuel is advocated for marine use is that the typical boat sits idle for long periods between uses; octane degrades gradually over time such that the 93 RON unleaded you put in the tank three months ago is only worth about 89 now, and will be down to 87 or 85 in another month or two. Adding fuel stabilizer will slow down the octane reduction process, but it's a better idea to just put in what you're going to use and try to use the boat more often. It's more fun, anyway! 
