Most Merc dealer's are gonna tell you, "regular octane gas". I was told by a mercury rep at last years Cobo show that all I needed to run was regular in my 502. No Way. I can definatly tell that my motor runs ALOT smoother when I run Premium gas. Granted my motor is alot larger than the 350, but I think you will notice a difference too. I will run midgrade most the time, putting premium in every 2-3 tanks.
My recommendation to you would be to run no less than midgrade at all times. My last boat had a 205 V-6 and it ran much smoother on mid than reg. I think even the smaller motors are made to run higher octanes. Marine engines are definatly different than cars!
Hey guys, the better the Octane the better the motor is gonna run. Dont forget, boat engines are always under load. There is no downhill or coasting. So they need more kick. Sure they will "run" on the lower grade, but you will get better economy with better fuel. Its worth the extra $$. Big go fasts or little cruisers alike.
Assuming these are factory stock Mercruiser or OMC engines I'm going to disagree and suggest you run what the manufacturer recommends. I've got twin Mercruiser 454's and have run 87 octane street pump regular since 1996 and 450 hours later no fuel related issues.
Now to go against what I just said most on the water gas stations don't pump 87 octane. Only 89 and higher.
Agreed, put in the manufacturer recommended octane. The motors are tuned for a peticular Octane. You won't get any better performance or fuel economy on a higher then recommended octane. I run a 502 on 89 Octane and the boat runs great.
To get a lot more information about this, than you could ever wish for, search the technical section of OSO.
From what I've gathered over the years, its always better to run the cheapest gas possible! Premium gas will not make a normal (marine) engine run any better, but regular octane gas will make a high performance engine run worse. In fact, Mercruiser recommends a maximum octane rating of 89 for all their HP500 (500 horse power) engines. Back in the beginning of the Marine EFI's engines, the electronic control module (ECM) would set the spark timing based on how much "spark knock" the knock sensor would pick up. Well, a lot of people started filling these new fancy EFI engines up with 93+ octane gasoline.
What started to happen was that the ECM was not sensing any knock since the octane was so high. The ECM was not smart enough to realize the owner was running premium, so it thought there was a bad knock sensor. Thinking this was the case, the ECM pulled out all the timing advance and ran a "failsafe" 0 degrees advance. You don't have to be a car nut to realize that: 0 degrees advance = significant power reduction. This was mostly the case on the first generation of 454 and 502 magnum motors. All of a sudden these 300 - 400 horsepower engines would barely be able to top 200 on the dyno with premium gasoline.
MerCruiser remedied this problem within a year or two, but is a good example of how running premium fuel in an engine will most likely yield zero results (and some times negative). In fact, a pound of 87 octane gas actually has more energy than a pound of 93 octane. The higher octane has simply been more refined to sustain the increased temperatures found in high compression and supercharged engines.
Bottom line, running more than the recommended octane on a stock engine is a money waster. However, if you are really bent on throwing away money, please send a check to:
Bayley's Boat Fund
5200 Auto Club Drive
Dearborn, MI 48126
Bayley is correct. You only need high octane in a high compression engine. If they are the standard 260 HP 350 run the manufacture recomended octane.
Many people think that a higher octain fuels make more power. That in it self is not true. High octiain fuel allows you to use higher compression ratios and more advanced ignition timing, without melting the tops of your pistons. The higher compression ratio and timing is what gives you more power.
The higher the rating the more heat it takes to get the fuel/air mix to ignite. In an ideal world you would run fuel with an oction rating just high enough to keep your engine from pre-iginition (spark knocking). That would give you optimal economy and power for your given engine.
Higher octain will not hurt your stock 350 but you will pay more per gallon, you will most likely lose a little preformance and milage (if you want to call it that when talking boats) and if your engine is getting long in the tooth and the compression is less then when it was new, it may be harder to start when the engine is cold.
Some of the pervious post stated that their engine runs smoother on premimum. This is true it may run smoother but that does not mean that it is making more power or milage. A smoother engine often gives the preception of more power. Think of it this way how many TRUE preformance engines run smooth? (bapity bapity bapity bap)
There is one other advantage to some premimum fuels. They have more deturgents in them to help clean inturnal engine parts. For this reason it is not a bad idea to run a tank of the pricey stuff once and a while.