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Author Topic:   Premium Gas
Sailor Rob posted 07-24-2006 09:08 AM ET (US)   Profile for Sailor Rob  
By mistake, my daughter had one of the six gallon tanks for my 2005 Montauk filled with premium gas. While water skiing, I switched to the tank with premium. I noticed the engine (Mercury 4 stroke) ran at 4200rpm where it used to run at 3800rpm in the same throttle position. That tank of premium seemed to last longer than my usual 89 octane fuel. Am I dreaming or have others experienced this?

swist posted 07-24-2006 11:28 AM ET (US)     Profile for swist  Send Email to swist     
Were both fuel loads the same with regard to Ethanol content? That would make a difference.

Premium gas has higher octane which means it can be compressed more before igniting it, thus it only provides higher performance in a high compression engine designed to use it. Using it in a engine designed for regular should produce zero difference in anything.

Having said that, the statement is true only if the two tankloads were identical except for octane. You never know what's in gas these days.

Most likely some condition on the boat was not the same - trim, loading, weather conditions, instrument irregularity, etc.

frontier posted 07-24-2006 12:20 PM ET (US)     Profile for frontier  Send Email to frontier     
Our local Yamaha dealer says premium gas is not only not necessary but used a lot in an outboard could harm it. Wrong additives for outboard use. He says 89 octane is perfect, regular is fine in most cases.
Sailor Rob posted 07-24-2006 12:55 PM ET (US)     Profile for Sailor Rob    
Based on these replies, I'll stick with 89 octane.

All conditions (trim, wind, wieght on boat) were the same as I switched tanks while out on the water. Ethanol was 10% in the premium (Exxon) as it is in the 89 octane. I could literally feel a surge in engine speed as the premium gas hit the carburetors for the first time.

Thanks again for the informed replies.

Plotman posted 07-24-2006 04:31 PM ET (US)     Profile for Plotman  Send Email to Plotman     
"performance" is going to depend on nothing more than the energy content of a gallon of fuel. Normal, all-petroleum gasoline has about 125,000 btu per gallon. Ethanol is about 78,000 btu per gallon. Therefore, E10 has ~ 120,000 btu per gallon (the weighted average of the two), so your mileage is going to be ~4% less burning E10 than what you would see with "straight" gas. In Minnesota, premium contains no ethanol, whereas regular and mid-grade are blended with 10% ethanol. What happens in your state may be different, as gaoline is a very regional market.

The upshot of this is that if premium, no-ethanol gas is less than a dime per gallon more expensive than E-10 regular, I am better off buying it as I am paying less per btu.

I agree with the above posters that on an apples to apples basis, there is no reason to buy premium if your engine doesn't need it, as the only advantage to a higher octane rating is to prevent premature detination in a high-compression engine. In a standard compressiion engine, there is no advantage.

Chuck Tribolet posted 07-24-2006 11:32 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
At least in the automotive world, the engine mangagement
computer often has a knock sensor that it uses to adjust
the spark timing. Both my '01 Nissan Pathfinder and my '87
Corvette do. They will run fine on regular, mid-grade, or
premium, but get better performance and gas mileage on
premium. The better gas mileage is about the same as the
additional cost of premium, so why not run premium.

Dunno whether the Mercury in question has a knock sensor.

But it takes a while for the computer to figure out that it
has better gas. Certainly some number of minutes, the 'vette
is reputed to take a tank full. Is the ethanol content
rated "up to 10%" or "10%"?


Sailor Rob posted 07-25-2006 10:20 AM ET (US)     Profile for Sailor Rob    
THe 90 horsepower/4 stroke Mercury in question does not have a knock sensor. The premium gas I used may have less than 10% ethanol as the sign on the pump states "may contain up to 10% ethanol". If there was no ethanol in the premium but 10% ethanol in the 89 octane, this may explain the surge in power when switched to premium as explained by the increased BTU's in the post above.

I plan to use 89 octane for the rest of the summer. Here in New Jersey 89 octane contains 10% ethanol. I wonder if Mercury will publish any guidance on how to winterize with 10% ethanol gas sitting in the carbs and lines. Will gas stabilizer be enough to counteract the corrosive properties of ethanol over a 9 month period? I do not run the engine dry before laying up as I believe not all gas leaves the carbs and the little remaining has a better chance of evaporating and gumming.

lakeman posted 08-02-2006 07:02 AM ET (US)     Profile for lakeman  Send Email to lakeman     
I have used virtually nothing but 87 in my Optimax for years, not because I cannot get 89 but when I need gas, most of the time, the marina only has 93 or 87. The 89 is recommended by my mechanic. However I have had no problems at all with the 87 octane.
However my wife's 97 Corvette goes into cardiac arrest with out the 93 octane.
I do, run year round, Pri-G fuel conditioner, I hope some real good reports on the Startron would surface, I would like to know how it is doing.
jimh posted 08-02-2006 08:27 PM ET (US)     Profile for jimh  Send Email to jimh     
Is this effect supposed to only occur with Boston Whaler boats designed after 1990? I am confused why this is being discussed in the POST-CLASSIC area.

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