Octane

[Chris F.]

More questionable advice from my dealer? What octane does everyone run in their boats? When I had a 640 with a 75 hp the dealer said run premium(91+octaine). Now I have a 901 with a 200EFI and this new dealer said run 87.

 

[Staci Matheis]

Run whatever the octane is called for in your owner's manual. With 2 strokes, running a higher octane won't necessarily increase performance and may actually hurt it.

 

[TOMMY RICH]

87... or you could make it run better by using 87

 

[Mike Miller]

Performance-wise, I'd agree - you shouldn't need more than what the mfr. says. But I always ran premium in my '89 Merc 60 and I could never get over how clean the plugs were and how little carbon was on the piston tops. I also used synthetic oil, so maybe it was the combination of both. I plan on donig the exact same thing when my new Tundra/150 XR6 shows up. Yes, it's more money, but the thing was pretty darn expensive to begin with...

 

[Bill Hamilton]

My dealer told me to always run mid-grade (89% octane). I always have and have never had a problem. Others run 87% and they don't have any problems either but since I have always run 89% in my last 4 boats I don't intend to change. The plugs have always run clean and at $10 per spark plug I don't have to change them as often. I generally run the plugs for two (2) years and not the customary one year.



For what it's worth!

 

[Staci Matheis]

The "problems" are not the type that you are likely to notice. As far as the individual can reasonably tell, their motor is functioning properly. But.....



There are ports in the cylinder walls of 2-stroke motors through which the air/fuel mixture and exhaust flow. The air/fuel mixture does not flow directly into and out of the compression chamber but into a chamber below the piston and surrounding the crank. The exhaust is both pushed into the exhaust pipe and drawn out with the help of the expansion chamber in the pipe. At one point of the cycle, both the new air/fuel mixture and the burned out exhaust gases are in the combustion chamber at the same time.



All the compontents are designed to work with fuel that burns at a specific rate.



Higher octanes slow the fuel burning rate. Slowing it enough could cause it to still be burning at the same time the new fuel/air mixture is flowing into the combustion chamber. Then both burn before the ports are sealed by the piston and you do not have as much to ignite once it is time to start the piston's down stroke.



The bottom line is that, unless you've changed the porting in your cylinder walls, they are cut specifically for a certain octane. Run what is recommened in your owners manual for maximum performance and fuel efficiency.



me!



I know that this is a poorly worded explanation but I'm at work and don't have the time to word it better right now.





(Reworded a bit later in the day. Still not great but I hope it's a bit more clear!)

 

[William H. (Bill) Barham]

87 for my 40.



Bill

 

[Mike Miller]

One question I often ask folks is this: if they were all the same price, which octane grade would you buy? Same question is valid for oils, boats, etc.



I know it's not real, and I'm never looking to start a head butting session, but sometimes looking at issues this way helps to separate cost and quality, because I think many of us look at these issues from different perspectives. A guy running thru 2000 gallons a year will surely have a different perspective than me who would LOVE to run thru a fraction of that.



My 9-to-5 as a product development engineer at an oil company has taught me some things, and believe-you-me I'm a cheap-A Pennsylvania Dutchie at heart, and thankfully it's a free country (mostly), but the bottom line is I will only buy premium fuels. I see the differences of running different fuels in engines every day. Yes, there are exceptions to rules, and all engines are different, and sure as heck 2-strokes are different than 4-strokes, but I know what I'm buying - for sure.



Good chattin' - gotta run...

 

[Jim B]

Octane requirements are a function of compression ratio as well as ignition timing advance. The only reason to need a high octane fuel would be if you have high compression, advanced timing or both to prevent pre-ignition. Most normal outboards wont meet these conditions and high octane will benefit you zero. Two or 4 stroke, dosent matter. Buy good quality gas in whatever octane is stated in the manual from a quality company like exxon-mobil, texaco... and a quality oil and you will be fine. Stay away from no name gas and cheap oil.



That being said i run 87 in my 150 xr6 with synthetic oil and never a problem. I also ran my old outboard with 87 and synthetic oil and never a problem and i beat the heck out of that motor. It was a rarity that it ever ran less then 5500 rpm the whole time i owned it.

 

[Bruce Freeman]

I proably should leave this alone but I can't. Staci is right,Mike M is right and so is Jim B. But bottom line you can not compare 2 stroke to 4 stroke. Follow the owners manual and you can't go wrong. On the 2 strokes the fuel/oil mixture also lubes the engine, so talking about burn rates or ignition timing are not the only factors. Yes the higher octaines have a slower burn rate or what ever you want to call it. 2 strokes have a higher compression rate but lower operating temp. The ash and carbon are a product of the oil and type of usage?

I am only a rookie with boats but what say you.

BF

 

[Barry Loos]

I use 87 octane because the engine is rated for it. One time I used 92 octane and kept getting an over-heating alarm. I diluted the fuel with 87 octane and have used it ever since and had no more alarms.



Da Bear

 

[JR F]

I run 93 octane In mine...JR

 

[Dan J.]

In the hi-perf outboards you'll need the higher octane for the increase in compression. Other than those select engines, 87-89 octane is the way to go. I run 89 in my OB's just to keep a slight edge on octane deprivation. (As recomended by Doug Cody, Mercury HP Tech/Instructor)

 

[Bob G.]

I use 87 octane fuel and Merc Premium Plus oil. I have 5 seasons on my Champion plugs and will probably change them this spring after burning off the fogging oil.



Yea, I know Pat, Champion spark plugs are junk!! LOL!!! You have never given a specific reason why they are junk, so could you give my XR6 the lowdown on why it runs so good with junk spark plugs?



Bob G.

 

[JR F]

Bob My Xr-6 would not Idle on the Champions..NGK It did fine...I have no Idea why It wouldn't Idle but It would shake and smoke and miss out..Weird huh..NGK It was smooth..JR

 

[Jim B]

Lol at Bob G. I always wondered the same thing myself in reference to the Champion plugs. My xr6 has also run great with them( changed once a year) and so did my evinrude intruder.

 

[Staci Matheis]

...uh..... "Octane Deprivation"..... ????????

Is that a high-falutin' way of saying "pinging"? LOL!!







I just did a Google and found:



WONCAR-L archives -- May 2004 - ... L0se Wei*ght Now with Phe*ntermine, A*dipex, B0ntril, Pre*scribed 0nline, shi*pped t0 Y0ur D00r octane deprivation mandrel cambodia jeffrey bowditch chablis ...

 

[Dan J.]

...uh.....???????? me - If you think that the actual octane listed in the yellow letters on the pump is actually what you are burning, you will be rarely correct. (Although the content should generally be close at a pump with steady tank turnover.) Gasoline, as with many other fuels, will lose their octane content over time. Ask anyone in the business at the Wood River refineries and they can explain it to you in simpler terms, if needed. I've been advised that the 89 octane fuel hedges the breakdown slightly without a major increase in residuals. Octane deprivation can lead to the "pinging" stated, but only after great length, producing instability within the molecule cohesion. (During my studies under Henry McCandles, V.P. of American Oil, it was commonly refered to as octane deprivation.)



You also might want to consider an alternate resource of information over the advertising sponsored Google search engine, but hey, what do I know?



FWIW - As always, Goff hit it on the head. (Especially with the Champion plugs! IMHO)

 

[Steve Rizza]

A question while we are on plugs.



Are the NGK BUHW plugs I bought at NAPA for $1.99 the same plugs they want $12 for at the marina ?

Same number,same plug ..right?



Steve

 

[JR F]

Yes Steve, I get mine from Napa and I get stuff at cost from the marina.... I like 93 octane....Comes form my hot rod days i guess....HEEEE

 

[Steve Rizza]

Thanks JR.

I happened to ask if they carried them and they had them in stock.I couldn't believe the difference in price.



Steve

 

[Jim B]

Dan i agree with you toally on tank turnover. Thats why im picky about where i buy my boat gas. I only run 87 octane but i also buy 90% of it from a friend of mine's exxon station. He sell so much gas there that he gets his tanks refilled 5 times a week on average. His gas never sits more then 24 hours.

 

[Staci Matheis]

Dan, I trust that the engineers at Mercury are taking all of those factors into consideration when they recommend octane levels. They know best what their motors need to run and will act accordingly.







P.S.

Clam down! I was just funin' with the Google search!

 

[Dan Sullivan]

I won't have my boat for a couple of weeks but this is what I know from having a truck.



My truck runs fine on 87 until the fall/winter months. In the North East (and I'm assuming other cold weather winter climates) gas companies up there ethanol levels which equates to thinned out gas. So I run plus and premium all winter. I found this out when my truck started pinging in November out of the blue.



My Ford service manager also recommended that I stay away from Mobile/Exxon and go with Amoco, Texaco, or Sunoco. He said the Mobile/Exxon gas has so much detergent in it, it's almost not gas anymore.

 

[Dan J.]

me, I also trust the engineers at Mercury Marine, neither have I differed from them, especially since many of them are my friends, including several working in the Suwanee facility about 10 minutes from my home and our dealership. That is why I replied to the original question (posted by Chris F.) of, "What octane does everyone run in their boats?", stating what level octane I use and referring to information expressed by one of Mercury's top engine tech./instructors.



P.S.

I am, and have been, completely calm. Please do not confuse my demeanor with my ability to provide an explaination of requested information.



Sully - If at all possible keep the ethanol diluted gas out of your boat's gas tank. The longevity of your motor will be greatly enhanced without it. (Two stroke outboards and alcohol do not mix well!)

 

[Staci Matheis]

LOL! I guess my head got so confused at the use of that high falutin' "Oxygen Deprivation" stuff that I completely missed the numbers! You have got to speak in plain, old, down-to-earth terminology like "pinging" for us uneducated, shadetree types! LOL!! Keep those fancy words for the educated members like Rich and Trepper!





me!