Fuel Octane Rating?

[Matchrocket]

What harm could be done to an outboard motor if you run 92 octane instead of 87 octane?

 

[mellowyellow]

Re: Fuel Octane Rating?

this subject has been debated at length here in<br />the past. do a serch on the subject above and<br />enjoy the reading. lot's of great points on both<br />sides. <br />good luck,<br />M.Y.

 

[JB]

Re: Fuel Octane Rating?

Howdy, Matchrocket.<br /><br />There are theories about added deposits, but some manufacturers say use 87 octane OR GREATER.<br /><br />I think the only difference you might notice is a thinner wallet.<br /><br />Good luck.

 

[Spidybot]

Re: Fuel Octane Rating?

In some regions (like mine) there is no gas available lower than 92. And all uneleaded.<br /><br />34 years of running outboards of various brands shows no problems - and no benefits.

 

[steelespike]

Re: Fuel Octane Rating?

Running higher than required octane is a waste of money.<br />As octane goes up gas actually is harder to ignite<br /> so it ignites correctly under the higher compression ratio it is designed for.<br /> As JB said the most obvious change will be in<br />your wallet.

 

[grandx]

Re: Fuel Octane Rating?

Octane needed to prevent knocking, pre ignition, detonation etc. Not needed in most outboard engines, if your engine is knocking with 87 it may need higher octane fuel. As far as damage from using higher than required... I believe only your wallet gets damaged like jb said. I have heard that your octane will fall as the fuel ages, I'm not sure if this is true or not, but when I plan on storing the boat for a while, I fill the tank with higher octane and use stabilizer. Just a one time/year hit on the wallet.

 

[Pursuit2150]

Re: Fuel Octane Rating?

Most marinas sell HI-TEST,OMC states 87 min, I've been running HI-Test for 7 years. Never a problem.

 

[Beernutz]

Re: Fuel Octane Rating?

With the slower combustion rate of higher octane fuel, couldn't you experience an actual power loss when using it in a low compression ratio engine?

 

[Dhadley]

Re: Fuel Octane Rating?

Beernutz, you are very correct! It may be very slight with 93, but it does happen. <br /><br />And the fuel isn't like it used to be. 93 also burns hotter than 87.<br /><br />Good observation!

 

[Beernutz]

Re: Fuel Octane Rating?

Thanx, D H.<br /><br />That's always been a theory of mine, but nobody wants to buy into it. For some reason, most people think that higher octane = higher performance. It doesn't. It's just a necessary evil to prevent pre-ignition in higher compression engines. I used to have to buy Sunoco 260 to keep my 12.5:1 Chevy engines from knocking.

 

[Dhadley]

Re: Fuel Octane Rating?

Exactly! The increase from 87 to 93 may not show up on the average motor but heres a funny (well its funny now!) story.<br /><br />When we decided to get into outboard drag racing I changed the set up on my ski boat and went to an end of the year event to see if I liked it. We had no intention of winning, just wanted to see if it was fun. <br /><br />I had a V4 crossflow and got it running about 62-6300 rpm. At the last minute a buddy, who had finished the stock car season, gave me 5 gal of his racing fuel. Something like 114-116 octane for his 13 1/2 to 1 stock car. Off I go to the races! <br /><br />Long story short, the motor is down 5-600 rpm. I stay up most of the night (miss dinner - the wife was real pleased about that!) working on the motor - carbs, ignition, spark plugs etc. Second day -- same deal.<br /><br />I never thought about the fuel until the ride home. Monday evening I drain the little bit out that was left, fill her up with 87 and presto! 62-6300 rpm. <br /><br />Good grief!

 

[sony2001]

Re: Fuel Octane Rating?

Therefore use the higher octane if it's going to sit in the tank for a while, or your engine PINGS.

 

[BillP]

Re: Fuel Octane Rating?

My 50 Mariner 4 stroke runs exactly the same rpm on 87 as 92. So have my OMCs twins, V4s and tripples over the years. I think losing power is a long shot for the average boater to worry about.<br />Years ago everyone I knew ran the highest octane available (for normal pump gas)and I can't remember any ill effects. Now 87 is normal.

 

[Dhadley]

Re: Fuel Octane Rating?

We do the same thing with the outboards we race. But on a stock motor the point is you may not see a difference unless you run a pyrometer. The combustion temps are definately higher with 93 unleaded pump gas over 87. The fuel alone may or may not be enough to cause damage but its sure one step closer.

 

[BillP]

Re: Fuel Octane Rating?

I often wondered why outboards don't use EGTs or CHTs. The head temps would be easy but exhaust might be a problem. On flats boats down here water temp and pressure is normal but nothing else for measuring temp.

 

[Forktail]

Re: Fuel Octane Rating?

Another piont:<br /><br />The higher octane ignites later and burns slower. I've found this actually retards the timing slightly. So, in affect, you can slightly advance your timing with the higer octane fuel.<br /><br />High octane really only makes a performance difference on high compression engines. Otherwise the 87 will perform better.

 

[Dhadley]

Re: Fuel Octane Rating?

BillP - pyrometers (EGT gauge) are not uncommon. Bob's Machine Shop sells a really nice dual readout for V6's. Nice folks too.<br /><br />Good luck!

 

[BillP]

Re: Fuel Octane Rating?

Dhadley...Bob's as in the guy who does the lower water pickups, jackplates, etc?

 

[Dhadley]

Re: Fuel Octane Rating?

Yes, that's him. Tell him I said hello!

 

[almost retired again]

Re: Fuel Octane Rating?

My respose to higher octane fuels. I worked for a boat rental place for many years, during that time I've tore down and overhauled a lot of engines. At first this place used the lower grades of fuel, the engines when disembled had lots of carbon build up and some of the damage could be attributed to that. Then the marina started using only 93 octane fuel and any engines that I tore down had much less carbon build up. Now I'm not an engineer, or a scientist, but something caused this change. Under normal use I think that I would use a cheaper fuel. However rental engines only have 2 speeds, stop and all ahead full in this situation I think that the higher octane fuels are best.<br />Octane tends to slow down the burn, giving a smoother and more complete burn rather than an ex plosion. Higher octane fuels along with retarding the timing will produce more h.p. and usually better fuel milage. This can cause dramatic differances in turboed engines.<br />In WWII the big bomers were having a difficult time getting airborn with a full load of bombs, so they water injected them to slow the burn and increase the h.p. So I think that increased octane does help depending on what you want, and what you are using it on.