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Author Topic:   Mid-grade octane gasoline vs High Octane
onlyawhaler posted 01-30-2004 11:28 AM ET (US)   Profile for onlyawhaler   Send Email to onlyawhaler  
I just read an article on the use of high octane gasoline in automotive engines. I didn't know that high octane gas actually burns slower and in most modern cars doesn't help in increasing performance. In high compression engines it helps.

My questions is what is considered high compression and do outboards run better on premium vs regular octane? How about traditional carb outboards vs more modern high pressure injection outboards?

What works better for you?


quattro20vt posted 01-30-2004 01:06 PM ET (US)     Profile for quattro20vt  Send Email to quattro20vt     
I've been told twice now to not waste your money on high octane gas for an outboard--once from the dealer when I bought my Searay with a carb'd Merc 135, and again yesterday when I picked up my repowered Whaler w/ a 225 Opti.
Peter posted 01-30-2004 01:20 PM ET (US)     Profile for Peter  Send Email to Peter     
Most outboards are designed to run satisfactorily on 87 octane. They have no knock sensors or motor controls that can adjust timing to take advantage of higher octane fuels. However, there are some exceptions such as the Mercury Optimax 250 XS, which requires a minimum of 91 octane fuel.

I have read somewhere in the last year or so that the rate of carbon build-up may be higher with the use of the slower burning, higher octane fuels.

kglinz posted 01-30-2004 01:22 PM ET (US)     Profile for kglinz  Send Email to kglinz     
Mercury doesn't list compression ratios, but generally I think all 2 stroke engines are quite low. Yamaha specs show all their compression ratios around 6 to 6.9 to 1... 4 strokes are generally higher. Around 9 to 1. Some of Mercurys "racing engines" require hi octane fuel, but thats probably because they run the timing way advanced.
Bigshot posted 01-30-2004 01:46 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I used to run 93 in EVERYTHING until I realized I was wasting my $$$. It is the norm for 89 octane at marinas so that is what I run, even when I use a gas station. Outboards are low compression so not necessary. In the old days only Super had the cleaners in it so it was beneficial to use. Now even 87 has all the same additives that 93 does so no worries about injectors, etc.

To be safe, run 89 like I do. $.10/gal aint gonna kill ya.

lhg posted 01-30-2004 01:51 PM ET (US)     Profile for lhg    
On both my "historic" 1985 Merc 115's (carbed, no oil injection) and my higher tech 1997 200 EFI's, Mercury clearly says to use 87 octane as fuel of choice. Don't forget the Quick Clean every 4th tank, regardless of your 2-stroke engine brand. This stuff will keep your cylinder bores, pistons, rings and exhaust passages shiny clean and free of any carbon build up.

I have noticed that many marinas only have the 89 or 91 octane gas, which is usually just an excuse for the high price. So sometimes I have to buy the 89, but see zero performance/economy improvement from it. The ultimate rip off is the Valvetech "Marine gas", just a sales gimmick. The earlier Valvetech, containing the "lead substitute", would wreck an outboard. SE Florida marinas, after many outboard disasters & tow-ins from this fuel (myself included), ran the Valvetech salesman out of town on a rail.

Besides the Optimax 250 that Peter mentioned, the Merc 300 EFI's also need 91 octane fuel, as do the EFI 7500 rpm racing 2.5 liter V-6's, putting out 260HP. None of which is an issue here.

David Jenkins posted 01-30-2004 02:03 PM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     
It depends on how long the fuel will sit in the tank.

Several months ago there was a thread on this forum about winterizing your boat. One of the questions addressed was whether or not to leave your fuel tank empty of full. Leaving it empty could cause water vapor to condense and put a layer of water in the bottom of your tank. But leaving fuel in the tank for several months caused the fuel to have a lower octane rating over time. So back in November I filled the tank with 93 Octane, thinking that this would solve the dilemma. When boating season returns the tank of 93 octane will still be at 87 or above. Right?

whalersman posted 01-30-2004 02:49 PM ET (US)     Profile for whalersman  Send Email to whalersman     

Here is some information I posted before about the recommended fuel for my 1985 Evinrude 150..
I have a 1985 Evinrude 150 that was designed to run on the middle grade of 93 Leaded octane fuel (from manual) .. The engine always sounded like it was pinging more then it should even though I was using that grade fuel but Unleaded. My mechanic said there is an outboard bulletin out that states you can put in "thicker" head gaskets to reduce the amount of compression in the engine and then use the lower grade 86AKI Unleaded fuel. Outboard Bulletin #2155.

I ordered a pair of this head gaskets and installed them myself.. Very simple. I cannot believe how quiet the engine runs now. It is unbelievably quiet compared to what it was.. Almost sounds like a 4 stroke when cruising as I have no problems having a converstaion with other people on board. I can now also run the lower 86AKI Unleaded fuel which costs about .10 cents less a gallon.. 60 gallons equals a $6.00 savings per tank.

I may not have the total 150 hp as it used to produce, but I surely can't tell the difference... Note, the newer engines aren't included in any of the above info..

For What It's Worth,

onlyawhaler posted 01-30-2004 03:29 PM ET (US)     Profile for onlyawhaler  Send Email to onlyawhaler     
Thanks for the answers

Ing, what is that quick clean you run through every 4th tank? Is that a brand product or a universal any board generic product? Is it available at West Marine?


onlyawhaler posted 01-30-2004 03:33 PM ET (US)     Profile for onlyawhaler  Send Email to onlyawhaler     
H Joe,

I think I would like to consider doing that. My Johnson pings at times like a old Camero on cheap gas. I tried this fall before winterizing higher octane gas, but it didn't help.

My Johnson is an 88. Is it the exact same motor as yours? I don't even know for sure if mine is a cross flow or a looper.


Peter posted 01-30-2004 04:00 PM ET (US)     Profile for Peter  Send Email to Peter     
If you engine pings, you may have excessive carbon build up. Have you decarbonized your engine?
Hobie1981 posted 01-30-2004 04:20 PM ET (US)     Profile for Hobie1981  Send Email to Hobie1981     

My 1970 MGB-GT loved the high octane, I doubled up the head gasket when I needed to replace it and then could run 86 in her with out any pinging or "run-on", after engine was switched off.

Glad to know this trick works also in your outboard. I wonder if anyone else has tried it in other brands of outboards.


whalersman posted 01-30-2004 04:22 PM ET (US)     Profile for whalersman  Send Email to whalersman     

I don't know if yours is the same or not... I would contact a dealer and ask them about bulletin #2155 and see what years that was for. Ask them if there are any other updates or bulletins on your particular model. You should also look in your engine manual and see what fuel they recommend. Leaded or Unleaded etc... Mine recommended Leaded which you can't get anymore unless you put in an additive..

My heads and pistons were clean when I pulled the heads off... I guess my engine wasn't designed to be run on unleaded thus the bulletin update...

It sure runs quiet now....

Joe Kriz

onlyawhaler posted 01-30-2004 04:25 PM ET (US)     Profile for onlyawhaler  Send Email to onlyawhaler     
No Peter,

I haven't decarbonized it and I need to do that. I am looking at different threads right now and there seem to be different answers.

One threads mentioned hooking up an inline value to the gas line and feeding it through that. Others are taking off the air cover and spraying it through the carbs while running.

One thread mentions waiting 15 minutes for the product to work, another leaving it (yamahas) leaving it in all night before starting up again.

I am not sure what to use on my Johnson 150. Any feedback would be helpful


lhg posted 01-30-2004 04:29 PM ET (US)     Profile for lhg    
Sterling, the product is Mercury QuickClean, and it is a Techron containing product in a black (naturally) 12 oz bottle that treats 70 gallons of gas. Bought right, it's about $8. It is a de-carboning, de-gumming and varnishing product that will keep your engine & fuel delivery system like new. I have seen the inside of cylinders/pistons where this has been used, and the engines are like new, no carbon deposits, using 87 Octane and Walmart/Pennzoil TCW-3.

It's widely available, including Boat US, West, etc. Merc dealers abviously have it also. Stongly recommended.

Carbon build-up is a major cause of detonation and resultant piston destruction.

Sal DiMercurio posted 01-30-2004 05:36 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Ihg, isn't "Power tune" Mercs decarbon agent ?
Sterling, the instructions are on the cans, no matter what you use.
Theres Power Tune from Merc, Ring Free from Yamaha, Engine Tuner from OMC Bombardier & Sea foam decarbon from your local auto supply, Sea foam dosen't come in a preasurized can but can be sprayed into the carbs if pored into a squirt bottle, the squirt bottle has to be for chemicals or the solution will disolve the innerds of the sprayer.
The top tech for Bombardier is a close friend & he recommends engine tuner left in overnight as all it does is liquify the hardened carbon so it can be blown out the exhaust & it wont harden again once it's saturated with the decarbon solvent, so 15 minutes or overnight is 6 of 1 & 1/2 dozen of the other.
I have left it in my engine for 5 days because the boat was 65 miles away when I put it in [ figured I was going fishing the next morn ] , & the weather went bad & didn't go back for 5 days.
Peter posted 01-30-2004 05:38 PM ET (US)     Profile for Peter  Send Email to Peter     
Sterling, for some guidance on decarbonizing, see .

Carbon acts as an insulator and so the combustion chamber runs hotter when coated with carbon than when its not so coated. I believe that this extra heat can cause detonation which will ultimately destroys the piston as Larry says.

Ed Stone posted 01-30-2004 06:15 PM ET (US)     Profile for Ed Stone  Send Email to Ed Stone     
Quickleen is a Quiksilver product.I use this along with
Opti-oil in my 135 Optis.They claim the Quickleen removes
deposits and prevents corrosion.I have 470 hours on my
pair of Opti's.I picked up the Quickleen at Bass world
for $7.99.
lhg posted 01-30-2004 08:15 PM ET (US)     Profile for lhg    
Sal - I have used Mercury's Power Tune, which is the one shot application sprayed in through the carbs, and then blown out with a 4000rpm run. I thick the fuel additive Quick Clean is a newer, superior product, which works continuously to do it's work. I asked my Mercury dealer about using the older Power Tune product, and he said that if you're using Quick Clean, the Power Tune is not necessary at all, nor as good. Power tune is now used for a badly carboned engine that needs a quick fix. Once used, it can be followed with regular QuickClean use.

Give it a try. It's really great stuff. I recently had some check up work done on my 2500 hour in-line 6's, and they were able to look into the cylinders/pistons. Said they were absolutely shiny new and clean, with no carbon, varnish, gum, etc. Definitely said Quick clean was the reason. My V-6 mechanics taught me to use this product back in 1990, from their experiences in rebuilding engines that hadn't used the stuff. They absolutley swear by it, and so do I. It works for any brand 2-stroke. Yamaha has a version of it also, but don't know the name. QuickClean is thicker yellowish liquid.

onlyawhaler posted 01-31-2004 02:36 AM ET (US)     Profile for onlyawhaler  Send Email to onlyawhaler     
Thanks guys. I appreciate the advice. This is a great site


Clark Roberts posted 01-31-2004 06:12 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
My 1995 Merc 200Xri (EFI) has a knock sensor that retards timing if pre-ignition is detected. I run 87 octain and use Chevron brand (techron additive) when available
Peter posted 01-31-2004 06:24 AM ET (US)     Profile for Peter  Send Email to Peter     
Clark, is there any noticible difference in performance when using higher octane fuel?
Clark Roberts posted 01-31-2004 11:23 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Peter, I can't tell any difference at all. I have ten outboards (2.2, 5,20,40,three 60's,115 4stroke, 150 EFI and 200 EFI)and run 87 octain in all. Clark... SCN
Clark Roberts posted 01-31-2004 03:03 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Peter, no noticeable difference to me!
Peter posted 01-31-2004 07:28 PM ET (US)     Profile for Peter  Send Email to Peter     
Thanks Clark, I wouldn't think you'd see a difference on anything but the EFI that has a knock sensor and can retard timing. But if the timing doesn't retard from a calibration for ~93 octane, but rather from a calibration for 87 octane, then I suspect you won't see a difference.

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