What octane do you use?

capt_jack

New Member
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Jul 10, 2007
180
GA
Boat Info
185 Sport
Engines
4.3 Merc
Just got a 2007 185 sport w/4.3 merc. Wondering what octane gas I should use? I use 93 in my bike and car, should i use the same in the boat? Is there any advantages or disadvantages?
 
I saw that little bit right there, but it diddnt really answer what im lookin for. From what I got from that thread was that if your boat owners manual doesnt say use higher gas then dont. lol, just wondering for those 185 or similar models what everyone uses and if there would be an advantage or not to running higher octane.
 
Burn what Merc says to burn in the 4.3.

Burning higher octane in any motor is wasting money and possibly degrading performance. The higher the octane the slower it is to burn. If the motor is not built or tuned to burn slower burning fuel, you're likely not burning it efficiently and therefore it's not performing like it should.

My 7.4 MPI would run much better on 87 octane than 91-93 octane. It was obvious when I ran higher octane. I learned.

Obviously I now run 91 at a minimum.

Of course a boat sitting for a long time with gas in the tank is a different story. But unless you have it analyzed, you don't know how much the octane rating has degraded from sitting. If you had a half tank sitting for 6 months, I wouldn't hesitate topping it off with 91-93 octane knowing the octane rating of the old fuel it probably down 2-3 points.

FYI, some car computer control systems will adjust for different fuel to some degree. If you start running higher octane, after a few tanks, it will have adjusted timing etc. to take advantage of it. Going back even to 2000, Ford ECM's did it. You could run 2-3 tanks of premium through my Expedition and MPG would increase slightly as it adjusted for the fuel. Even cars that require high octane generally will adjust for lower octane if that's what's getting run. But those are cars that are designed for peak performance, so they start out expecting to run premium.
 
Thanks! Now to just find what is reccomended to run in the 185 :)
 
Is the 4.3 you have a MPI or carbed? If it's a MPI you won't have any trouble using 87 or 89 octane fuel. Most marina's sell 89 on the water. So I always wind up with a combination of fuels in my tank during the summer. Both have worked with no issues. I have not ever tried putting 91 octane in my tank simply for the reason I don't want to pay for it for no good reason. There is none that I am aware of.
 
Sorry I thought it was covered in Dave and Jeremy's posting in the link but here is what the Mercury Manual of your boat says.

Page 40

Fuel Requirements:
Important - Use of improper gasoline can damage your engine. Engine damage resulting from the use of improper gasoline is considered misuse of the enngine, and damage caused thereby will not be covered under the limited warranty.

Fuel Rating - Mercury Mercruiser engines will operate satisfactorily when using a major brand of unleaded gasoline meeting the following specifications:

USA and Canada - having a posted pump Octane Rating of 87 (R+M)/2 minimum. Premium gasonline [92 (R+M)/w Octane] is also acceptable. Do NOT use leaded gasoline.

It goes on to talk about using reformulated gasolines and gasolines containing alcohol.
 
yea, i actually just read the manual and found that same page. I guess we can conduct some tests :) I will run high octane for a few weeks then switch it up to low for a few weeks see what kind of performance differnce there is.
 
Hey crock,

Is that grill guard for catchin' 'gators? If so, I will have to get one for my Titan. Then I'll have to head down to my bro's house in Houston. No 'gators here in N. Texas!
 
ruf1967 said:
Hey crock,

Is that grill guard for catchin' 'gators? If so, I will have to get one for my Titan. Then I'll have to head down to my bro's house in Houston. No 'gators here in N. Texas!

Then you aren't looking hard enough. :grin:
 
Run the minimum that Merc recommends then change if something causes a need. Some older 4.3's were prone to pre-ignition when hot so Premium would help control that. But these day's until you get up into the high HP Blue motors, all Merc's only require 87 octane.

Most marinas carry 89 which is good for adding a little buffer and giving you some room for octane degredation which happens. Depending on the tank, you could lose 1 octane per month or so.

FYI, mine sat for almost 7 months this year with the same fuel. It ran fine, although obviously I took it easy and never went WOT (when detonation would occur) but when I was able to top it off with good new 93 octane, it was very obvious it liked the fresh 93 much better.

If you fill on the street, use 87. If you fill on the water, use the 89 if that's what they sell. If you won't be burning it off in 1-2 months, don't fill to capacity so that you'll go through the gas and keep fresher fuel. I usually fill to about 1/2 tank max unless it's a road trip/rally or something where I know I'll be burning most of it in a week or so.

I'd venture to guess that 75% of premium fuel sold at stations on the street is wasted money. I don't know how this day and time, but a lot of people are still stuck on the "premium is better" myth. Octane has nothing to do with the additives that make one fuel better than another.
 
My 4.3 (carb.) had issues with pre-ignition. I went from 87 to 89, and now run 93 and let it idle for a few seconds prior to shutting down. Works like a champ now. stabil is a must have!! :thumbsup: I never leave the dock without it. :grin:
 
mrtopknight said:
My 4.3 (carb.) had issues with pre-ignition. I went from 87 to 89, and now run 93 and let it idle for a few seconds prior to shutting down. Works like a champ now. stabil is a must have!! :thumbsup: I never leave the dock without it. :grin:

When you say pre-ignition are you talking about it not shutting down right away? I've heard it refered to as dieseling, not that it's the proper term....anyway mine does that if I come off plane and shut down, when picking up a skiier/wakeboarder etc..., but if I come off plane and idle to a dock it shuts down no problem. That's one of the reasons my next boat will be fuel injected!
 
4.3's have had issues. Both dieseling and vapor lock. I think there was finally a solution from Merc. on the dieseling which was by far the worst problem for them.

I'm certainly no expert, but it had to do with heat and head design. It would "run on" because the heat and head design would create a situation where it could ignite fuel and vapor without ignition, therefore running on. Higher octane would sometimes solve it because it takes longer to burn and would not ignite without ignition source other than high cylinder pressure and temp.

Eeerrrrr, somptin like dat. :smt043
 
I am by no means an expert but I was told by a very well trained Sea Ray mechanic that if it is fuel injected he has never seen a problem w/ people using a lower octane gas, even from the "discount" type gas stations. For carb engines he did strongly recommend using a higher octane gas (89 or higher) and not from a "discount" type station (by discount I mean AM/PM, Safeway, Costco etc.).
 
I have never had a pre-ignition problem, I do have the dieseling problem, I just let it idle a few before shutdown and that usually takes care of it, if not then it is key off and key on until it decides to shut down, one day I will cut the idle speed back a little.
 

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