93 Mercury BlackMax What Octane to Use?

[TheBlitz]

I just purchased a new/used boat and it has a 1993 Merc BlackMax 135 on it. Seems to be running great had it out several times this year. I am curious what you guys suggest for octane to use. I recently put in 94 octane but boy did I feel that $$. I have spoken to a few friends and they said with older outboard motors just to fill up regular 87. What do you guys suggest?

 

[bubbaajack]

I have the same motor. It has been running on Texaco 89 mid-grade with carefully measured amounts of Star Tron Enzyme added. Buy a service manual and keep it well maintained and adjusted. It will run well and long. NEVER run it with overheat alarm sounding or no water from tell tale! Water pumps and thermostats/poppets are cheap and easy. Maybe run some Seafoam through it but that's all I have done. Great motors!

 

[Chris1956]

regular fuel is fine for all production outboards.

 

[Chris1956]

Star Tron is snake oil in my opinion. I simply did not believe their claims.....

 

[H20Rat]

Higher octane results in less power unless your engine is designed for it. ESPECIALLY in that engine, the result of the higher octane was a placebo. (that engine has no knock sensor, no way to alter the timing or mixture based on fuel or ignition conditions.) Octane is essentially a measurement of how easy the fuel vapor ignites. By running higher octane, you are slightly delaying ignition, and basically retarding the timing through fuel. That equals lost power.

More importantly, find non-ethanol fuel if possible. In some cases, this means premium fuel.

 

[Texasmark]

Star Tron is snake oil in my opinion. I simply did not believe their claims.....

Well Chris that's your opinion and you are surely entitled to it.

 

[Pony]

Regular Octane is not only fine, but the best option. If you can find recreational fuel that doesn't have ethenol added.....use that.

If anything the higher octane is causing potential issues with timing and how it combusts and isn't giving you a performance increase. $$ down the drain.

I run regular in my 04' optimax with Mercury fuel cleaner/stabilizer.

 

[Texasmark]

Another reason for running the "cheap gas" is that it gets used frequently meaning you are more likely to get a fresh batch. We had one gas station in the area that had opaque soft plastic covers over the buttons for weather proofing. The "regular" was worn completely through including the paint that was on the button. The mid and premium were obviously much less used as you went up in price.

I'm a firm believer in "Snake Oil" too. Have noticed a lot of OEM mfgrs. are finally including a sample in their operating literature package on new units. I have used it for 20 years and swear by it. Proven itself in all kinds of farm and non-farm diesel and gas engines, 2 and 4 strokers, naturally and turbo charged, and carbureted and injected engines. For 2 strokers, Sea Foam has proven the favorite as was the case on this site where I learned of it back in '04.

 

[pmilana1]

I've been using 87 octane 100% pure gas...I live in an area that has has several gas stations offering the no ethenol version. and a fuel stabilizer...No problems so far,,,owned the boat since November last year

 

[WIMUSKY]

I use premium, around here it's only 91, because it doesn't contain ethanol. And, if your boat sits a lot, unfortunately like mine and at least 7 months over winter, it is my understanding the octane level decreases over time. At what rate, I have no idea...... And yes, I'm a believer in Seafoam......

As far as "older outboards" from the 60's/70's I would always run premium since the octane levels at the pump were higher then across the board.......

 

[Frank Acampora]

WOW! The internet strikes again with misinformation. While you can consider octane to be a measure of how easily fuel vapor ignites, it is really a measure of resistance to ignition due to heat of compression or advanced timing. Gasoline is rated in octane because originally the hydrocarbon Octane (C8H18) was used as a baseline. All other mixtures of hydrocarbons (gasoline) were measured against octane with octane being arbitrarily set as 100. Thus Octane has more resistance to self ignition under compression than any gasoline sold today. In the old days with a lot of muscle cars running compression ratios of 12 to 1 or higher., Sunoco used to sell "Racing" fuel at the pump. I believe it was rated 105 octane and thus had a higher resistance to "knock" than pure octane.

All gasoline essentially burns at the same speed (in a given engine) no matter what the octane number. Thus a higher octane gasoline while having a greater resistance to "Knock" will not retard timing. Indeed one of the reasons for using higher octane gasoline is advanced timing. Using higher octane gasoline in an engine not designed for it is simply a waste of money. Any engine with a 9 to 1 or lower compression ratio will run quite happily on 87 octane gasoline.

 

[smokeonthewater]

Eh you can still buy higher than 100 octane gasoline.

 

[H20Rat]

WOW! The internet strikes again with misinformation. While you can consider octane to be a measure of how easily fuel vapor ignites, it is really a measure of resistance to ignition due to heat of compression or advanced timing.

All gasoline essentially burns at the same speed (in a given engine) no matter what the octane number. Thus a higher octane gasoline while having a greater resistance to "Knock" will not retard timing. Indeed one of the reasons for using higher octane gasoline is advanced timing. Using higher octane gasoline in an engine not designed for it is simply a waste of money. Any engine with a 9 to 1 or lower compression ratio will run quite happily on 87 octane gasoline.

I don't believe anyone except you referenced speed of burn... I mentioned resistance to ignition, which you agreed to. High octane fuel in an engine designed for low octane can cause misfires, and ANY delay in ignition is technically retarding the timing.

In any case, it the OP's, it is throwing money away. Assuming both grades are non-ethanol, running higher octane fuel will not increase power.

 

[bruceb58]

Octane is a measure of fuel burning in an uncontolled manner vs a controlled manner and that does mean higher octane fuel will tend to burn slower.

I agree that it is a waste of money to use it unless you have an engine that has its timing set for it.

 

[bruceb58]

As far as "older outboards" from the 60's/70's I would always run premium since the octane levels at the pump were higher then across the board.......

In the 60s and 70s they used a completely different Octane rating than they do now so the numbers don't mean the same thing.

 

[WIMUSKY]

In the 60s and 70s they used a completely different Octane rating than they do now so the numbers don't mean the same thing.

I'm thinking they still had higher octane than now since compression ratios were higher and wouldn't run very well on today's gas.... I run my timing as advanced as I can w/o pinging......As far as my car......

 

[NYBo]

I'm thinking they still had higher octane than now since compression ratios were higher and wouldn't run very well on today's gas.... I run my timing as advanced as I can w/o pinging......As far as my car......

Nope, Bruce had it right. There are/were 3 measures f octane rating: Research, Motor, and the average of the two. We use the average these days (R+N/2; this is what that sticker on the pump means).

 

[444]

I have n 82 Merc Black Max with a 175. I run 91 octane because that's the only one available around here without ethanol. I would not hesitate to run 87 non-ethanol. Really for me, the difference in cost per tank full is only around 10 bucks a fill. There have been times I've run the 10% ethanol stuff without problem.