88 Octane?

[JimS123]

I am running a one-owner 1984 Mercruiser 3.0 L 140 HP engine in my Stingray bowrider. The engine has run flawlessly from day one and I intend to keep it that way. I have always been meticulous about maintenance. It now has about 1200 hours. About 10 years ago I upgraded to Pertronics electronic ignition.

The owners manual calls for 88 octane, so I have ALWAYS used the middle grade gas (89), not the low grade 87.

I notice that Merc now calls for 87 octane in current motors. I don't lknow when the change was made. Should I switch to the cheep stuff, or keep up using the 89?

 

[Rocky_Road]

Re: 88 Octane?

You already answered your own question..."(t)he engine has run flawlessly from day one..." Why would you change?

Actually...there is a good reason to use one grade higher than the engine manual calls for. All gasoline loses some of it's octane, from it's 'born on' date to the point that it is burned. Unlike your personal car, your boat's gas load might sit for weeks, or even months, before it is burned. There is no downside, only good, from spending the extra money for the 89 octane.

Just my opinion...but I also run the middle grade!

 

[phoneguy925]

Re: 88 Octane?

I would say if your owners manual says your engine is rated at 88 then dont go lower than that you will end up leaning out the engine and cause detination. with newer ignitions and oil ingection systems they have been able to finaly make the engines to run on lower octain as well as more fule eficiant.

 

[Rocky_Road]

Re: 88 Octane?

P.S.

The loss of octane is even more pronounced in E10 formula...ethanol tends to seperate, and there goes the octane rating.

 

[JimS123]

Re: 88 Octane?

Thanks for the replies.

Obviously, its an inboard 4-stroke, so oil injection does not apply.

I just wondered why Mercruiser now specifies 87 for the same engine. What changes did they make? The only thing I can see is the points / condenser ignition was replaced.

 

[180shabah]

Re: 88 Octane?

Octane is gasolines "resistance" to ignition. higher octane rating = more "resistance" to ignition, lower octane lights off more easily.

Older gas is HARDER to ignite than fresh, not easier.

That said, if 89 has served well for 25 years, stick with it.

 

[mkast]

Re: 88 Octane?

Octane is gasolines "resistance" to ignition. higher octane rating = more "resistance" to ignition, lower octane lights off more easily.

The octane rating is a measure of the resistance of gasoline to detonation (engine knocking) in spark ignition, internal combustion engines.

What is the rated compression of the engine in question?
What is the factory specification for total timing advance?
For some reason, consumers in this country, feel better using the higher octane fuel, false sense of security. In reality, with the engine in servicable condition, using higher octane fuel does nothing for the engine.

 

[JimS123]

Re: 88 Octane?

For some reason, consumers in this country, feel better using the higher octane fuel, false sense of security. In reality, with the engine in servicable condition, using higher octane fuel does nothing for the engine.

I agree. That's why I asked the question in the first place. Merc specified 88 octane way back then, but doesn't any more. Why?

As far as your other questions, I don't have the answers. I was hoping somebody else would.

 

[Zackman]

Re: 88 Octane?

You want fuel to burn not explode (detonation). The higher the octane the slower the burn and the less likely dentonation is to occur under higher load conditions.

Engines are rated based on the fuel octane used during testing. To get maximum Hp rating out of this engine, the manufacturer needed to use a higher octane fuel so that they wouldn't need to retard the timing to prevent detonation. Newer engines have a knock sensor, so the engine timing can be adjusted if detonation is detected, allowing the safe use of lower octane fuel.

If you run your boat at full throttle or with heavy loads, you should use the recommended octane. If you take it easy with your boat, you could safely use the lower octane fuel.​

 

[Rocky_Road]

Re: 88 Octane?

...For some reason, consumers in this country, feel better using the higher octane fuel, false sense of security. In reality, with the engine in servicable condition, using higher octane fuel does nothing for the engine.

No one is disagreeing with this.

But...HIS engine specifies 88, and he is using 89. Hardly overkill. In fact...given the natural degradation of fuel octane from storage, he is right on the money.

Now...if he switched to 87 (which is, even fresh) is not to his engine specs...he would be making a costly mistake.

If you are pumping 93 octane into a Honda Civic, then your point would apply!

As for the change in newer 3.0's...my guess would be the compression first, and timing advance second. Or, more likely, the combination of the two. Factory detuning of engines has been going on for some time now...all for the 'sake' of the environment!

 

[mkast]

Re: 88 Octane?

1984 88 octane fuel doesn't exist. In 1984, the factory listed 88 octane as minimum.
Gasoline has been reformulated a number of times since the early 70's.
Engines, regardless of year of manufacture, are rated with the fuel available at that time.
Mercruiser considers anything over 8.5:1 compression as high performance.
All things being equal, (interpret that as engine is setup at manufacturer's specifications) who isn't going to run 87 octane in an engine with less than 9:1 compression?

 

[Maclin]

Re: 88 Octane?

There are 2 methods of rating octane, (R+M)/2 and RON. RON shows a higher number for the same actual octane. (R+M)/2 is what is on the pumps here now in the states. 87 would be 91 RON, 89 would be 93 RON, etc. It seems that most older boat engine manuals specifed in RON, some both but all of them should say which method. If the manual says 88 RON then 87 R/M is ok, and 89 R/M is not needed

 

[Fishermark]

Re: 88 Octane?

As your engine ages and loses compression you will have less need of high octane fuel. You can do what you want of course, but if it were mine, (and since you asked ), I would say go for "regular" 87 octane. Buy a can of octane booster just in case you notice the engine pinging... but I don't think you will. I think you will find your engine runs just as good on regular as it does on mid-grade.

 

[abj87]

Re: 88 Octane?

There are 2 methods of rating octane, (R+M)/2 and RON. RON shows a higher number for the same actual octane. (R+M)/2 is what is on the pumps here now in the states. 87 would be 91 RON, 89 would be 93 RON, etc. It seems that most older boat engine manuals specifed in RON, some both but all of them should say which method. If the manual says 88 RON then 87 R/M is ok, and 89 R/M is not needed

Yep i posted this up before in another thread... this should be a sticky.

 

[APPALOOSA2]

Re: 88 Octane?

I will stick with 89 in mine as a boat engine is always under a load unless idling. Less carbon build up also.

Work a newer engine in a truck designed for 87 under a heavy load and you will hear it start pinging and the computer kicks in to fix it.
So your valves don't last as long oh well !

No thanks

It's hard to hear a boat engine as the noise level is much higher , for the 10 cents extra times what 20 gallons .

Its like buying a 45,000 dollar vehicle and putting a 1.49 oil filter on it, does it work ...yes somewhat. No thanks

 

[deves3000gt]

Re: 88 Octane?

If you wouldn't mind me jumping in on your thread. I have a boat that i just purchaced with a 3/4 tank but it has been sitting for over a year. what would you guys think about filling it up the reast of the way with 93 pluse a booster. it runs fine on land all rpm's now. As for the orignal question i'd stay with what works.

 

[Rocky_Road]

Re: 88 Octane?

If you wouldn't mind me jumping in on your thread. I have a boat that i just purchaced with a 3/4 tank but it has been sitting for over a year. what would you guys think about filling it up the reast of the way with 93 pluse a booster. it runs fine on land all rpm's now. As for the orignal question i'd stay with what works.

I'd be sure that there is a fuel/water seperator installed...before the fuel pump. If there is one now...then be sure to change the filter element!

As for adding to your 'older' gas...I might get 'flamed' for this, but I would probably do the same (but skip the octane booster...they are almost ineffective).

 

[deves3000gt]

Re: 88 Octane?

Thanx Rocky yeah there is a water seperator and i plan on changing it tomorrow.

 

[Rocky_Road]

Re: 88 Octane?

Thanx Rocky yeah there is a water seperator and i plan on changing it tomorrow.

Then you should take a look at this short article about the numerous filters out there...there are two PDF pages, and the second page rates the different brands:

http://continuouswave.com/whaler/reference/fuelFilter.pdf

 

[JimS123]

Re: 88 Octane?

Thanks for all the good comments guys. I periodically run thru the rpm range and check speed performance at each level. I do that just to see if i need new plugs, etc. I have charts showing the results each year.

I'll do it again this year to get a baseline, then I'll run a few tanks of 87 thru her and see the new results.

Thanks again.