Mercruiser 260 HP 350, octane requirement?

[cadunkle]

What octane does a 1989 Mercruiser 260 HP 350 Chevy require? I've run 93 in it the few times I've taken this boat out just to be safe, as it doesnt' say on any of the tags on the engine the octane requirement.

 

[mtnrat]

Re: Mercruiser 260 HP 350, octane requirement?

Regular would be fine.

 

[sundowner205]

Re: Mercruiser 260 HP 350, octane requirement?

I have an '89 OMC Cobra 5.7 - the same Chevy block, etc as your Merc. My OMC Factory Manual says to use nothing lower than 89 octane...my boat is moored and the two fuel docks here don't sell anything lower than 89, however, I always put in 92.

I would be more concerned with the level of ethanol in the fuel around where you fill up. If you can find a station close by that sells ethanol free fuel, go for it!

This is some interesting info - scroll down to pages 2-3 to what Mercury has to say regarding "Alcohol in Gasoline" - ethanol is alcohol...an interesting read.

http://www.ethanolrfa.org/objects/pdf/2003marine.pdf

 

[thedillybar]

Re: Mercruiser 260 HP 350, octane requirement?

the spec is 87

 

[HT32BSX115]

Re: Mercruiser 260 HP 350, octane requirement?

This is some interesting info - scroll down to pages 2-3 to what Mercury has to say regarding "Alcohol in Gasoline" - ethanol is alcohol...an interesting read.

Actually THIS is what Mercury says about what fuel should be used in Mercruiser engines.

If you look in your OMC service manual there is timing setting that will allow you to use as low as 86 AKI fuel in your 5.7L engine. Depending on your serial number you simply time the engine to the setting specified in the manual. (it will make slightly less HP but I'll challenge anyone detect the difference) The timing change made a 5 HP difference (335 vs 340hp) with my King Cobra 460. NO ONE could tell a 5 HP difference in any boat unless they started with 5 hp!!!

Every manufacturer in that older 2003 article, indicates that it's "OK" to use "gasohol" if the ethanol level is 10% or less. 10% is the max allowed in the US according to EPA.

If you'd like to determine how much ethanol you have in your gas there is a very simple test.

You can buy a test kit or just make your own.

http://www.fuel-testers.com/state_guide_ethanol_laws.html

Also, many states do not require gasoline retailers to identify that their fuel contains ethanol. You may be buying E10 and not know it. In fact, you may have been using it for years and not know it. MTBE was banned several years ago and states that used it are now selling E10. Chances are you're using it.


Cheers,


Rick

 

[buckw13]

Re: Mercruiser 260 HP 350, octane requirement?

i have the same engine but 1990. I run 87 and have no problems. Every now and then i will add some 89 but have never noticed a difference

 

[Roman S]

Re: Mercruiser 260 HP 350, octane requirement?

Strangely enough I looked at the labels on my 1987 5.7l and the recommended octane rating was 88.

 

[cadunkle]

Re: Mercruiser 260 HP 350, octane requirement?

Alright looks like I'll try 87 octane then. Next time I hit the water I'll fill one of the tanks with 87 and see what she does. I'll have a buddy drive while I sit next to the engine and listen for any knocking or pinging.

Also I should check the timing as I haven't done taht since I got the boat. I did check/clean plugs, clean the carb, and go over a million other little things but the balancer and timing indicator are a bit crusty so I avoided setting the timing. I believe Merc calls for 8* initial, anyone know waht total timing should be? Also, do these engine like any more timing than 8* initial or wahtever the total is? In all the Ford engines I've run small block iron headed engiens typically like 12*-14* initial and around 36* total. Not sure what Chevy engines like, and I'm sure in a marine appalication the numbers are a little more cnoservative to avoid predetonation.

Also, what is the compression ratio on this 260 horse Mercruiser? Just curious.

 

[HT32BSX115]

Re: Mercruiser 260 HP 350, octane requirement?

Marine engine timing IS ABSOLUTELY more conservative! Don't even think of advancing "a little more" just to see what happens. You won't like what can happen. There's pictures on this board somewhere that show detonation-destroyed pistons.

Also, sitting next to the engine is a risky way to determine if the engine is detonating. it's usually all done by the time you hear it...(that's why they put knock sensors on these engines nowadays)

Detonation in a car engine usually doesn't cause damage right away....and you can hear it because it's MUCH quieter.....I'll also add that a car engine is NEVER loaded to as high a power level as any marine engine nor for as long..

But detonation in a marine engine (because of the MUCH higher load at any power setting....much like an aircraft engine), can cause immediate (or near immediate) catastrophic damage like broken pistons or holes in them.

If you're planning to do your own maintenance etc now would be a good time to get an OEM service manual.


Regards,


Rick

 

[180shabah]

Re: Mercruiser 260 HP 350, octane requirement?

87 and get a real FSM