Octane for Mercruiser 165 hp

windsors03cobra · Mar 18, 2009

Which octane for the Mercruiser 165hp Chevy 250ci ? I think its 8:1

Thanks

Silvertip · Mar 18, 2009

Re: Octane for Mercruiser 165 hp

87 octane pump gas.

APPALOOSA2 · Mar 18, 2009

Re: Octane for Mercruiser 165 hp

This will probaly open up a can of worms but I would not put 87 in my lawnmower much less my boat. The 250 straight 6 is a older motor which required leaded fuel.
Now I would ASSuMe you are not the orginal owner, so you don't know if the valves and seats have ever been redone to go with unleaded fuel.

If I did not know the answer I would either pull the head off and take it to a machine shop that rebuilds heads or I would buy lead substitute and 89 octane.

Car engines don't work half as hard as a boat engine.

My 2 cents.

ziggy · Mar 18, 2009

Re: Octane for Mercruiser 165 hp

this is the sticker (on my valve cover) on my 75 165hp L6....

i don't know what method they use in my hood to determine the octane rating. but i do believe that premium fuel (w/ higher octane number) has more anti detonation properties to it. and since detonation isn't good. i use premium... w/ a lead additive for my non hardened valves..

windsors03cobra · Mar 18, 2009

Re: Octane for Mercruiser 165 hp

I have the head sitting on a bench. Its is in really nice shape, very low hours.
I guess I'll run the mid grade.

bigred95stang · Mar 19, 2009

Re: Octane for Mercruiser 165 hp

Can I just chuckle about the fact that there is an oil for use below 32 degrees on a boat......

If you have the head off, go ahead and just have the seats replaced and be done with it.

abj87 · Mar 19, 2009

Re: Octane for Mercruiser 165 hp

You should be able to run 87 pump octane, a while back they changed the way it was rated from the research method to (R+M)/2 which is an average of the motor method octane and research method octane. which ended up lowering the number by 5 points.

that sticker lists it as 93 motor octane which in the new method is about 87.
Other than wasting money there is NO reason to run higher octane than required.

You will still need the lead additive, or you can get hardened valve seats installed.

edit: yours is 77 which was built during the phase out of lead in gasoline so who knows what if you need the additive or not, check the manual if you have one.

Fishermark · Mar 19, 2009

Re: Octane for Mercruiser 165 hp

You don't need the lead additive and you don't need any higher octane than regular 87. Anything higher wont hurt, but you certainly don't need it.

Silvertip · Mar 19, 2009

Re: Octane for Mercruiser 165 hp

That 250 six is an 8:1 compression motor. It needs 93 octane fuel like I need another hole in my head. Besides, there will be no benefit to using it. If it were an ECM controlled motor with a knock sensor, yes. That engine grew up in the emissions erra of retarded timing, smog pumps, and wimpy cams. It simply does not need high octane fuel.

bruceb58 · Mar 19, 2009

Re: Octane for Mercruiser 165 hp

APPALOOSA2 said:
This will probaly open up a can of worms but I would not put 87 in my lawnmower much less my boat. The 250 straight 6 is a older motor which required leaded fuel.
Now I would ASSuMe you are not the orginal owner, so you don't know if the valves and seats have ever been redone to go with unleaded fuel.

If I did not know the answer I would either pull the head off and take it to a machine shop that rebuilds heads or I would buy lead substitute and 89 octane.

Car engines don't work half as hard as a boat engine.

My 2 cents.

The octane number and the lead in the fuel are two different issues completely.

Regarding the octane, if the engine does not ping, it is fine. If it does, you retard the spark until it does not ping under load.

Regarding lead and valve seats. If the engine does not have hardened valve seats, run the engine "as is" until you start losing compression due to seat failure. Why take the head off now when in all probability, it will last a long long time...possibly for as long as you own your boat.

rockyman · Mar 19, 2009

Re: Octane for Mercruiser 165 hp

i have a 1978 inline 6 cyl in my 1981 galaxy 19.5 ft and i have ran the regular unleaded the mid grade and the premium in mine and it runs and sounds the same on all three even though sticker states the same as yours does .

HT32BSX115 · Mar 20, 2009

Re: Octane for Mercruiser 165 hp

Regarding lead and valve seats. If the engine does not have hardened valve seats, run the engine "as is" until you start losing compression due to seat failure. Why take the head off now when in all probability, it will last a long long time...possibly for as long as you own your boat.

Quite true....it WILL last a good long time.

My previous boat was a 1966 Mark Twain (150 [GM L6] Mercruiser) that my dad bought new.

That engine never had "hardened" seats and it almost exclusively got unleaded fuel most of it's life.

When I sold it in 2005, it was 39 years old and never had the head off.......

I think you'll get away running regular unleaded without "lead substitute"

Marvan · Mar 20, 2009

Re: Octane for Mercruiser 165 hp

Can the same be said for my 1971 Merc S/6 no points, electronic installed took it out for first time season on the Sacramento river, and it ran fine for about an 1/2 hr and started backfiring really bad. I winterized like the good book said, it has been setting for 2yrs but I would start it up and run it for 5-10 min and shut it down. Do you think that gas has broken down and thats the cause if the backfiring? I've never ran a lead additive just the Octain boost. Should i start?

Fishermark · Mar 20, 2009

Re: Octane for Mercruiser 165 hp

Marvan, your problem has nothing to do with the lead issue. It could be old gas however. You will be better off starting a thread with a question of your own.

Octane for Mercruiser 165 hp

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