Recommended fuel for 1973 inline 6

[Campfire]

The boat is running like brand new but one thing has always bothered me since I got it a few years ago (for free…. Yeah, right…). There is a placard that states “Leaded fuel required. Minimum 93 octane research method”.

The engine is a 1973 Mercruiser 250 ci inline 6.

All I have ever run is 87 octane gas station gas and 89 octane from the marina. The compressions are down about 15% from the Mercruiser specs but fairly even.

Are the lower compressions the reasons why I have not had any issues or is it simply luck? Should I be using a lead additive in my fuel?

 

[alldodge]

In the 60 and 70's premium was 98 to 102. Later in the 70's it went down to 95 to 98 and the 80's to 94 to 95.

In short the 91 of your motor equates to 87 today

 

[Scott Danforth]

did you redo the head to change to hardened valve seats?

if not, what ever fuel you get, you should be running lead substitute in it.

 

[Bondo]

Ayuh,..... While I don't disagree with anything said above, those motors will run for decades on 'bout any clean gas ya pour into 'em,.....
'n the odds are, you won't live long enough to put enough hours on that motor, with lead free gas, to cause sunken valves, 'n if ya do, have it rebuild then with new seats,....

'n if you can find ethanol free gas, Use it,....
Ethanol seems to amplify any sorta issue that a gasoline fuel systems might endure,......