87 or 89 Unleaded?

[VPC20HD]

Hey everyone,

I have an 82 C20 that originally had a "M code" 350 sb with all the emissions stuff (California).

After research, my understanding is that you were only supposed to put 89 unleaded with that engine.

When my dad first purchase the truck, he didn't know this. He always put 87 and eventually had to replace the engine.

In 2006, he put in a new 350 crate engine from Chevy and hasn't really been driven since.

My question is, Do I still need to put 89 in it or am I good putting 87?

 

[Ricko1966]

87 is fine better actually unless your 89 is ethanol free. More octane means more resistance to burn. Good for fighting preignition,but not better for making more power or better milage unless you tune specifically for the higher octane.

 

[JGN62]

I also have a 350 "M" with all of the California emission equipment. I initially ran 87 octane, but after listening to the knock and ping on a highway trip, I quickly pulled into a service station and mixed the tank with 91. Within minutes the sound had lessened to the point that I was able to relax and drive. I now run 89 to 91 based on price and availability. The engine runs smooth and quiet now. Performance is the same with 87-91, but if you push the engine with heavy sustained knock from 87, you will do damage over time.

 

[Grit dog]

After you’re sure it’s timed and tuned properly, the lowest octane that doesn’t cause pre-ignition, pinging, is the right answer. Or dieseling. Although dieseling can be caused by other things aside from the need for more octane.
IMO there isn’t a factory 80s 350 made with enough compression to demand higher than 87 octane if it’s running right.