Slight knocking at 4000 rpm, what octane should I use

[utadventure]

1974 Cobalt, 165HP OMC, 14 X 14p prop.

WOT tops out at 4500 rpm. We ran for a while today at 4000, and I started noticing a slight knock.

Two possible issues/solutions that come to mind are:

1 - use a higher octane fuel (currently using 85)

2 - timing could be slightly out and need to retard it a little bit

I'd appreciate any other ideas!

Dave

 

[Lyle29464]

Re: Slight knocking at 4000 rpm, what octane should I use

I would run 87 octane. drop the cruise to 3000, 3500 absolute max. Great engine just does not like WOT or near.

 

[sw33ttooth]

Re: Slight knocking at 4000 rpm, what octane should I use

+1 my motor doesnt like 4k+ "5.8l omc cobra" you can cruise 3800-3900 runs just fine i use 89 -mid-grade 100% of the time its what my owners manual calls for.

 

[Fordiesel69]

Re: Slight knocking at 4000 rpm, what octane should I use

You can use 87 if the engine is timed correctly. I run mine at 3700 for hours. After 2-3 min of WOT it can hear a slight ping but I do not run it like that often.

The L6 is just like anything, detonation can destory it in seconds. But otherwise one almost indestructible monster.