octane levels

GoodysGotaCuda said

Just seems to be common knowledge. Not only compression ratio factors into it. Things like piston design, head chamber design, ignition timing, outside conditions, like you mentioned aluminum heads.

Seems to be '10:1' is 'max' for 91/93 octane with cast iron heads. And 11:1 is 'max' for aluminum heads for their tendancy to remove heat faster from the combustion chamber. Excessive heat = can lead to detonation. Octane is rated in 'antiknock' resistance. Charles law, higher pressure = higher temperature. Lower pressure = lower temperature. The more pressure you put into that cylinder (compression ratio), the more heat, and the more 'antiknock' (octane) you need to safely run it without detonating.


Atleast thats my understanding of it at the moment. :D

Click to expand...

The head chamber design and the duration of the cam has everything to do with the static compression you can get away with.

I have a 360 with iron 64cc magnum heads and zero deck flat top pistons and a XE268 cam. This makes for 10.6:1 compression and the engine runs just fine on 89 octane, never tried 87 and mostly use 93 because it's only a $1.50 difference on a fill.

This works because of the closed chamber and ideal quench with the flat top piston.

Anyway, a big cam bleeds off a lot pressure so an engine with a near stock cam and 10:1 compression may ping like the dickens but put a cam with a lot duration and no problem and a real BIG cam you may get away with as much as 12:1.

So, you can't just through numbers around, you need to know the chamber design, the size cam and the head material to have any ideal of what you can run.