We've been running nothing but the 91 octane available in AZ in our 69 Z since I refreshed the engine 5 years ago and have been daily driving it. Over 40k miles now, Runs perfectly fine, but these little engines actually have enough camshaft in them to bleed off quite a bit of cylinder pressure. But I knew that going in from building similar stuff like this in the past.
I built it back with exactly 11:1 compression, the original iron heads, a slightly tweaked version of the original 30/30 cam that has a little more lift and tighter lash, and a few subtle things internally for more performance. It loves 36 degrees of total timing with an additional 10 degrees dialed in with a vacuum can. In fact it gets 14 mpg around town and 17 highway with the muncie. After putting in a 5 speed highway mileage jumped to 22 mpg. It's been a fantastic running car on 91 pump.
On the BBC's like the L78's and LS6's, I could see where it might need some help in the gas department. Those factory cams didn't act quite as big in those engines.
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