87 Octane

[Simonpie]

The plus of the Model A is the low compression, so detonation is rare. The minus is the gigantic combustion chamber, with a spark plug miles from the far edge of the chamber. This promotes detonation. I'd say- rule of thumb - that a stock compression engine is fine on low octane and a high compression engine would need higher octane. I haven't heard detonation in mine, but most cast iron engines are pretty audible. If it sounds good, don't worry about it.