[Fred43]

I have a 1966 Mustang with a 200 CID engine and 3-speed transmission. What octane is required.

Thank you.

[02mingrayv6]

use 91 or 93 its more puree

[Bocephus357]

Welcome to AFM! Check the classic talk and tech sections for tips and info

[Ivy66GT]

Your engine should have a 9.2:1 compression ratio which would have been regular fuel in 1966. In today's fuel numbering system a 9:1 nominally needs a 91 octane and a 10:1 needs 95 at sea level. As you go up in elevation the needed number goes down. Your 600' elevation only reduces your needs back to about the 91 octane level from the 92 it would be at sea level. Those are all near, or above the current day 'premium' rating. If your engine is tired its octane needs may be less (from lost compression) or more (from carbon buildup). There is no advantage to using a higher octane than your engine requires unless you like paying more. Most premium is sold to people whose cars don't need it but then few new cars have the compression of your old one. Find what works for your engine.

If you lived at a higher elevation, like some of us, that engine would definitely be a regular car. With thinner air, compression is effectively reduced and less octane is needed. At our 6150' your engine would only need about a 92-9 or 83 octane. Even the worst regular up here is 85.

If it makes knocking noises more than just occasionally you need to feed it more octane.

[andrewmp6]

Depending the timing you can get away with 87 octane.

[nigel]

Welcome to the site.

[Fred43]

Thank you all. As my hearing won't detect spark knock, I'll play ity safe with 91 octane minimum.
Thanks again.

[66Sprint200]

I have been using regular since we had my engine rebuilt & my carb replaced. Every once in a while when I have the money I will treat my car to premium gas, but I have never noticed any real difference.

[Ivy66GT]

Originally Posted by 66Sprint200

Every once in a while when I have the money I will treat my car to premium gas, but I have never noticed any real difference.

In that case regular is likely all you need. Every engine is a little bit different. Using more octane than required is only a treat for whoever sold you the gas. Your engine could care less unless you try to feed it too little octane. It is not uncommon for the premium grade to have less energy content, i.e. worse mileage, than the regular. That is especially true if the extra octane comes from added alcohol which only has 2/3 the energy content of real gasoline. Last summer somewhere in mid-America's corn belt I saw they were selling regular and premium for the same price since the only difference was more added ethanol.

Timing, etc., can affect the octane requirement but these engines knock if they need more octane. At sea level 91 is usually only a mid-grade but that isn't a hard rule either. At altitude 'premiums' can be as low as 89 octane.