I recently purchased a 66 Mustang GT fastback 289 hi po. Anyone know what the required fuel / octane should be? One muscle car rebuilder suggested sunoco 94 and to add a pint of no-lead substitute in addition and octane booster, such as 104+.

Thanks

 

you need to run the highest you can get in your area and listen close for pinging. if it does, you need to drop the timing back some until it doesn't ping. adding 104+ will get to be expensive over time when you can retard the timing. if the valve seats are original to the heads, you do need to run a lead substitute to keep them lubricated as the no lead fuel doesn't do the job on older, non hardened valve seats. the seats can be replaced with ones designed for unleaded fuel so you can get around the additive need.

 

Back in the day. 1965. I ran shell premium, but then our gas had lead. Good luck.