Many many years ago, when I had my Bronco (1985, 5.0 EFI), and I put 87 octane fuel in, it pinged a bit. 91 octane was fine. I used to fill up in the US, and had to use 92 or higher octane to keep the ping away. A short time later, I replaced the timing chain, and voila, the ping was completely gone. I've always assumed that it was the stretched timing chain causing the ping.