Its a 97 f150 4x4 4.6 v8 auto with 205k on the clock. It has a k&n open element kit and flowmaster exshaust, other than that its all stock. Now the problem. Whenever I'm accelerating up a grade or on level ground, I start to hear the engine ping. (I'm not putting the hammer down when this is happening, just a good solid acceleration.) I run 91 octane fuel from chevron and from unical 76. I seem to notice it more when using 76 than chevron. This bothers me cause I take such great care of this truck and besides this problem it run great. Please help a blue oval brother in need.

 

I have the same thing on my 98 4.6 but using 91 octane my pinging goes away.