Hey guys I have a knock in my 97 4.6 212k miles, it is only there if I run 87 octane, 93 makes it run perfect. Can I buy a thermostat that keeps the engine slightly colder that would allow me to save $.2 a gallon and not cause any negative side affects in the engine?

 

Not really. I would address the knock. It can usually be caused by clogged EGR ports in the intake. Verify that your EGR system is working. At an aile, apply vacuum to the EGR valve. The engine should start running rough or die. If not, the ports are probably clogged and causing pinging.
You might also clean the MAF. If dirty, it can cause it to run a bit lean and also cause it to knock.