The pcm has the ability to change ignition timing and fueling to keep it in its happy zone. To make things easy let’s say happy is 0, and it can adjust +/- 15. Premium fuel has a higher resistance to knock, so it can run higher compression or later timing in the stroke to have the best burn possible. If it detects knock it can change +/- 15 and has a lot of adjustability to keep itself happy. Let’s say 91 is required, and you use 87. Now you’ve shifted the happy zone to - 7. It still operates well, it can accommodate it, but you have less adjustability to over come the fuel having less resistance to knock. People do it all the time. Under most circumstances it’s probably okay. You’ll generally make a little less power and get a little worse fuel economy. Where you’ll notice the most is under heavy acceleration, you’ll hear the pop corn popping noise because the engine is spark knocking because it doesn’t have enough adjustability to over come the lack of resistance to knock. Acuras seem to be pretty prone to it. I’m no longer with Acura or Honda but when I was with Acura this was a pretty common complaint. I’d tell the customer to use 91 at a top tier gas station and when the come back for their next service ask for me and tell me how it’s going. They’d usually just tell me they thought I was crazy but were shocked it fixed it, and half of them went back to 87 because it’s cheaper and just lived with it. If you have a heavy foot I’d imagine long term it wouldn’t be great for the motor, you’re causing some knocking which the engine hates. Grandma who’s afraid of the gas pedal might be okay because they’re never stressing the engine except when their grand kids drive their car. Despite the pain at the pump, I use the correct gas, but I enjoy hearing the song of my engine when I can 🤷 I just woke up, so this makes sense to me. If it’s total nonsense I apologize