Ideal octane for a 420a

[Checkers]

Hey guys, I know it seems like a stupid question, but I can't seem to find anything definitive on the subject. I have a 99 Eclipse GS, and it has a 420a engine in it but not the 420a from the factory. I'm not sure what year the engine is from, because the previous owner told me it was from an '02, but that's a 3G and I don't think they have 420a's. Anyways, he told me he always used 91 octane to "keep the engine clean" so I've been using it so as not to shock the engine with a different octane. What octane should I be using, 87? And how do I safely transition if I need to?

 

[JKRacing]

I apologize for bringing up this 2 year old post. As for miles on my 420a I have no clue the body has just shy of 200K however the previous owner had done work to motor. whether this included a rebuild or not I am not sure. I do know that the head is a 98 not a 97 like my car is. So not too positive on motor miles. Cat is brand new within the last 1000 or so. Oh and as for my mother she only ever put 1500 miles on the car, all highway. I bought it from her cause it was too small for her. Full tune up was done just after my mother bought it so about 4000 miles ago. this included water pump, timing belt and all tensioners, injectors, spark plugs, wires, thermostat. Engine Oil, Trans, Power Steering, Brake/Clutch, and Coolant all changed. And it still ran like junk on 87. So go figure there.

 

[DSM1G90]

And it still ran like junk on 87. So go figure there.

Interesting ... had the timing checked out .... ? Wonder if the ECU is getting tired on you if that's never been changed out or at least checked over...