I remember by old 240k miles VX motor hated to have 87 octane in it. To make it not ping I would always have to shift sooner when trying to keep the RPMs low, or I would just run mid-grade.
After I installed the 120k mile motor, it loves the cheap gas. It runs better on the cheap stuff than the old one ever did on the more expensive gas. (Both motors had a full tune-up done on them...I'm guessing the old one had a lot of carbon buildup?)
On the never-ending quest for better gas mileage...
The question is mostly for VX owner, but in same time to everyone.
I do run Supreme fuel in my VX since I own it (91-92 octane). Where I live, there are lost of hill to climb and since I don't want to downshift, I put supreme fuel so the car run better (especially at low rpm because VX does run often below 2000rpm). Also, since I'm crussing 60mph (100km/h) on highway, I'm telling myself that I have a better FE with supreme fuel.
I would like to have your thoughts on this.
A mechanic whose opinion I respect said that high octane fuel will make your car burn cleaner (less polluting) but have worse FE. This is what we see with Hybrids. They get okay FE, but could do better if they weren't sooo much cleaner burning than conventional cars. Soo... not sure that it will yield you better FE, but it is better for the environment. Mean how that works, eh? heh
I should test it sometime with mine. Does yours get the same or better FE with high octane?
Wait, your Civic Si? I wonder if the sporty nature of your car has anything to do with it. Most sports cars can only use high octane gas. And the Si is a pretty sporty car. Well, that could be the reason, I dunno. My point is, your car prolly isn't going to be representative of non-sporty cars using high octane gas.
Our cars weren't built for higher octane, stock combustion would be off...if anything I would say it would hurt mileage and possible emissions (slower burning fuel doesn't combust completely without timing changes and such).