If all you do is bump compression one full point, you will see somewhere between 4-5%, depending on actual compression, before changing anything, if you do absolutely nothing else.
And no, if you paid any attention to what I said, none of it is crap.
I've tested gas stations for water content. Doesn't take a lab. Fill a can, pour some in a mason jar, seal it and shake the crap out of it, then watch it. You can see water separate, if the ground tanks are faulty.
I've seen the numbers in MPG increase, running mid grade over min. This is likely due to octane rating increase, allowing a better tune, combined with more consistent results coming from the stations. Water content doesn't change within the same gas station, just use a known, good gas station (test it yourself)
There is a difference between the capabilities of an engine that runs 8:1, against 9:1.
It just takes a minute to think about the change and how it will change the combustion.
Assuming you've done everything you can to the engine, at 8:1. Timing advanced as much as it can be, air to fuel ratios are sound, throttle positions are correct, etc... Now think about how that fuel is going to react under load, under 9:1 ratio, with absolutely no other changes to timing, fuel, etc.
If you've got the engine tuned so that 8:1 is pulling as much out of each cycle as it can get, there is no room for more advance in the combustion. Anything you do to that combustion is going to cause pre-detonation.
Can you tune a car to run on regular, at 9:1? Yes. It requires LESS timing, which will hurt performance and defeat the purpose of moving up in cylinder PSI.
When you change CR, you change how the engine deals with fuel. This changes the entire tune. Higher cylinder pressure will respond on a higher percentage level to all performance tuning.
It's worth it, if not just for the hardened seats. They are induction hardened, so don't look for ductile iron seats on the exhaust. The entire combustion chamber is already there. The lack of pitting on the seats will tell you, on a used set that is in good shape.
Any increase in cylinder psi/ drop in cc is going to help the engine make more power, per cycle, increasing your potential fuel economy, as long as you tune it for better fuel, it will return the favor.
If your dynamic is down at 8:1, 87 being min-rating octane (I would assume) in your area, I think you are asking for it with that vehicle.
I thought I wanted min octane (85 up here), until I started toying with mid-grade and higher fuels and saw results. Even if it's a wash in dollar per mile, you'll find more consistent results at stations. In my experience, less water in the tanks, too.
If your TBI corrects from o2, you might actually see more out of mid-grade than my carbed engine would.
I'd play with some mid-grade on that engine to see what it does and stick with something that will work on 87 with the build.
It would be nice to hear from someone else running TBI. On the Toyota truck with port injection I saw results in, it went from 20-22mpg, by just changing fuels, on 8.7:1 CR.
Min octane sucks for everything, if you're willing to tune. I promise.
There are guys fetching awesome MPG in big block cars on the HotRod Pro Tour, with O/D, running premium, because they dial their engines and set the car up right.
