SuperSeven said:
I know that My Prelude S, with it's F22 engine ... and i'm fairly certain anything V-8 needs premium.
wow... no offense dude but these are incorrect and ignorant statements, respectively.
Here we go....
In the compression stroke the sparkplug fires a set number of degrees of crank rotation before top dead center (TDC). This happens as the pressure within the cylinder is rising, and after ignition the pressure heads quickly to its maximum. Cylinder pressure peaks several degrees after TDC which means the piston is already on its way back down the cylinder at peak pressure. Imagine that curve. (I'll post a graph)
Using a lower octane gas shifts the curve to the left because peak cylinder pressure is acheived earlier in the power stroke. The majority of the work in the power stroke occurs between TDC and when your exhaust valve opens (the rest of the work happens between firing and TDC, but that is minimal)
So when you shift the curve to the left you reduce the area under the curve post TDC, which is where all your power is. Also, consider it as decreased relative pressure as the volume of the cylinder is increasing, which basically means that gas is just filling the space the pistion leaves behind, rather than forcing it back down.
A knock sensor looks for earlier peak pressures and retards the ignition timing accordingly. Your ECU wants the mixture to mount pressure later to get the 87 pressure curve closer to the curve for 91. That's theory....realistically every ignition system is advanced as far as possible to match its other components and that becomes the upper limit of the range of octanes your engine can handle. That is why putting 91 in your CRX HF gets you jack dick, other than closing the gap between what the gas station sells you (85ish) and what 87 really would be.
Then depending on the ignition system's spark distribution system (spring, vaccuum, oil line...) there are limits to how much ignition retardation your system can do. So it will retard timing within a range, and that becomes the range of octanes you can use without pinging.
That said I am tempted to believe that Honda would give us at least an 87 to 91 range...but that is untested as by me.
Short story long - I think you can use 87, but despite all my blathering, I don't know.
EDIT - your best bet is to empty your tank and put a gallon of 87 in to test it. If it pings, fill her up with 91. If you just lose power then that means 87 is within your engine's ability to retard timing.