Oh, And Furthermore...
[Does this stuff ever become obsessive to you, or is it just me?
]
It occurs to me that maybe our dialogue is too slanted toward how much horsepower is "enough" horsepower. The more fundamental trade-off is between horsepower and payload, since the design gross weight is fixed. Bigger engines cost more in terms of weight (oh, and dollars).
Again, in the context of my notional RV-6's, let's look at the trade-offs and see how they effect engine selection. (I appreciate that this thread was about an RV-8. I don't mean to hijack it, since it's safe to presume that the same considerations will apply.)
Most -6's come out of the shop weighing around 1050 lb. plus or minus a bunch. Van's design maximum gross weight is 1600 lb. Of course, some people fly them heavier - in part because they shoehorn so much horsepower into them - but that was the gross weight designed into the structure. Consequently, the payload breaks down roughly as follows:
Empty: 1050 lb
Me and wife: 300 lb
Full fuel (38 gal): 250 lb
Baggage: 0 lb
Take-Off Wt: 1600 lb
It seems that we have a classic design for a light airplane; a design that allows a functional trade-off between range and payload. In relation to engine selection, and in light of the loading scenario above, would you trade payload for increased performance by installing a bigger engine? The weight difference between an O-320 and O-360 is on the order of 20 lb, and that's 20 lb that you can't carry in the cabin. I don't presume to have an answer, because it's a question of your intended mission. If I let testosterone speak, I'd opt for horsepower any day. The extreme example of that logic, in an RV-sized package, is the Extra 300. Incredible performance, but you can't carry anything.
Another way of looking at it: For roughly the same weight (dollars aside) I could have either an O-360 with a fixed-pitch prop or an O-320 with a constant speed prop. My bet is that the propulsive efficiency of the constant speed would more than make up for the lower horsepower.
Just thinkin' aloud. Opinions most welcome.
Rob Erdos