O320 and O360
But engine prices do differ a bit and so do their fuel consumption. You can get a 0-320 or smaller for a -9 for thousands less than a 0-360.
I think you have to be careful with this statement as there are alot of common misperceptions out there. It is only true if you are comparing a new 360 with a used 320. If you are comparing new with new, which is what I think the poster was asking. the price between an comparably equipped parallel valve O360 and an O320 engines is negligible. You pay only a few hundred dollars for the additional displacement. Also note that if you go with a new Lycoming clone from Mattituck or Superior, the cost for fuel injection is also negligible, about $300.
As far as fuel savings, An O360 will burn more than an O320, but it pushes the plane faster as well. If you throttle back to the same speed, you will burn about the same amount.
Here is another misnomer. I've had many tell me to buy an O320 because it is lighter. Ive recently investigated this carefully and an O360 is only 7 pounds heavier than an O320. Precision airhawk fuel injection adds 3 pounds to either the 320 or 360 if you go that route.
Another thought, If you throttle an injected O360 back to the same speed as an O320, it will burn less fuel than the O320. Of course you can also get an injected O320 as well.
You are barking up the wrong tree if you are building a 9 to save money.
I think both are great planes, but given the choice, the RV-7 is a no brainer for me. You can put a parallel valve 180HP engine in it (the 200HP is really pricey and much heavier), and maintain acro capability. It is just too much fun to pass up!!! You will also have a faster airplane as you can push the throttle in all the way without passing Vne.
Remember. speed is your friend!!!
Regards,