I went on a rant and stole Axel's thread, I'm giving it back.
Here's the summary. I'll post here in the future as it progresses.
Dave Anders(most effient RV-4 ever) has some great info, search his name on here. From memory, the biggest improvements he found has getting air smoothly out of the cowl and under the belly. He made various ramps forward of the firewall and tweaked the exit geometry. So the exit air thing doesn't surprise me.
Here's a link to an old post I made here years ago.
I think these are notes from a talk, they're hard to follow, but appear to loosely diary the updates he made before for making his CAFE killer N230A.
I'd love to put together a site that was just dedicated to what he did and how. He doesn't post much to VAF or anywhere, what he has posted is astounding. 243mph top speeds with 2" of ram boost (perhaps exaggerate but not much). It seems he's made over 100 modifications over the years, I know I've seen lists of alterations and the delta.
Oh btw, I'm buying back the fastback RV-4 N254MM I built. If we ever meet we should take pictures together some time. I'd like to share ideas for getting it even more efficient.
I have a pretty well matched Catto prop, IO-320 (balanced injectors) that does 5.5gph at 140ktas @8,000 running lean-of-peak ~2300RPM. I'd like to focus on the air frame improvement next.
Cowl inlet/exit (James cowl?)
The canopy is a mess in general, I need to rebuild it, when I do I'll lower it about 2"
EFII injection system
I home in combination this gives another 8-10 kts on that 5.5gph cruise or ~4.8 gph 140ktas.
Other than the fastback and the ubiquitous PR wheel pants, my engine/airframe are totally stock RV. The engine started life as an O-320 that I converted to injected and has new slick mags. I have the very stock 2:1 exhaust. I'd like to put together a thoughtful upgrade plan and execute one at a time and re-test after each and get numbers just as Dave did.