Some more info on
Radical's designs for 2006 ... they fully intended to modify their 2.8L V8 "twin Hayabusa"
Powertec RPA engines to 3L versions for their SR9 LMP2 project. Schedules were too tight and they opted for the AER LMP2 custom one, but apparently the original idea was fully viable. How competitive a normally aspirated 3.0L engine would've been against the 3.4L ones (largest displacement allowed for the class), I don't know.
Anyway, it all came back to me as I discovered another version of putting two Hayabusas together, namely
Hartley H1 V8 ... specs are close to Powertec's version:
75 degree odd fire V8
2.8 LITER, 170 cu. in. displacement
84mm Bore X 63mm Stroke
4 cams, 32 valves via internal silent chain
530mm wide x 485mm long x 530mm high
400HP @ 10,000 rpm with stock street cams
245 ft-lbs torque @ 7500 rpm
200 lbs engine weight
Billet nitrided steel 180 degree crank
4340 Carrillo H-beam rods
Billet 6061-T6 aluminum crankcase
Dry sump oil system with 4 stage pump
7.25" or 5.5" Twin disc clutch
Image routed from Hartley website
The engine itself weighs less than most four cylinder ones, and is really compact. First I got to thinking that by adding two cylinders ("another V"), modifying the ignition, cam and crank accordingly and shortening the stroke minutely one would get one mean normally aspirated 3.4L V10! (Easily 500HP+ with custom parts) But as modifications go, that might prove as hard as building an engine ground up? It wouldn't be so daunting, perhaps, if the Hayabusa was a V engine originally, but it's a straight four, so adding a modification dimension to that construction ...
Well, as long as ideas are thrown around ...