Here’s the specs on one of the naturally aspirated Mugen V8 race motors. Lots of rumors of that the replacement for the NSX will be sporting a street version of this set-up. 400hp and 250+ tq out of a 4.0 would defintely be doable.
Mugen MF408S engine
Debuting in the 2002 ALMS (American Le Mans Series), this 4 liter V8 engine has seen action every year since then. In 2004, it was used in the JGTC Series by the VEMAC RD408R GT500 class racing machine.
Specs for the MF408S in the VEMAC RD408R
Layout: 90 degree V8 Naturally Aspirated
Displacement: 4000cc
Horsepower: 590hp / 9500rpm
torque: 383lb/ft / 7500rpm
Also, theres some pics on this page of the engine above fitted into a Japanese Legend (aka Acura RL in the US)