With a naturally aspirated engine, when you really throw money behind the R&D and engineering, you get things like:

Ferrari 599GTB Fiorano - 5,999cc V12; 620HP; 103.35HP/L
Ferrari F430 - 4,308cc V8; 490HP; 113.74HP/L
Lamborghini Gallardo LP560-4 - 5,204cc V10; 553HP; 106.35HP/L
Audi R8 - 4.2L V8; 420HP; 100HP/L
BMW M3 - 3,999cc V8; 414HP; 103.53HP/L

When the budget is more modest you get things like:

Ford Mustang GT - 4.6L V8; 300HP; 65.22HP/L
Dodge Charger R/T - 5.7L V8; 370HP (2009 spec); 64.91HP/L
Dodge Charger SRT8 - 6.1L V8; 425HP; 69.67HP/L
Corvette Z06 - 7.0L V8; 505HP; 72.14HP/L
Dodge Viper SRT/10 - 8.4L V10; 600HP; 71.43HP/L