Car 2: Alfa Romeo 164 Procar. As the only car in its category, it is completely unique - a special Formula 1-style V10 engine, producing 605 hp, was used. It has a Performance Index value of 700.
Car 3: Ferrari 166 F2. This 1967 F2 car has a 1.6 litre V6 that produced 210 hp. Compared to its contemporaries, it lacked power but had a reasonable chassis. It has a Performance Index value of 448.
Car 4: Miller V16. This was an entry for the 1931 Indy 500. Despite showing lots of promise, and a healthy power output of 300 hp, reliability issues prevented it from getting the results its pace showed. It has a Performance Index value of 392.
Car 5: Penske PC23 Mercedes-Benz. A car that literally changed the Indy 500 rule book after it - perfectly legitimately - exploited a loophole involving the fact that pushrod engines could be utilized with a higher engine capacity. Its Mercedes-Benz engine produces a whopping 1,024 hp. It has a Performance Index value of 841.
Car 6: Bugatti Type 35B. The early-1930s GP racer everyone wanted. With 135 hp, it seems a bit modest now, however! It has a Performance Index value of 327.
Car 7: Cunningham C-5R. This 1953 GT car incredibly finished 3rd beyond the all-conquering D-Type Jaguars of that year's 24 Hours of Le Mans, thanks partially to its potent 300 hp Chrysler V8. It has a Performance Index value of 437.
Car 8: Ferrari 308 GT/M. This Michelotto-built car was actually a Group B car, however it was only ever entered in one rally - the 1984 Monza International Rally, where it lead until it suffered a puncture. With 363 hp, it's not surprising! It has a Performance Index value of 633.
Car 9: Ferrari 712 Can-Am. As the name suggests, it is a 7 litre, V12-engined Can-Am car, producing 750 hp. Due to Ferrari's low interest in Can-Am, however, the chassis was very underdeveloped, so it must've been a massive handful! It has a Performance Index value of 709.
Car 10: Chaparral 2K Cosworth. This car's claim to fame is that it introduced ground effect to the CART circuit. Only reliability glitches prevented it dominating its first season, where it had to wait until the last race for its first win. In 1980, however, it comfortably took the title. With 780 hp, this is a terrifyingly fast car. It has a Performance Index value of 883.
Car 11: Porsche 909 Bergspyder. This hillclimb car weighed just 385 kg, so, with 275 hp, it was massively quick. Due to the lack of any real competition, however, it was used sparingly, with the heavier Porsche 910 used more frequently instead. It has a Performance Index value of 584.
Car 12: Lotus 56. This car used a 500hp gas turbine, AWD transmission and a highly aerodynamic shape to great effect. Sadly, legislation was constantly brought in to severely restrict the car's performance and to favour piston-engined cars by the USAC. It has a Performance Index value of 658.
EDIT: Whoops, looks like it was missing the Lotus 56, and had two unnecessary Bugatti Type 35s instead. That's now fixed!