[2][3][4][5][6][7][8][9][10] The Mercedes-Benz 500I engine was slightly lighter than the Ilmor 265D Indy V8 it replaced in the Penske PC-23, although because of its longer inlets, the 500I had a higher overall centre of gravity, thus changing the overall balance of the car a bit.

[11][12] Much to the surprise of competitors, media, and fans, Marlboro Team Penske arrived at the Indianapolis Motor Speedway with a brand new, secretly-built 209 cid Mercedes-Benz pushrod engine, which was capable of a reported 1000 horsepower.

[13] Despite reliability issues with the engine[13] and handling difficulties with the chassis,[14] the three-car Penske team (Unser, Emerson Fittipaldi and Paul Tracy) dominated most of the month, and nearly the entire race.

[14]) allowed the Penskes to run significantly faster, giving them the pole and outside front row on the grid for the 78th Indianapolis 500.

However, in 1991, USAC quietly lifted the requirement, and purpose-built pushrod engines were permitted to be designed for racing at the onset.