The Mercedes-Benz 500I had one real shot at the Indianapolis 500, and it won. Ilmor built the pushrod V8 in secret for Team Penske in about 10 months. Then Al Unser Jr. put it on pole and drove it to victory in 1994.

The engine came from a gap in the rules. At the time, Indy racing let pushrod engines run more displacement and more boost than overhead-cam engines. Those breaks were meant for engines based on production blocks. However, Ilmor found a way to use them with a pure racing design.

Ilmor’s Mario Illien later put the result in one sentence. “I promised 940bhp, we achieved 1024bhp at 9800rpm,” he said.

Where the Mercedes-Benz 500I idea came from

Illien told the story to Motor Sport magazine in 2007. He says the idea came up over dinner in Phoenix in 1993, with Roger Penske and Paul Morgan at the table.

The rules were the key, according to Motor Sport. USAC’s pushrod formula allowed 3.43 litres, a 30% larger capacity than the overhead-cam engines. It also allowed 22% more turbo boost. Earlier, those engines had to use a stock road-car block.

For 1994, though, a stock block was no longer mandatory. As a result, a purpose-built pushrod engine became legal. Illien expected the rules to change once the engine raced, so the team had only one chance. “We had one go at it, that was all,” he told Motor Sport.

Ten months, in secret

The timeline was tight. According to the Mercedes-AMG F1 team’s history of the project, Ilmor had just 10 months to build the engine. Work began in the summer of 1993. Motor Sport also says Illien designed it in “a period of only 10 months”.

Secrecy mattered just as much as speed. “Everything had to be secret,” Illien told Mercedes. In fact, the car and engine did not break cover until April 1994, only weeks before the race.

Forix’s account adds two details from inside the project. First, Ilmor engineer Jeff Williams pushed for a softer valve-train design against Illien’s wishes, and that change unlocked the power. Second, Paul Morgan flew engine parts from Ilmor’s Brixworth base to London in a World War II fighter, so they could catch a Concorde flight.

Mercedes says the engine carried the name Mercedes-Benz 500I. Its nickname was blunter: “The Beast.”

How the pushrod V8 was built

The engine was a turbocharged V8 with pushrod valve gear. Forix’s 8W history of the Penske PC23 calls it a “purpose-built V8 turbo engine” with pushrod valve actuation inspired by stock blocks. In other words, it borrowed the old layout but none of the production parts.

Illien chose a 72-degree angle between the cylinder banks for packaging, according to Motor Sport. By comparison, Ilmor’s overhead-cam D engine that year used 82 degrees. Displacement was 3.43 litres, or about 209 cubic inches.

The power target was 940 bhp at minimum. Instead, Illien says the engine made 1,024 bhp at 9,800 rpm. Motor Sport adds that it had a rev limit of about 10,500 rpm.

Pole and victory at the 1994 Indianapolis 500

Penske ran three PC-23s at Indianapolis that May. Its cars locked out pole and third on the grid, with Unser Jr. at the head of the field, according to Mercedes. Meanwhile, Paul Tracy lined up 25th in the third car.

Emerson Fittipaldi dominated much of the race in the other Penske. Mercedes says he led 145 laps. However, his race ended with 15 laps to go. Unser Jr., who had lost the lead during the first pit stops, then took it back and won.

That gave the Mercedes-Benz 500I a win on its debut. It also capped a strong year for Penske. According to Motor Sport, the team won 12 of 16 Indycar races in 1994.

One race, then the rules changed

The loophole did not last. Mercedes says USAC changed the rules for purpose-built pushrod engines a few weeks after the race. Forix describes the decision as one that made the engine uncompetitive for 1995 and beyond.

So the 500I never got a second Indy 500. In fact, Motor Sport notes that Penske failed to qualify for the race the following year. Its 1994 Indianapolis record still reads pole position and victory.

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This article was created with the help of AI and was reviewed and edited by Fast Lane Only editors before publication.