More than two decades after Penske dominated the 1994 Indianapolis 500 they still talk about 'The Beast'.

It was a 209 cubic-inch engine developed by Ilmor, badged by Mercedes-Benz and built by Penske Racing in such secrecy that the team members not involved in the project had no idea what was going on down the block at a warehouse that had been turned into a "mad scientist laboratory".

"That engine almost brought me out of retirement," says four-time Indianapolis 500 winner Rick Mears, who retired from racing after the 1992 season.

Team owner Roger Penske decided to exploit a rule in the United States Auto Club (USAC) rulebook for the Indy 500 that gave purpose-built pushrod engines 55 inches of turbocharger boost pressure instead of the 45 inches of boost for the overhead cam engines that every team used in CART competition.

John Menard's team took advantage of this rule by using the Buick 'stock block' engine throughout the 1980s and 1990s. The Buicks had great speed and won the pole in 1985 with Pancho Carter and 1992 with Roberto Guerrero (and twice more with Scott Brayton in 1995-96). The problem with the engines, however, was they rarely made it through 500 miles in the race.

Penske determined with the long straights and flat turns at Indianapolis, an engine with torque off the corners would be decidedly beneficial in the 500. He thought a pushrod engine with 55 inches of boost would give Penske Racing the 'unfair advantage' it always craved.

In 1993, Penske met with Ilmor's Paul Morgan and Mario Illien, telling both about his grand idea. Illien assured him he could design an engine and the secret project began. Shortly after signing Al Unser Jr in September 1993 for the following season Penske whispered in his new driver's ear: "We've got an engine for Indianapolis. It's a rocket."

At that time, Penske was based in Reading, Pennsylvania in a series of buildings instead of the huge factory it has today in Mooresville, North Carolina. Penske knew he had to keep the engine a secret, even from the crew members on his own race team.

Back then, Chuck Sprague was the general manager, Clive Howell was the team manager and Karl Kainhofer - one of Penske's first employees - was the director of the engine and dyno department. They put together a secret team that set up an engine shop in a Penske Truck Rental warehouse down the block from the race team. They would go to work once the people working the day shift had left, working through the night and disappearing without a trace by 7am.

"It was kept secret for quite a while," current Penske team manager Jon Bouslog recalls. "I probably was in on it four months before we actually ran the car. The whole operation was down the street. We used to store stuff down there and suddenly we weren't allowed to go down there any more and people started asking 'why?'. We were told we just didn't need to know.

"The first time I saw the car, it was cloaked with large curtains sectioning off the area. Underneath the car was some of our mechanics screaming how they couldn't see the tunnel bolts in the dark and were using flashlights to try to work on the underside of the car..."

Penske team manager Jon Bouslog

"I had a really good friend on the team who was building gearboxes for the car and he didn't say anything. There was definitely some fear of anybody letting it out.

"Nobody knew what was going on. Nobody knew what to ask questions about. When you came in the next day at the engine shop it was like a normal day. There were no extra parts lying around. There were no signs of any overnight activity. They did a very good job of making sure nobody knew."

"I wasn't part of the secret group," Howell says. "It was a small group of guys and we didn't really have much crossover there. It was the group of guys working on that car and nobody else really went and helped out because the closer knit you kept it the better, and the easier it was to keep it a secret.

"I could go down if I wanted to but we were racing at the time. My first involvement was when we went and tested it."

"I was one of the members of the engine department and we were supplying racing engines as well as special projects," says Mark Swavely, brought in to develop speciality pieces for the engine.

"Once pen went to paper and manufacturing began we started to get pieces to build test engines for the dyno and there was a separation of our staff. There were guys that were going to stick with the daytime hours and guys were going to work night time hours with the secret project.

"I wasn't on the night time hours initially but once months had passed and development had taken place we would build an engine, dyno test it through the night, disassemble the engine in the daytime, communicate back to Ilmor and develop that engine over the phone.

"We flew parts back and forth. A lot of times, we bought seats on the Concorde to get pistons and other engine parts to the shop from England.

"The development curve was steep. The engine became common knowledge once we had to go into production mode and build those engines for Indianapolis and that is when everyone got involved.

"We were broken down into specialities for that project. My speciality was the valve followers and the arrangement with the pushrods and the rockers. It involved many, many needle bearings to take all the friction out of that assembly.

"The pushrods were 6-8 inches where a standard pushrod is 12 inches long."

When Bouslog was allowed into the secret shop for the first time, he never forgot the scene.

"The first time I saw the car, it was cloaked with large curtains sectioning off the area and once you got inside of the curtains, there was the car propped up on the stands with one, large spotlight shining on it giving it an eerie experience," he says.

"Underneath the car was some of our mechanics screaming how they couldn't see the tunnel bolts in the dark and were using flashlights to try to work on the underside of the car.

"The secret gang was headed by Guy Oder, our test team chief at the time, as well as our engine department - Mark Swavely, Karl Kainhofer and Kevin Walter. They were the ones that would come in after hours and build engines and run them on the dyno overnight. We would wonder why we didn't see these guys - 'It's work day, why aren't they around?'"

When it was finally time to reveal the engine, Penske flew into the shop and called the team together, as former Penske engineer Nigel Beresford recalls.

"Roger gathered everybody together in the small dining area and talked about the importance of maintaining the secret," Beresford says. "He warned everyone that if the secret got out then it would be like 'cutting your pay check in half' - everyone would stand to benefit well if the project was a success - i.e. we won Indy - but we would all lose a lot if it got banned."

Of course, during the short development phase there were problems. Howell said the engines had trouble synching the triggers and often would not start. The initial tests of the engine were held in the cold, icy conditions of wintertime in Nazareth, Pennsylvania. Ice scrapers and snowploughs had to clean the track before Paul Tracy could test the engine in sub-freezing conditions at Nazareth Speedway - a quirky oval that measured less than one mile in length.

Tracy's hands were frozen and Team Penske's Teddy Mayer had him put thermal socks over his driving gloves, making it difficult to steer the car and change gear. The engine would often blow up after just six or seven laps.

"It wasn't until a very short time before the Indianapolis 500 we actually did 500 miles at Michigan during a test," Bouslog says, referring to Unser completing 520 miles away from the world's focus while the rest of the Indy field was taking part in the second day of practice.

"That was a huge sigh of relief. Up until that point when we were into the month [of the race] the engine still hadn't done 500 miles. It was a huge sigh of relief to finally get that 500-mile mark before the race."

When the team arrived at Indianapolis, Penske urged his drivers not to unleash the full potential of the engines out of fear of being pegged back for the race. But the competition realised if the 209 engine did not break, the other 30 drivers had no chance to win against the Penske trio of two-time Indy 500 winner Emerson Fittipaldi, 1992 winner Unser, and Tracy.

"They used to give us speed reports that had the trap speeds of each car at certain points of the track," Bouslog says. "I remember looking at the times and you would see 238mph, 235, 237, 234. Then one of our cars would go by and you would see 255.

"When everybody saw that, their eyes popped out."

The three drivers had to be delicate with the engine, however, because it produced well over 1000 horsepower - nearly 200bhp more than the other engines at Indy that year."

"Our reliability during the month of May was remarkable," Swavely says. "The one sensitive thing about this engine is it had a lot of torque and if a driver wasn't anticipating that he could break the tyres loose and over-rev the engine very quick.

"With the pushrod engine, it wasn't good at being over-revved. It wasn't tolerant of that. We had one of those instances during the month. But I don't think we had any failures with any components related to the engine."

"They reduced boost five or six inches the next week, then the next week they outlawed the engine. If you do too well, typical, you get slapped..."

Roger Penske

Losing control of the car wasn't the only risk that came with the increased torque. Penske discovered during practice its tyres were turning on the wheelrims during a run, causing a handling imbalance. With the help of a local sandblasting facility, coarse masking strips were applied to the wheels and the tyres to increase the strength of the bond and prevent the tyre-slip. A potential race day risk was averted.

The race was no contest as the green flag dropped with Fittipaldi and Unser charging away at the front. By lap 38 of the 200 they had lapped the field they led 98 of the first 100 laps between them. Tracy - who started 25th after a crash in practice, dropped out with turbocharger failure.

Fittipaldi was cruising to victory when he decided to pass Unser with 16 laps to go to put his team-mate one lap down. His moment of greed was costly as he lost control coming out of Turn 4 and crashed into the wall.

"He got a little greedy and it wasn't Emmo's day - it was Al's day," Bouslog says. "That's just the way that place is. It gives it to you and takes it away all in a split second, which is the great thing about it."

Unser would go on to win the race - his second Indy 500 victory. Rookie Jacques Villeneuve was the only other driver to complete all 200 laps.

A few months after this spectacular success for Penske, USAC changed the rules, effectively outlawing the engine for the 1995 Indianapolis 500 - a race Penske remarkably failed to qualify for.

"They reduced boost five or six inches the next week, then the next week they outlawed the engine," Penske says. "If you do too well, typical, you get slapped. It was like USAC put a thumb on the scales."

While Penske was frustrated, Mercedes - which provided much-needed funding to the project when it signed the deal in November 1993 - didn't regret spending so much money on the project.

Former Mercedes motorsport boss Norbert Haug told Autosport it was worth it "10 times over" even to just race the engine once during Indycar's heyday.

"It's a good feeling when you have 250 horsepower more than everyone else," he added. "That shouldn't always be the basis in motorsport, but in this case it was just a loophole. The car was still accelerating from the start/finish line into Turn 1. This was a great project - nobody suspected that something like this would go on.

"Winning the Indy 500 - that means something. It was worldwide promotion for Mercedes-Benz, so it was easy to say yes when Ilmor suggested it to us, and compared to what people guessed, it did not cost that much."

The Mercedes 209-cubic-inch engine became one of the greatest one-offs in the colourful history of the Indy 500. Even Andy Granatelli's famed STP-Turbine engine competed in two Indy 500s in 1967 and '68 before it was outlawed.

Major change was on the horizon. Indianapolis Motor Speedway president Tony George had announced the creation of the Indy Racing League in March 1994. The 500 would be its anchor event. In 1995, the IRL announced rules with production-based engines beginning in '97.

Turbocharged engines didn't return to the Indianapolis 500 until 2012 - four years after Champ Car and the IRL unified to create today's IndyCar Series in '08.

But under the current rules, there is no opportunity for any race team to create another 'Beast' of an engine.

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