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Oct 15, 2024

The Mazda 787B Had a Wild Variable-Length Intake System

If you want good low-end torque from your rotary, you don't need intake trumpets—you need intake trombones. Most of you know the Mazda 787B, the first Japanese car to win Le Mans, the only

If you want good low-end torque from your rotary, you don't need intake trumpets—you need intake trombones.

Most of you know the Mazda 787B, the first Japanese car to win Le Mans, the only non-piston-engine car to win the race, and a machine with an unforgettable shriek. You probably know that it had a four-rotor engine, the R26B, but did you know about its variable-length intake system? And have you seen it in action? It's pretty wild. Mazda is justifiably celebrating the 30th anniversary of that Le Mans win, and it put up a video showing off various bits of the car, including the intakes in action.

The length of an intake runner has a pretty significant effect on the way an engine delivers power. In simple terms, longer runners help generate better low-end torque, short runners provide high-rpm power. Using a variable-length intake system means engineers don't have to compromise. You get the best of both worlds, and that's why many road cars have variable-length intake systems.

Rotary engines are notorious for lacking low-end torque, so Mazda engineers used variable-length runners in its racing engines to help them better compete with piston-powered machines. With the 13J four-rotor in the 767, Mazda first used a two-stage intake, before upgrading to a multi-step intake for the R26B. The final iteration of Mazda's rotary-powered Group C prototypes, the 787B, had the most sophisticated intake system yet.

After 1991, rotary-powered cars would no longer be allowed to run Le Mans, so Mazda put a ton of effort into making the best rotary racing engine it could. Ritsuharu Shimizu was the engineer in Mazda's rotary program tasked with getting as much power out of the R26B as was possible. His solution was fully variable intake runners that changed length based on engine speed. The runners were moved by electric motors and pulleys mounted on top of the engine controlled by the ECU. Essentially, Shimizu turned intake trumpets into intake trombones, and it worked.

Mazda was able to achieve 690 hp at 9000 rpm, and crucially, 448 lb-ft of torque at 6500 rpm. Amazing figures, but still some ways behind the big piston engines in the Sauber-Mercedes, Jaguar, and Porsche entrants. But, Mazda lobbied the sanctioning body to lower the maximum weight for the 787B, giving it a power-to-weight advantage and better fuel economy. Road-car rotary engines are famously finicky, but Mazda's racing units were well-developed by this point, and the #55 787B gradually made its way through the field after starting in 19th. When he was told of the win, Shimizu apparently cried.

Toyota has since joined Mazda in winning Le Mans, meaning the small Hiroshima company is no longer the only Japanese automaker to have achieved this feat. But, it's highly doubtful that another rotary-powered car will ever win, so the 787B will forever stand alone.

A car enthusiast since childhood, Chris Perkins served as Road & Track's engineering nerd and Porsche apologist.

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