ENGINE | Here’s how rolling noise was measured on a Porsche 928.

VALENCIA. It’s like design Crazy Max or prototype Return to the futurebut in fact it was a special vehicle with a long history, designed to measure tire noise.

Eventually it was used less and less, but in Weissach they did not want to remove it. It feels like it has always been here. There probably wasn’t a Development Center employee who didn’t come across 928 sterling silver with its many additional details.

It has been used to measure tire noise for over 30 years. To this end, it has been redesigned to limit engine and transmission interference as much as possible: some noise sources have been radically reduced, and others that could not be suppressed at source have been isolated. Today it is in the Porsche Museum.

“This is a real dinosaur,” says Harald Mann, referring to its long history, probably unique to the Weissach test vehicle. “And today it can still serve its purpose.” It has been used in numerous tests to measure tire noise to meet legal requirements for acoustic emissions. The choice of the 928 to transform and remain in service for so long was no accident.

“It didn’t matter whether the engine was in the front or the rear, or how much interior space there was. For our tests, we primarily needed more power at low speeds. That’s why 924 was dropped. Also a 944, with a typical gearbox sound under light load. And the air-cooled 911 was too loud, says Mann. “It was important to keep the vehicle noise as low as possible.”

Harald Mann, a mechanic by training, worked for Porsche for 40 years, more than 30 of them on test benches in Weissach, mainly in the field of car acoustics. This is a complex but interesting area.

“Partial noise sources, engine and transmission mechanical noise, tire rolling noise, intake and exhaust noise, add up to a volume that should not exceed a certain value,” explains Mann. In the late 1980s, the test limit was 75 dB(A). “In the future it will be only 68 dB(A).” This is a big problem, especially for sports cars, because they need wide tires designed for high speeds. These wheels are never silent.

The quietest 928 in the world

“In reality, it is almost impossible to control the mechanical noise of the engine and tires. In the end, it’s always a mixed bag: if the engine and gearbox are particularly quiet, the exhaust might be a little louder, for example. If the tires are making excessive noise, the intake noise will have to be reduced.” Since the manual transmission sports car was designed solely to measure the noise levels of different tires, it became the quietest 928 in the world.

Engineers took several steps to isolate and minimize noise sources from the engine and transmission, as well as intake and exhaust noise. The radiator was installed in front of the bumper, without a fan. The air intake has been moved to a large cylinder with as much insulation as possible, equivalent to an intake muffler. From there, pipes led to the engine in a compartment that was completely sealed. To keep the V8 cool when needed, two fans located in the hood vents extract hot air at the touch of a button.

Drive shaft and gearbox insulation.

Down below, the driveshaft was completely wrapped and the lower body was insulated around the gearbox. “Manual 928 was ideal for its purpose,” says Harald Mann. “Torque and power are critical. This allows the vehicle to pass the measurement points at lower engine speeds, which reduces noise levels.” According to test specifications, the 928 accelerated from 50 km/h to at least 61 km/h. At this moment he had to walk in front of two microphones located 20 meters away. For vehicles with a five-speed manual transmission, values ​​measured in second and third gear were averaged. “Naturally, the fact that the 928’s transmission was already silent from the start also played a role. In everyday life, this is important for the impression of quality conveyed to passengers.”