Saab’s Clever Knock Detector Changed Turbocharged Engines Forever

Turbocharging is everywhere now. Every manufacturer from Chevy to Porsche bolts a turbo on something these days, cranking out ridiculous power from tiny displacement engines with almost zero drama. We barely think about it anymore. But rewind to 1980, and boosting an engine reliably was genuinely hard—so hard that a small Swedish car company’s solution became the blueprint for how turbo engines work today.

The Problem: Turbo Knock Was a Silent Engine Killer

Before Saab cracked the code, turbocharging was a party trick with real consequences. Cram more air into a cylinder, mix it with fuel, and ignite it—you get more power. Simple physics. But there’s a catch: too much boost pressure slams the piston upward with so much force that fuel ignites before the spark plug tells it to. That’s knock, or detonation. And if you don’t catch it fast, it shreds your engine from the inside out.

Automakers in the ’70s and early ’80s had crude ways of managing this. You’d either run higher-octane fuel (which costs more and wasn’t always available), dial back the boost (which means less power), or hope your driver didn’t floor it on cheap gas. None of those solutions were great. The Saab 900, which arrived in 1979, needed a better way if it was going to compete as a genuinely quick car. So Saab’s engineers built one.

APC: An Accelerometer Smarter Than the Driver

Saab’s Automatic Power Control system, developed in 1980, was beautifully simple. Install a knock sensor—basically an accelerometer that listens for the vibration signature of detonation—in the engine block. Wire it to an electronic control unit. Connect that unit to a solenoid valve that sits in the wastegate line. Done.

Here’s how it actually works: the wastegate is the valve that controls how much exhaust gas spins the turbo’s turbine wheel. More exhaust spinning the turbine equals more boost. Less exhaust means less boost. APC’s sensor constantly monitors boost pressure in the intake manifold, checking for knock. If everything’s clean—no detonation—the system lets boost rip up to the factory limit of 7.25 psi. The moment knock is detected, the electronic brain tells the solenoid to crack open the wastegate, dumping excess exhaust and dropping boost pressure in fractions of a second. No detonation, no engine damage, no loss of power that lasts more than a millisecond.

Turbocharging had been around for decades by then, but APC was the first closed-loop system that could adapt in real-time. As Car and Driver noted back in 1980, the system was “one element of a feedback loop, which also contains a compact electronic brain and a solenoid valve.” It sounds mundane now, but in 1980 this was genuinely clever stuff.

The Real Genius: Running the Edge on Any Gas

Here’s where APC gets interesting. Because the system could detect knock instantly and back off boost, Saab could push the Saab 900 Turbo’s compression ratio up to 8.5:1—a jump from the earlier 1978 99 Turbo’s 7.2:1. Higher compression means better throttle response when you’re cruising normally and better fuel economy. It also means more aggressive boost because the sensor catches detonation before it starts.

The numbers tell the story: the pre-APC 1978 Saab 99 Turbo made 143 horsepower and 174 pound-feet of torque but took 9 seconds to hit 60 mph. The APC-equipped 1980 900 Turbo made roughly the same horsepower but dropped that 0-60 time to about 8.2 seconds. Same power rating, noticeably quicker acceleration. That’s the magic of a thicker torque curve and better throttle response across the RPM range.

And because APC adapted to whatever octane fuel you fed it, owners had flexibility. Running premium? More boost. Stuck with regular? The system scaled back automatically. No engine destruction, no babying the throttle. That was radical.

Why This Matters Today

Modern knock control is more sophisticated—multiple sensors, real-time fuel mapping, cylinder-by-cylinder adjustment—but the core principle Saab established in 1980 is still the foundation. Every turbo engine on the road today uses some version of what Saab invented: a sensor that listens for detonation and a mechanism that adapts boost in response. EPA fuel economy standards partly exist because turbocharging is now reliable enough to be a mainstream efficiency tool, not a power-at-all-costs gimmick. And that reliability traces back to APC.

The Swedes didn’t invent turbocharging—that credit belongs to earlier experimenters and manufacturers—but they invented the system that made it safe, responsive, and smart. They took a technology that was temperamental and unreliable and turned it into something you could mass-produce and sell to regular people who’d never heard of a wastegate. That’s engineering that deserves respect, even if it feels invisible today.

Sources: Jalopnik