The Porsche 911 Speedster's Independent Throttle Bodies Help Reduce Emissions

The guy behind this new 502-hp 911 explains how independent throttle bodies kill two birds with one stone.

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Máté Petrány/Road & Track

To create the new 911 Speedster, Porsche's Motorsport department didn't just take a sawzall to the roof of a GT3 and call it a day. There were lots of detail changes as well, because that's what engineers do. One change is the inclusion of individual throttle bodies for the Speedster's 4.0-liter naturally aspirated flat-six, which serve an interesting dual purpose.

At the 2019 New York International Auto Show, Porsche Motorsport's road-car boss, Andreas Preuninger, told us that his team first investigated individual throttle bodies as a way to make the engine even better. "Individual throttle bodies are known in racing for throttle response, performance, better part-load throttle behavior, more torque in the mid-range, and simply faster reaction to changes," he said.

But, the individual throttle bodies also help promote tumble for the intake charge as it enters the combustion chamber, which the manifold can't do on its own. "That makes for a better combustion, cleaner burn, and less emissions," Preuninger said. "[It kills] two flies with one clap. It's a win-win situation."

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Individual throttle bodies aren't common in production cars—BMW was probably best known for using them in various M car motors, the last being the E90 M3's V-8. Preuninger said ITBs aren't really useful in turbocharged cars—hence why BMW ditched 'em—and not really designed for road cars. "You take a [race] engine apart every half a year, or something like that, and you can adjust all the time," he said. "But a street-car has to be foolproof."

It was worth the effort for the dynamic improvements ITBs brought and the reduction in emissions. They help a lot with cold starts, where emissions are especially high.

There are other engine changes too. Preuninger said the fuel injectors now operate at 3626 psi, an increase of 725 psi over the 2019 GT3 powerplant. A new stainless steel exhaust system saves around 22 pounds compared with that of the 2019 GT3, too, and that weight savings comes at a crucial spot—low and near the back of the car. The GT3's exhaust had three mufflers, two on either side of the car, with one in the center. Now thanks to a particulate filter and catalyst, Porsche only needs a single rear muffler. It also has variable baffles that can adjust back-pressure and noise levels on the fly.

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There's a very good chance we'll see this engine in the next 911 GT3, though Preuninger declined to confirm it definitively. Addressing the persistent rumor of the next GT3 going turbo, he did say "I don't see any reason" for ditching natural aspiration.

In the Speedster, the 4.0-liter six is paired with the same six-speed manual that came with the 911 R and is available for the current GT3. In a lot of ways, the Speedster is the open-top counterpart to the mighty 911 R. "When [we came up with the] idea to pair a Speedster with GT technology—which I wanted to do for ages—and when we finally built up a car, it was in parallel with the 911 R...an open and a closed version of a purist's car."

With the 911 R, Porsche tuned the rear-wheel steering system from the GT3 and GT3 RS to promote more agility in favor of total stability and race-track performance—it's the same story here. Like the R, the Speedster also gets carbon-fiber bodywork, including front fenders, hood, and rear decklid, while the front and rear fascias are made from a composite material. Those bits— the six-speed and the manually folding top—help offset some of the weight gained from the stiffening measures you need to take with an open-top car.

And you might think the Speedster won't be as good to drive as, say, a 911 R or a GT3 because of the lack of stiffness from not having a roof. Preuninger says it really isn't an issue, since the 991 Carrera 4 bodyshell the Speedster uses as a base is already very stiff, and the Motorsport team made additional strengthening measures for this car. He claims you won't even miss the roof on track.

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Of course, you'll pay for all this goodness—$275,000, which is almost double the price of a regular GT3 and almost a hundred grand more than the MSRP of the 911 R (though that car has since become worth over $300,000). Thankfully, Porsche will build almost 1000 more Speedsters than 911 Rs, with 1948 set to be produced.

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