Air-Cooled 911 Intake Revamp
Almost any engine will appreciate more air. It is, after all, among the four prerequisites any box of internal combustion needs in order to fire up and stay running. As it turns out, it’s also also on the short list of things you ought to care about when it comes to doing things like making more power and doing so more efficiently.
You and your 1.5L Civic hatchback learned a couple of decades ago that sticking some sort of high-capacity air filter onto the end of that D-series’ rubber intake tubing helped wake it out of its 70hp stupor. You also learned that once things like that engine’s fuel supply, ignition system and compression were also brought up to snuff, things like that wonky little air filter of yours all of a sudden started making more of an impact.
When it comes to Porsche’s 3.6L air-cooled engines native to 964 and 993 series 911s, according to Southern California Porsche tuning firm Bisimoto, they’re primed and ready for extra airflow.
The Factory Intake and Why It Stinks
Bisimoto doesn’t make blind assumptions, and you won’t hear about its lead engineer, Bisi Ezeriohia, finger tapping his way around some web forum looking for answers. Instead, he’s gone ahead and done things like integrate a couple of pressure sensors into the 3.6L engines’ intake tracts in order to make some deductions of his own. “Upon testing factory 964 and 993 engines with pressure sensors placed in [their] intakes,” he says, “observing significant vacuum after 2,700 rpm proved that the engines demand more air than the factory intake system can provide.” He goes on to point out that the engines’ MAF sensors also pose a restriction and that, frankly, the whole layout just looks outdated.
“This led to a quest to remove these restrictions to unleash reliable power while retaining the beauty of the air-cooled engine,” Ezeriohia says. “It combines the classic beauty of an air-cooled setup with the modern amenities [of] GT3 engines.”
The whole conversion starts with you getting rid of the engine’s obvious bits like its intake plenum, airbox and throttle body, as well as its not-so-obvious bits like its IAC valve, TPS, and MAF sensor.
Individual Throttle Bodies Make Everything Better
If one throttle body’s good, then six of them ought to be better. Bisimoto’s intake conversion is based upon six 48mm Jenvey throttle bodies, each made to fit against the 3.6L engine’s cylinder heads by way of specially made, CNC-machined aluminum adapters. Individual throttle bodies are known for their ability to provide unparalleled throttle response but their most significant advantage is their ability to introduce equal amounts of air into each cylinder.
Unlike intake manifolds that distribute the air charge by way of a plenum and runners that, by design, don’t always introduce the same amount of air to each cylinder, individual throttle bodies eliminate all of that. Sticking a throttle plate just inches away from those intake valves and without any sort of plenum between them means that, for example, cylinders number one and number five will no longer be fighting over whatever air’s inside that intake manifold.
Making the GT3-Based Intake Work
You think you’re finished after you’re done bolting up those Jenveys, and you couldn’t be more wrong. Rather than exposing those six throttle bodies to all sorts of debris that’s liable to get sucked inside, Bisimoto’s system incorporates a couple of plenums and a manifold of sorts, which is where all of that GT3 business comes from.
Here, specially designed rubber boots transition the throttle bodies toward the GT3’s cast-aluminum plenums that connect to either the 996 GT3’s rubber center housing or the 997 GT3’s cast-aluminum one—your choice. In either case, the throttle body that’d ultimately bolt up to either of those center housings and create all of those problems associated with conventional intake manifolds we just talked about is ditched because, well, nobody needs a throttle body in front of their throttle bodies. The whole shebang is finished off with Rothsport fuel rails and a set of Bosch fuel injectors.
Later 997-based GT3 manifolds are made entirely of cast aluminum, unlike 996-based systems that are made up of rubber center sections. Both work, and both feature resonance flaps that vary incoming air volume to spread torque across the entire powerband.
Right about now you’re wondering about that MAF sensor you tossed and how in the world that engine’s going to run without one. The answer lies with the AEM Infinity engine management system that Bisimoto says you’ll need. The pre-tuned, speed density-based computer relies an a MAP sensor instead, and gets its signal from the provided vacuum canister that also has provisions for a brake booster reference. All told, the special-order kit will set you back at least $8,500, according to Bisimoto, and goes up from there depending on engine management and wiring harness options.
Horsepower Never Lies
All you care about is whether or not this whole conversion is worth it, though, and if you believe Bisimoto’s dyno data, you’ll see that it is.
According to Ezeriohia, it’s those specific intake runner lengths of the GT3 plenums that allow for all of this, or what he says makes for “ideal inertial supercharging, with emphasis on the 5,500 to 7,300 rpm range.” The whole package, he says—and that dyno data would tend to agree—lends itself well to the top-end sort of numbers you care about all while preserving the low-end torque you know you need.
It isn’t just about horsepower, either. Converting to something like AEM’s Infinity and modern Bosch fuel injectors means sequential fuel injection, improved fuel atomization and more immediate throttle response can now be expected out of something like a 30-year-old flat-six.
What You’ll Need
- 996 or 997 GT3 intake manifold assembly, actuation solenoids and IAT sensor
- Custom Bisimoto air filter chamber, individual throttle body adapters, and vacuum chamber
- Jenvey individual throttle bodies
- Rothsport Racing fuel rails
- Bosch EV14 injectors
- AEM Infinity engine management system and TPS