Intake System FAQ
Why Rasant ITB's?
After years of trial and error with other manufacture throttle bodies we ventured into making our own version of ITB's. Our main goal with our products is what we call "OEM +". We want a similar feel of what Porsche would put on a modern aircooled. For us that means great driveability and reliability. I think one of biggest advantage an average user of our throttle bodies would notice is an idle air circuit. This allows for a prompt cold start through and electronic idle air control valve. The other systems on the market do not have a way to add idle air for cold starts, this means you typically have to crack the throttle with your foot to get it started. Another big advantage is our progressive throttle linkage. It's easier to setup, very consistent from bank to bank, and reliable over time.
Plenums VS Trumpets?
This is one of the first questions customers have when discussing a system for their air-cooled flat six. Trumpets have that classic look but what are these mid range advantages I hear about with plenums?? Typically on a street car we recommend plenums as they're easy to add adapt air filtration, and have improved mid-range performance through it's resonant valve center section.
GT3 or 964 plenums with a center resonant valve significantly improve HP and torque. Combined with ITB’s for snappy throttle response it’s a fantastic combination. The plenums feature a resonant valve in the center. The valve opening and closing effectively changes the intake geometry, optimizing for bottom end or top end performance. This valve is controlled through the ECU.
- Easy to add air filtration (done on the back side of the plenums)
- Resonant valve varies intake volume, improving performance on top and bottom end
- Custom finishes available
Trumpets are also a good option, but they lack the midrange performance of the plenum version and are harder to add air filtration. To us it’s more of an aesthetic choice to run the trumpets. They lack some of the performance and practicality of plenums but help retain the vintage look for builds where that’s important. We offer a mesh filtration option but it's not as protective as the traditional air filter we're able to add to plenums.
- Classic look yet integrates electronic fuel injection
- Difficult to add adequate air filtration
- Missing some of the mid-range performance of plenums
Our throttle bodies come in two varieties: 2-bolt for earlier engines and 3-bolt for 3.6L+ engines. This refers to the number of fasteners between the throttle body and cylinder head.
The 2-bolt variety comes in two throttle sizes: 42mm and 48mm.
- The 42mm version is ideal for engines between 2.7-3.3L. Available as trumpets or mating to a 964 plenum.
- 48mm and above are best for a 3.4L+ engine. Available as trumpets or mating to a GT3 plenum.
The 3-bolt variety is available as a 48mm only. It mates to trumpets or a GT3 plenum.