What is a Plenum


Pro Tuner
Staff member
2002 Clio 172
I'm totally baffled by the Plenum? What is it?

Do all cars have them and what should I know about them when it comes to car tuning?
It is the part of the inlet manifold between the throttle body and the individual runners that feed each cylinder.
Does it control air flow, fuel or contain a throttle/butterfly valve or is it just a pipe?
The main purpose of the plenum is to equalize the air flow to the various cylinders, but its volume and shape, as well as the shape of the bell mouths, which are the opening to the runners, are also important. Generally, a plenum volume of approximately 80% of engine capacity for naturally aspirated engines to 150% of engine capacity for turbocharged engines works best.
In terms of function, the best plenum design would have the air duct feed the center of the plenum. Unfortunately, due to space limitations and production costs, manufacturers tend to build plenums that are fed from one end, with the plenum blocked off at the other end. This results in air rushing to the far end of the plenum and creates a slight imbalance of air flow to the individual cylinders are the air will tend to flow past the first cylinder and collect at the far end of the cylinder, which is usually at the last cylinder. Consequently, the first cylinder will run slightly lean while the last cylinder will run slightly rich.
The easiest solution to this problem is to fit a second throttle body to the far end of the plenum and fit a double air filter, intake system. But this only works on naturally aspirated engines that have sufficient space for a second intake system. On supercharged and turbocharged engines this solution is not feasible and you would need to modify the plenum or fit an aftermarket intake manifold with a more efficient air flow and air distribution design. There are three things you must consider when modifying the plenum or selecting an aftermarket intake manifold.

  • First, the plenum should increase in size rapidly well before the first cylinder.
  • Second, the plenum can taper towards the end from after the first cylinder but it should not taper to less than 1½ times the diameter of the intake runners.
  • Third, the plenum should extend well beyond the last cylinder.
I really appreciate your answers guys, this is a great forum and I feel I can ask just about anything and get some sensible feedback and answers.

I wish I had your knowledge and experience but I'm slowly learning thanks to you all @Dale69 @old-git @TCJBOLDIE and all the regulars on here who have been so patient with my non stop questions.

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