So i was talking to my dads buddy about replacing my plenum and i just have one simple question.........what the hell is the reason for a plenum plate? what does it do? trap big things from falling into the engine? thats my guess but its a shot in the dark so i have no idea....thanks

 

Because of the way the manifold was designed.... there is a BIG open hole on the underside of the manifold. The open area right below the throttle body is technically the plenum. So, they had to seal the big hole.... and used a steel plate, and cheap arsed gasket to do so. Needless to say, that didn't work out too good.

 

It's my understanding that the primary function of the plenum plate is to develop air and oil leaks that cause lean fuel ratios, pinging, fouled plugs, and clogged cats.

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And on a more serious note: as Brian alluded to, the engineers at Chrylser probably just thought it would be cheaper to manufacture the intake manifold with a plate there instead of making a more complicated mold that would have required more exotic casting methods to produce. I don't think they had in mind making it possible to get inside the manifold to modify runners/dividers, although that's a positive by-product of what might just be regarded as a bad manufacturing/engineering decision.

 

Purely for ease of casting and cost control of the manufacturing process.

 

The literal plenum is the space, cavity, holding area, or whatever you want to call it, for air.. the plenum is simply the air reserve canister... The plenum pan is what everybody else has said, and has a removable bottom to expose the plenum.. It's there for ease of manufacturing the rest of the intake manifold.

The plenum is the biggest part of your intake manifold, speaking in terms of size.. the runners, actual intake venturi, and fuel injection ports make up the rest.. the plenum (aka ready air reserve) is needed to allow immediate and equally pressured response to all the cylinders instead of creating a stronger stream to one or another.. think of that air being like water, and streaming the path of least resistance.. you can't see the turbulence in air, but that doesn't mean they aren't there.. by having a big **** reserve right there, that holds enough air to equally satisfy each runner, you reduce turbulence creating a situation that allows the air to take the that path of least resistance- and in this case feed the four cylinder runners closer to the TB/venturi and starving the others of equal volume..

There are better designs out there, but none as cost effective to produce- and mother mopar is all about reducing costs..