Stupid question, but please answer

I was wondering why cars use intake manifolds and not headers (like the ones used for exhaust). I know there's a reason why, but I don't know it. Somebody help me out? TIA

That's not a stupid question at all, in fact it's a very good question. The basic physics behind efficient airflow through an engine are identical regardless which side of the combustion chamber you are on, but the differing jobs of the intake and exhaust manifolding do require specific design points for each duty.

Specifically, and from a pure performance perspective, the largest difference in jobs (on port fuel injected cars, carb and throttle body injected cars need intake manifolds which flow wet mixture well... an entirely different job than air alone) comes with pressure differential; the intake manifold should route an incoming air charge to the highest possible pressure when compared to ambient at good velocity, while the exhaust system should route air with the lowest possible internal pressure and maintaining higher velocities. Bernoulli's Law states that pressure varies inversely with velocity in a moving air column, so by increasing velocity one sacrifices internal pressure. The obvious way around that in an intake manifold is to add a plenum, whereby you effectively store a large volume of air (large vs. single cylinder displacement in said engine) at low velocities, which then gets ramped up in speed again when traveling into the final intake runners. Any engine for which you can find long intake runners and no plenum is one which will make no high RPM power, since charge velocity moves into an inefficient area for cylinder filling at such short intervals of time.

Additionally, you have the increasing demand for tuned intake manifolds which not only harness the pressure resonance of moving air columns which are constantly starting and stopping (I can describe this in detail later), but do so over several RPM points... so called staged intake systems. BMW is the first to market a continously variable staged manifold, and the potential gains in power and efficiency over large RPM ranges should be impressive. Simply using a psuedo- reversed exhaust header means neither of these first two points in intake design would be possible, and engine performance and efficiency will suffer accordingly.

There are plenty of other points to be made, but from a street performance standpoint, I feel these are the most significant.

Thanks for the informative reply!

.. Forgive my ignorance (and insolence? :)), but could you explain further on how a continuously variable staged intake manifold works?

Thanks again!

^