Technically, the compressor inlet pressure is normally ALWAYS below ambient (if only by a small amount), as there is generally no other force/driver to move new air into there once the compressor pumps the current air out (into the upper deck/manifold).  Thus, for air to flow into the compressor inlet, the pressure there must always be below ambient (at least by a small amount).

 

This (small) inlet pressure differential is what drives/determines the airflow into the turbo, and is resisted by all other friction/restrictions in your intake system (scoop, diverter, filter, tubes, bends, etc).  So, the more restrictions you have the less airflow you’ll have.

 

Of course, the less inlet airflow, the greater the ambient pressure differential created at the compressor inlet, and thus the more ambient air force available to increase the inlet flow, until an equilibrium is reached for the current conditions - but at the expense of compressor efficiency and available maximum compressor output pressure/flow.  This situation is exacerbated at low ambient pressure (high altitude) – but is always present at all altitudes.

 

This inevitable situation should be a cause for total inlet system design concern/consideration for operation at all altitudes, as we generally want to minimize the compressor inlet pressure drop (vs ambient) and still have good/strong flow.  Just something to keep in mind.

 

Note: “proper” inlet scoop and divergent tube manifold design can help lessen/overcome some of this, by attempting to harness the energy from the free/ambient airflow stream (dynamic pressure) to increase flow (or at least lessen the decrease) while minimizing the need for inlet/ambient pressure drop to feed the compressor.

 

.02 technical nit.

 

 

 Jim Writes:
<< They were from Brazil and they told me that they flew their IV-P's at 33,000 feet with no problems.>>>

… As you climb the air cleaner flow increases and the delta P across it rises causing the compressor inlet pressure to drop below ambient. Full power air demand is ~500 CFM at sea level. You need ~2.4 times that at FL280. I use a pair of K&Ns that are rated to 800 CFM each.