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Pressure vs. Flow

Pressure is a force applied to all surfaces based on the level of compaction of air molecules... flow relates to the mass or volume of air as it moves through an engine.

So pressure is important because it tells us about latent heat production due to compression of air - but flow will tell us how the mass of air moves through a motor.

Pressure of storage of air versus pressure of an engine:
Pressure of a fixed volume of air relates to the environmental factors surrounding the storage of air. So an 80 cu/ft cylinder holding 3000 psi at 80F will increase in pressure by 10% for every 10% increase in temperature (see general gas law). NO MATTER WHAT - changes in temperature effect changes in pressure and vice versa; this is important as relates to timing.

Flow is king on a street car (factory internals) because it relates to the mass of air an engine can move. Power is derived from flow because and engine is not a static environment.... so using our 80 cu/ft cylinder as an example... it would be emptied in less than 5 seconds on an engine it boost!!!!! So a 1 degree temperature change is not as important as a 1% change in flow (an 80 cu/ft cylinder versus an 8.1 cu/ft cylinder).

Pressure = heat = timing changes is totally different than (flow = power = fueling changes.)

Pressure and heat are tied while flow and power are tied... as long as you can account for heat you can run whatever pressure you desire.

Picture a positive displacement compressor... the pressure it produces efficiently is better than any centrifugal compressor because it gains pressure on each RPM - at the sacrifice of heat generation. This compared to a centrifugal compressor which may only increase the Pressure Ratio fractionally per RPM, however the efficiency over the flow characteristics of an engine are better..... So overall the engine moves a higher mass of air in the operating range. But in both these examples - the air charge gets heated and the latent heat has to be accounted for.

Compressors use pressure because that is what they do; make air pressure. This is why there are all these pressure signals for a turbo; it doesn't care about flow it needs pressure to tell it what to do. Engines use flow because that is what they do - move air in and out and turn a shaft to make power.

This is why when people talk about running XYZ psi of air it means nothing - that just refers to the compressor not the engine. With the exception of timing, and once you get REALLY high pressure - building the intake to contain it; measure flow and keep an eye on pressure.