Air Flow Measurement

[Mazda Ebrahimi]

Stephen Dubovsky wrote:

> 
> Ok, why the heck does the airflow change directions?  Some sort of shock
> wave like in a 2cycle tuned pipe exhaust?  Wouldn't this just indicate lousy
> intake manifold/runner (whatever you all call them) design?  Seems it would
> give peaks and valleys to the torque curve (like resonances in the exhaust do).
> The change of direction in air flow is a result of compromises you have to make in 
living with one cam profile over the RPM range of the engine.  Intake valves are left 
open for the first few degrees of the compression stroke, because the air flow comming 
into the cylinder has a positive momentum.  So even as the piston is on its way up, you 
manage to pack more air into the cylinder.  This momentum effect is much less pronounced 
at lower loads, and since the camshaft is also rotating slower, it allows some air to be 
pumped back into the intake runner.  I have seen the results of one experiment, where at 
low RPM, as much as 50% of the air volume in an individual intake runner was being 
pumped back into the intake plenum!  At part throttle, the butterfly shields the MAF 
sensor from flow reversion, but at lower RPMs and WOT, flow reversion past the MAF could 
be a problem.  I think this is what you guys are talking about. That's my two cents 
worth!