Re: Rams and other sheepish things

[Scott Fisher <sefisher@domain.elided>]

Michael Smith writes, in AD V7 #873, "I meant to underline my
point about inertia effects which I do not believe explain the
supercharging effect of intake resonance."

Indeed, Michael, the inertial effects are not the only thing in
operation in inertial ram resonant supercharging.  However, for the
novice, they are the easiest to visualize, and therefore a good place to
start.  It should be apparent that the science of intake tuning is very
complex, very rich, and is not something that *can* be explained in a
single email message; I try to pick the pieces that I think I can get
across in a few dozen lines, and then point to books when necessary.

However, you go on to state: "For practical purposes airflow is
continuous through an engine."  A little reflection will no doubt have
you realizing the inherent flaws of this statement.

For starters, the intake cycle occupies no more than 25% (plus or minus
cam overlap timing) of an individual cylinder's duty cycle -- that is,
intake is one of four phases in a 4-cycle internal combustion engine's
operation.  75% dead time for each cylinder is a long ways from
continuous.  (Of course, if you were talking about the air flow from the
perspective of the intake to the whole system, you are correct; maybe I
just wasn't making it clear that I'm talking about individual cylinder
tuning, which is where inertial ram comes into effect.)

More subtle than that, though, is the point that it is precisely this
discontinuity at the individual cylinder level that causes intake
resonance.  Here's how:

The intake valve opens, the piston starts dropping, and air moves down
the intake tract.  Sometime later, the intake valve closes.  The air
which *used* to be moving down the intake tract now has to stop;
furthermore, air being elastic, it's going to bounce back out the intake
tract.  This "bounce" takes the shape of a wave of positive pressure
which reflects in the reverse direction to desired flow, moving at the
speed of sound for the local pressure and temperature.  When that
positive wave reaches an opening -- say, the inside of an airbox, or the
mouth of a carb -- it will dissipate into the air in the opening,
reaching pressure equilibrium.  This is the source of the "intake moan"
we've been talking about.  I've seen cases where the reverse wave was
strong enough to cause "stand-off" -- that is, a cloud of vaporized fuel
*outside* the mouth of the carb -- when everything wasn't quite right. 
It's enough to make you pay close attention to air filters (such as
those from ITG) which mention that they have a fireproof inner liner.
:-)  Oh, and note also that the same rules of pressure, inertia and
resonance all apply to exhaust system design, though that really relies
on another interesting physical phenomenon worth a short sidebar.

When a wave of positive pressure travels down a tube (whether that's an
exhaust pipe, an intake ram tube, or a trombone) and exits into a larger
open area (whether that's a muffler, a Spica air box, or the orchestra
pit at Carnegie Hall), there is a corresponding wave of negative
pressure that travels back *up* the tube, in reverse of the direction of
flow, at the speed of sound.  For right now, let's ignore the Spica and
the Glenn Miller Orchestra and concentrate on the exhaust system, 'cause
it's the easiest to see what's going on in our favor.

A well-designed exhaust system will put a series of these open areas --
collectors, resonators, and eventually the tailpipe -- at carefully
measured distances down the system.  When it all works right, an exhaust
pulse travels down the header tube and reaches an opening, for example
the place where that tube joins another.  At that instant, a pulse of
negative pressure (meaning a low-pressure zone) travels back *up* the
header tube at the speed of sound for the pressure and temperature of
the gas in the tube.  

If that low-pressure zone can be timed (tuned) to hit the exhaust valve
at just the right moment, it can actually help suck out the burned gases
from the cylinder, reduce the amount of effort it takes the piston to do
the job, and turn into more power at the wheels.  That's one way that
performance exhaust systems work.

Where things get interesting is in deciding what the "right moment" is;
in general, a longer tube works just like moving the slide away from you
on the trombone, and produces a lower-frequency wave that works better
at lower RPM.  A shorter tube is like when Glenn Miller pulls in on the
trombone's slide and the note is higher; in an Alfa, the effect is that
this negative pulse hits the exhaust valve sooner than with a long tube,
meaning it works most effectively when RPM is higher.  

That, btw, is one reason bolt-on headers appear not to work: they may
either be tuned to work at a particular frequency, or they may require
adjusting the exhaust-cam timing to take maximum effect at some other
RPM, or they may be resonating at a frequency that messes up some other
operating characteristic of the engine.  It may take a good deal of
testing on a dyno to figure out just what other changes are required to
get your money's worth out of a bolt-on header.

Over the weekend, I came up with a good analogy for making only *one*
change to an engine (e.g., bolting on a header OR adding ram tubes OR
changing a cam OR you get the picture): it's like lowering only *one*
corner of your car and expecting it to handle better.  You've got to
have all four corners working together to handle well, and you've got to
have intake, compression, combustion and exhaust all working together to
make power.  

- --Scott Fisher

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End of alfa-digest V7 #877
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