Induction length (was re: Strange Suds (manifold question))

[Scott Fisher <sefisher@domain.elided>]

Tom Callahan asks:

> How does the length of the intake manifold affect the torque 
> and power? I can understand if there is significant variation 
> between one cylinder and the next, but length?

Though we don't normally sense it, air has mass.  Mass, in turn, has
inertia.  Inertia means it takes energy to get air moving from a
standstill, but it wants to keep moving once it's under way, just like a
bowling ball.

A long intake runner has more air inside it -- hence more mass and
therefore greater inertia -- than a short one.  This allows a careful
engine designer to time the intake cam to take advantage of this extra
inertia; the inertia of the air moving down the long intake runner can
actually force a little more air into the cylinder, typically at the end
of the cycle.

And that's the key to affecting torque and power -- the length of the
intake runner affects not so much how *much* power the engine makes, as
*where* in the RPM band it makes it.  Here's why.

Consider the piston at the top of the cylinder, just getting ready to
drop down for the intake stroke.  As the piston moves down, the intake
valve opens and air starts getting sucked into the piston.  But the
piston's speed isn't uniform, because it's connected to a circular
crankshaft.  The piston moves slowly at first, then speeds up as it
moves to the center of the cylinder bore, then it starts slowing down
again.  By the time the piston gets to the bottom of the stroke, it's at
a dead stop for an instant before it moves upwards again.

Intuitively, you'd think that once the piston starts moving up, you'd
need to close the intake valve to keep from forcing air back out.  But
that's where the inertia of a long intake runner comes into play.  If
there's enough inertia in the air in the intake manifold, it can
actually force more air into the cylinder even after the piston starts
to rise.  This is called the inertial ram effect.

However, this is most effective when the engine is turning relatively
slowly, when there's a comparatively long time that the piston is almost
stationary at the bottom of the stroke.  As engine speeds rise, the
inertia in the intake runner works against you, because now there's less
time for this inertial ram effect to do any good.  Not only that, but
the added inertia of the longer column of air means it takes longer for
the air to come up to speed when the intake valve first opens. 

Engines that are intended to run at high RPM (like the inline four Alfa
engines, especially those with Weber carburetion) tend to have shorter
intake runners, because the lower inertia means the air responds more
quickly when the intake valve opens and the inertial effect can start
sooner.  It does, however, require a very different kind of cam timing
from the long-runner, slow-piston engine.

By the way, this is the theory behind several modern German cars which
have begun using variable-length induction systems; they use longer
columns of air at low RPM to take advantage of the inertial ram effect
provided by the long intake tube, but then the intakes shorten as the
RPM level rises so as to take advantage of the lower inertia in the
smaller column of air.  

And of course, it's also the same theory behind the variable cam timing
on the last few years of Alfa Spiders, though they tweaked the point
that the cam opened and closed rather than the length of the induction
system.  I'm not an expert in Alfa's variable cam timing system, and I
know that it was designed (or at least marketed :-) largely as an
attempt to meet emission requirements, but it, and the Honda VTEC system
which has similar goals and effects, was a step in the direction of
optimizing an engine for a much broader powerband.

Hope this isn't too long-winded (no pun intended), and that it helps
more than it hurts...

 --Scott Fisher
   Sunnyvale, CA

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End of alfa-digest V7 #670
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