Transients

[G. Scott Ponton]

<unlurk>

    First the disclaimer. I tend to speak in a way that some people have a
problem with. I can come across as being condescending and the like. This
isn't my intention so I hope you will all take this for what it is worth. I
am putting on my flame retardant suit and placing my CSH nearby just in case
they are needed.
    I may be completely wrong here as it has been nearly 25 years since I
studied fluid dynamics on a daily basis. I think, through experience and
experimenting, that many times the actuality of the interaction of air flow,
fueling, intake tract sizing and cam specs is severely misunderstood. VE is
also many times misunderstood as to how it relates to the engine during a
variety of operating speeds and conditions.
    There is no way to give a short and simple answer to all the questions
that come with this subject. All of the points made have an effect and they
are all tied together to effect the throttle response of any particular
engine combination. But ...... ( knew that would be there didn't y'all) The
first step in understanding this is simpler than it seems. Throttle response
= how quickly we can go from an idle condition to a full power condition,
Correct?
    If we "fix" certain variables, air density and humidity, for the sake of
the argument, what is the difference between idle conditions and full
throttle conditions in the combustion chamber and intake manifold? How does
intake tract volume affect this? How does the cam spec effect it?  Where
does rod/stroke ratio come into the picture? What affect does compression
ratio have on this? In carburated apps how does the size of the carb affect
this?
    Each of these questions is interconnected. Each has an effect on the
"total" picture. They all need to be addressed in turn and yet at once. VE
is a curve that is related closer to the rod stroke ratio and cam spec than
anything else. Yes, tuning the intake runners, exhaust manifolds/headers for
size and length has an affect too. It's just not as large an affect as the
proper cam selection for each engine. It all relates to the time needed for
an event to occur.
    Assuming everything else remains the same, VE at idle is directly
related to the density of the air charge in the cylinder/intake. The swept
volume of a cylinder doesn't change between idle and WOT. Only the density
if the air does. It is only after the throttle is opened that we run into
how the other components affect the VE of an engine. ( I am trying to keep
this related to the idle/WOT transition only).
    So ...... At idle we are metering the density of the air in order to
control engine speed. As we go to WOT there is a transition period equal to
the lenth of time it takes for the air, at atmosferic pressure, to move past
the throttle and reach the cylinder. This is why IR intakes tend to have
better throttle response than common plenum intakes. The throttle plates are
closer to the cylinder therefor less time to affect a change in air density.
    If we confuse the issue at this point with upper engine speed VE and the
like we will miss the point. The cam, compression, and exhaust system have
the greatest affect at this point. Each affects the time needed for the air
charge to reach the cylinder at full density. If we add too much or not
enough fuel at the wrong moment the throttle response suffers do to the
cylinder being either too lean or too rich.
        I have used up enough bandwdth for now. Obviously this is a very
simplistic view of all the variables and how they interact. Hope it will
help some.
Back to lurk mode.

Scott