John,
Thanks for the additional and helpful “visual” example. If you contemplate
this problem further, bear in mind that the diameter at the opening of the
stock snorkel on my car does not have a diameter of 2", but an area of
2.07". The diameter is an even smaller 1-5/8". George reports that the
inside diameter of the throttle is 2.75", with an area of 5.94", roughly 287%
greater than that of the snorkel intake.
Not taking into account the loss of area due to the butterfly valve’s cross
section, it would seem that the snorkel opening would equal the throttle
opening when the throttle opening is 34.8% open (2.07/5.94). Does this mean
that for any throttle opening greater than 34.8%, the snorkel is the limiting
point in the intake? I realize that under constant speed conditions the
throttle opening may be well shy of 34.8%, but in stop and go traffic, or
even dodge-and-dart multi-lane commuting, wouldn’t the throttle frequently be
opening more widely than that, and could that be why I’m sensing a greater
responsiveness in my car now?
Sorry for the long explanations … but you ask tough questions.
John P.
Hey, isn’t that what musicians are for? 
By the way, for those following this thread, if you try the mod, you can get
a sense of the performance difference simply by removing the snorkel on your
air filter housing. However, unless you replace the snorkel with a hose from
the filter housing to the fender well, the sound increase will be very
noticeable, and under heavy throttle the sound is rather unbecoming of a
Jaguar. If you run a 3" hose, the sound level falls significantly (on my
car, anyway), and you will restore the supply of cool air your engine needs
for optimum performance.
Don
'88 XJ6
In a message dated 2/15/01 5:22:55 PM Central Standard Time, JRPRING writes:
Don,
Thanks for the vote of technical confidence … I’ll try to reasonably
address the questions and concerns you raised in your two previous posts.For starters, its kind of hard for me to perform a one-to-one comparison
on
the potential benefits of an “increased intake port size” on the air filter
housing between our vehicles. Your XJ40 is a 1988 3.6 and 2.88
differential
ratio while mine is a 1990 4.0 with a 3.55 differential ratio. The later
models definitely have the advantage in greater “bottom end” torque and a
numerically higher gear ratio. These cars will be quicker and more
flexible
in daily driving but are likely to suffer a little on fuel economy.If I owned a 3.6 with a numerically lower differential ratio, I might be
tempted to seek “cost effective” performance upgrades. I honestly don’t
have
a lot of complaints in regard to the performance of my 4.0. And neither
does
my wife … as she uses it as a daily driver.Getting a mental picture of the “dynamics” of the intake system is not
that
difficult. Just picture a five foot length of 3.5" pipe. Near one end
we’ll
envision a butterfly valve (throttle plate) while at the other end we have
a
reducing coupling (snorkel) that limits our inlet diameter to 2". Now
consider that we have an air flow rate through the pipe that simulates the
XJ40 moving down the highway at approximately 50 to 60 mph. Under this
constant flow (speed) condition, the butterfly valve is only open
approximately 15 to 20% of its total travel. Its a major restriction in
air
flow as the effective open area through the valve is small. But its enough
to satisfy the current demands of the system (engine). Under this
operating
condition, the pressure drop across the reducing coupling (snorkel) is
minimal as the flow is minimal.Change the operating conditions … as in full throttle acceleration and
the
major pressure drops swap position in the intake. The throttle has minimal
drop in the full open position but the snorkel is acting as a significant
restrictor. That’s why you will always see “open throated” competition
vehicles and “compromised” daily drivers. If the potential extra induction
noise is of minimal concern to you … go for it. It probably is an
acceptable tradeoff for you. To quantify the benefits of your proposal …
the system would need to be modeled or extensively tested. That is a
considerable task.Vacuum or Pressure Questions … if you think in absolute pressure terms,
its easy to understand. No need to worry about pushing or pulling air.
Air
simply moves from high pressure (high energy condition) to low pressure
(low
energy condition). Don’t think we need to approach this problem on a
molecular level.Density / Specific Volume Effects … you are absolutely correct that as
the
density of air decreases (or its specific volume increases … reciprocals
of
each other), engine horsepower decreases. The relationship is linear I
believe, as a function of elevation. Viscosity (or the ability to flow) of
any fluid has a direct input on fluid pressure drop through a system and
its
typically controlled by temperature and pressure. Under normal conditions,
the fluid viscosity is usually fixed by the operating conditions and does
not
become a major player in the equation like velocity … its considered a
constant for all practical purposes … something you just deal with so to
speak.So yes, there is less pressure drop in the intake system at higher
altitudes
so theoretically you could have more air flow. But the overall effect is
less engine power produced even with greater volumetric air flow due
entirely
to the lower air density. If you want more power under these conditions
…
you need supercharging (a general term for both exhaust driven
turbochargers> and mechanically driven superchargers).Sorry for the long explanations … but you ask tough questions.
John P.