Okay, now we're getting to the meat of this sandwich! Right about now,
we cross the lines between very rough calculations and go to the
extremely detailed gas flow laws, equations, and properties that
encompass the exact wat the gas enters the system (big hot burst from an
exhaust valve) to the way it exits the arse end of your truck. As
mentioned below, the ideal system is not necessarily the one that flows
the most - other important factors enter into the equation, like pulse
spacing, scavenging (using the uneven flow of gases to pull upstream
exhaust gases out of the engine), and heat gradients. If you've ever
seen a modern performance snowmobile, you will see how important
scavenging is (yeah, I know - 2 strokes have a different set of rules).
They even insulate the pipes!
Jeez, did I add anything to the discussion or just make heads explode
across North America? All I'm trying to get across is that it gets
REALLY complicated to do this accurately: we're talking pages and pages
of equations here.
Cheers,
Cale
Kyle Kozubal wrote:
>
> > Fluid dynamics, Cale is a thinking man !
> >
> > Okay, what happens to the exhaust gases as they travel further away from
> > the engine ? It cools off. What happens to gases as they cool ? They
> > contract. When the gas contracts , it slows down. This minimumizes
> > scavenging. How do we speed it up to scavenge better ? We reduce the
> > pipe size. I think exhaust systems should be designed with ever
> > decreasing diameters every couple feet to keep the flow pulses velocity
> > up for scavenging and to hold some pressure for the break into the
> > atmosphere. If the gasses are below atmospheric pressure like with a
> > pipe too large, why would the exhaust gas exit ? The higher pressure
> > inside the pipe sucks the exhaust out into the atmosphere.
> >
> > A true dual exhaust on a V8 engine makes it run like two four cylinders
> > mechanically attached, and scavenging is cut in half with only half the
> > pulses pulling twice as far apart.
>
> This is very good technical info!! Now may I bring up a question, which
> may/may not be worthy of this very good technical discussion: What does
> exhaust pipe length do for performance? Is having the shortest possible
> exhaust system the best? For legality reasons/emissions, is there a length
> the exhaust pipe must be on truck? I want to say it just has to go past the
> axle, but then again there are Fords from the factory which have the exit
> pipe coming out on the passenger's side, BEFORE the rear tire/axle. Anyone
> know?
> Kyle
> 93 Dakota 4x4 V6
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