From:
jamesrkays_REMOVE_285279_THIS_@yahoo.com
All the preceding is true. Usually , the longer the
distance from the inlet valve mating surface to the
lip of the inlet snorkle, the greater the mid to low
end torque, ie better on the street or short track.
The shorter the the tube, the greater the high end HP.
A classic example of an attempt to get the best of
both worlds would be mixed stack heights of late 60s &
early 70s CanAM Chevies.
The radiused port is just good engineering practice
for improving the flow of any fluid as it enters a
tube or hose. It reduces the initial turbulence as the
air is entering allowing a greater CFM of flow. You
will find the same inlet devices on FI, well designed
turbo chargers, and the inlets to any well designed
pump system. Jim
--- doctorduke <<a href="/group/cosworthvegas/post?protectID=091233212180056219138097203245147187078145038136183193071193172194143142">no_reply@yahoogroups.com</a>> wrote:
> --- In <a href="/group/cosworthvegas/post?protectID=219233066105193209050199029077192253163098100046209130">cosworthvegas@y...</a>, "soloii_74"
> <<a href="/group/cosworthvegas/post?protectID=200158253185078031036061203043129208071">kconnair@c...</a>> wrote:
> > --- In <a href="/group/cosworthvegas/post?protectID=219233066105193209050199029077192253163098100046209130">cosworthvegas@y...</a>, camsx2 <<a href="/group/cosworthvegas/post?protectID=091233212180056219138097203245129208071">no_reply@y...</a>>
> wrote:
> > > What is the function of an air horn / velocity
> stack on a weber
> > > carburetor? I have noticed that some carbureted
> CVs have them and
> > > some do not.
> > >
> > > Fred
> >
> > Fred,
> >
> > On a carburetor in an independent runner type
> set-up, like dual side
> > drafts on a CV (or nearly any other similar
> set-up), the air horn
> > acts as part of the overall intake passage, and
> helps to contain
> > what is called "standoff". Standoff is what
> happens when a fast
> > rushing fuel/air mixture encounters an intake
> valve, which has been
> > slammed shut in its face. The energy of the
> intake stream is re-
> > directed, and it heads back out through the intake
> passage and on
> > through the carburetor. Normally this whole
> process will end up
> > with a main jet and idle jet requirement that is
> less than if the
> > carburetor were on a plenum, since fuel is drawn
> (via the bi-
> > directional vacuum signal) during air movement in
> both directions.
> >
> > When the intake opens again, the air/fuel mixture
> above the
> > auxiliary venturi is drawn in and enters the
> combustion chamber,
> > with a "ramming" effect, dependent on rpm, and the
> length of the
> > intake runner system.
> >
> > If no velocity stack were present, some of the ram
> tuning effect
> > would be lost, along with some of the fuel to
> adjacent cylinders.
> >
> > If you watch one of these intake systems without
> the velocity stack
> > in place, you might be able to see the little
> cloud of atomized fuel
> > droplets, which seem like they are floating just
> outside the
> > carburetor venturi. If you use a mirror look into
> a velocity stack
> > on this type of system, you will see the same
> cloud of atomized fuel
> > droplets, but contained in the air stream within
> the velocity
> > stack.
> >
> > This whole effect is why the jetting on
> independent runner
> > carburetion is normally leaner than the jetting on
> a plenum type
> > system, and also explains why the amount of
> carburetion seems rather
> > large in relation to engine size.
> >
> > The benefit of the ram tuning from this type of
> system is improved
> > torque (for a specific rpm range). The negative
> of using a true
> > independent runner type system is the loss of a
> constant vacuum
> > signal for vacuum accessories, and a much larger
> carburetion
> > requirement (since each cylinder only has one
> venturi to draw from,
> > not a common air/fuel mass in a plenum area.)
> >
> > Hope this helps.
> >
> > Kevin.
>
> To add to the above, individual runner induction
> systems require
> substantially more total flow area, because each
> venturi is only
> supplying one cylinder, so it is only flowing
> one-quarter of the time.
>
> The CV throttle body flows 400 plus CFM @ 1.5" Hg
> and each cylinder
> draws from this entire flow potential because the
> inlet strokes have
> very little overlap, so an IR system would require a
> total of 1600 CFM
> to have equally low restriction. Because Weber
> carburetors
> essentially lengthen the inlet tract over the EFI
> manifold, they
> create more inertia tuning pressure at medium revs
> and produce more
> torque, but because they are more restrictive than
> the EFI's very
> large, "venturiless" throttle bodies, generous
> plenum and runner area,
> they won't produce as much power, everything else
> being the same.
>
> A production CV aspirates close to 200 CFM at 6500
> and a blueprinted
> CV with some pocket porting will aspirate about 225
> CFM @ 7000.
>
> The slide throttle Lucas mechanical fuel injection
> system used on the
> early racing CV engines built by Cosworth
> incorporated 2" diameter
> velocity stacks, and because the throttle valves
> slide out of the air
> stream at WOT, the inlet tract is essentially a
> straight pipe offering
> the absolute minimum restriction to flow.
>
> Duke
>
>
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[This is message #9215 by user jamesrkays on Yahoo! Group Cosworth Vegas:
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