No. The vacuum that the piston chamber sees is totally affected by the
amount of air going into the engine. By the way, they're constant
"velocity" carbs. The idea is to keep the velocity of the air going through
the carb relatively constant, and the piston goes up and down to allow that.
Think of the piston as an automatic throttle, and of the disc plate as an
air valve. If the air valve is open, but the engine is at low RPM, it can't
use as much air so the automatic throttle stays fairly closed. If the
engine is at high rpm, but no load (think of over-run as you lift off going
into a corner), there is also very little need for air, so the piston stays
low. The only time the piston is high is under mid to high rpm, high load
conditions. Remember that air must be in balance with fuel, so if the
engine is doing little work, no fuel is required, hence no air, hence the
piston is low.
The really interesting thing (to me, anyway, and I'm easily amused!) is that
the damper in the piston causes the whole thing to act like an accelerator
pump when the throttle is opened quickly. The throttle is opened, vacuum
goes high because the engine is asking for more air but the damper is
keeping the piston closed, so the engine sucks harder on the fuel so the
mixture goes rich until the piston catches up with where the air flow wants
it to be! Thats why you always have to run oil in the dampers.
Thats also why with Webers you some times have to run bigger jets than you
would expect in small engines. The larger engine sucks harder, making more
efficient use of smaller jets!
At 06:30 AM 24/05/97 -0400, you wrote:
>Every once in awhile I come up against a question I can't answer. It
>keeps me humble. Since I'm feeling pretty humble right now, please help
>me with this:
>
>On constant vacuum carburetors like SU's and Side draft Strombergs, is
>the piston opening at a given rpm the same whether or not the engine is
>under load?
>
>Stated another way, if my car is on jack stands running at 3000 rpm,
>will the pistons be the same distance up as they are when going down the
>track under full throttle at 3000 rpm?
>
>Please respond only if you really know, not just with an opinion -- your
>response will preclude my present plan of mounting a graduated mirror
>viewed through a hole in the dash......
>
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