Hi Ray: Part I
Great question because it makes us all stop, in our busy worlds, and reflect
on the basics which is always a good thing. I'll do this in too parts simple
because my two fingered typing can't even keep up my slow mental process :-)
First off remember that your proposed exhaust sucker (ES) would not be
sucking anything out of the cylinder. The piston already pushes 100% of the
cylinder volume out. (Even if there is overlap, which is a no no on a turbo,
the piston is doing all of the real work). This is important to remember for
a comment later on. Your ES would only lower the residuals left in the
combustion chamber and the exhaust port volume up to where your ES resides.
So assuming a compression of 10:1 the volume in the combustion chamber is
only 1/10 of the cylinder volume and the volume in the port is maybe 1/3 of
cylinder volume. (Assuming a 302 SBC, 618cc cylinder and a 220cc exhaust
port volume). So you are talking about reducing some exhaust residuals equal
to less than 1/2 of the cylinder volume. Is it worth it? Well, lets think
about it. Pushing out the burned mix does cost the piston some HP. So if you
could lower the delta P in the ex port, theoretically during the very short
overlap period, the ES could be helping the piston during about the last few
degrees of travel BTDC. Is there a very slight HP savings there . . . maybe?
However, the intake opens a few degrees BTDC and you would then be sucking
out your fresh air/fuel mix, which is not such a good move because your ES
is most efficient with hot expanding gas. So far no free lunch . . .! :-)
-Elon
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