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Bob:<br>
Exactly so. You've just described how, at low r.p.m., the valve
overlap actually hurts volumetric efficiency, by allowing some of the
intake charge to escape back into the intake tract, and be contaminated
by exhaust gasses. This, in fact, can be seen as a "standoff" of fuel
vapor above a carburetor intake, as the intake charge is forced out
"backward" before being ingested. With larger engines, you can
actually "tune" the engine as to where you want the peak torque to be,
by changing camshaft timing a few degrees. (Be careful, though; you
don't want the valves kissing the piston tops.)<br>
<br>
Bill Glaze<br>
<br>
Bob Richards wrote:
<blockquote
cite="mid20050524014748.25812.qmail@web30301.mail.mud.yahoo.com"
type="cite">
<pre wrap="">Bill,
Ok, if I understand you correctly, the low pressure
helps suck in the new fuel/air mixture during the
valve overlap.
Come to think of it, isn't this similar to the way a
pulse jet works?
I guess any kind of backpressure would defeat or
minimize the process, which I think is the case with
our setups.
Bob R.
--- Bill Glaze <a class="moz-txt-link-rfc2396E" href="mailto:billglaze@triad.rr.com"><billglaze@triad.rr.com></a> wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Bob:
One small correction: the outgoing exhaust pulse,
which is under
pressure, creates a small vacuum behind itself as it
escapes; this
creates a low pressure area that helps draw the new
charge into the
combustion chamber. I've used a "tuned" exhaust on
engines as large as
540 cu. in. The tuning takes a little time on the
race course, but it's
really worth it. It doesn't work quite the same
with a blown
(supercharged) engine, because the blower pressure
masks the small (by
comparison) vacuum behind the outgoing exhaust.
Bill Glaze
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