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That's exactly what does happen, because ignition is anything from 10 =
degrees upwards BTDC. It's a trade-off between how much power you lose =
by too much of the fuel burning while the piston is still coming up - =
over advanced, against how much you lose with fuel still burning far =
into the expansion stroke - retarded. There is a sweet-spot, but that =
continually changes with rpm and throttle opening, which is why we have =
centrifugal and vacuum advance. Centrifugal increases the advance as =
the rpm increases and the likelihood of detonation decreases, vacuum =
advance backs off the timing as the throttle is opened and more mixture =
goes into the cylinder giving higher cylinder pressures, which increases =
the likelihood of detonation.
PaulH.
----- Original Message -----=20
Well, it seems to me that if you advance the spark to a point on the =
compression stroke where the piston is still coming up, the expanding =
ignited mixture would be fighting the piston, and it would actually =
reduce power. I=E2=80=99m not sure if that is what you are talking =
about, though.
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<DIV><FONT size=3D2>That's exactly what does happen, because =
ignition is=20
anything from 10 degrees upwards BTDC. It's a trade-off between =
how much=20
power you lose by too much of the fuel burning while the piston is still =
coming=20
up - over advanced, against how much you lose with fuel still burning =
far into=20
the expansion stroke - retarded. There is a sweet-spot, but that=20
continually changes with rpm and throttle opening, which is why we =
have=20
centrifugal and vacuum advance. Centrifugal increases the advance =
as the=20
rpm increases and the likelihood of detonation decreases, vacuum =
advance=20
backs off the timing as the throttle is opened and more mixture goes =
into the=20
cylinder giving higher cylinder pressures, which increases the =
likelihood of=20
detonation.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>PaulH.</FONT></DIV>
<BLOCKQUOTE=20
style=3D"BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; =
PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<DIV style=3D"FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV><FONT size=3D2></FONT><BR></DIV>Well, it seems to me that if you =
advance=20
the spark to a point on the compression stroke where the piston is =
still=20
coming up, the expanding ignited mixture would be fighting the piston, =
and it=20
would actually reduce power. I=E2=80=99m not sure if that is what you =
are talking=20
about, though.</BLOCKQUOTE></BODY></HTML>
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