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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.
--
Max Heim
'66 MGB
> On Apr 1, 2020, at 8:17 PM, Barney Gaylord via Mgs =
<mgs@autox.team.net> wrote:
>=20
> Okay, let's see if anyone knows the answer to this one. Why is it =
that "You do eventually get to a point where no more advance will help". =
Why do we stop spark advance at ~32d BTDC at road speed, and keep the =
advance the same for all higher engine speeds?
>=20
>=20
> At 07:56 PM 4/1/2020, Richard Ewald via Mgs wrote:
>> It takes about 3ms to burn the fuel in a cylinder from when the spark =
ignites it.=C3=82 The engine designers=C3=82 want the peak pressure to =
occur at between 5-10 ATDC.
>> At idle speed 3ms is about 15 degrees.=C3=82 =C3=82
>> So if you want your peak pressure at 5 ATDC, your base timing=C3=82 =
is 10 BTDC, if=C3=82 you want=C3=82 your peak pressure at 10 ATDC your =
timing is 5 BTDC.
>> If you look at tune up specs for non smog controlled engines, most =
have base timing in the 5-10 BTDC range.
>> Now if you double the engine speed, it still take 3ms to burn, but to =
keep the max pressure at 5-10 ATDC, you have to light the mixture =
sooner.
>> You do eventually=C3=82 get to a point where no more advance will =
help.=C3=82 That point varies with the engine=C3=82 design,=C3=82 cam =
profile, combustion chamber design, headers etc.=C3=82 =C3=82
>> Rick
>> ....
>=20
> _______________________________________________
>=20
> Mgs@autox.team.net
> Donate: http://www.team.net/donate.html
> Suggested annual donation $12.75
>=20
> Archive: http://www.team.net/pipermail/mgs =
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>=20
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>=20
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<html><head><meta http-equiv=3D"Content-Type" content=3D"text/html =
charset=3Dutf-8"></head><body style=3D"word-wrap: break-word; =
-webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" =
class=3D"">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.<div class=3D""><br class=3D""><div class=3D"">
<div style=3D"color: rgb(0, 0, 0); letter-spacing: normal; text-align: =
start; text-indent: 0px; text-transform: none; white-space: normal; =
word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: =
break-word; -webkit-nbsp-mode: space; -webkit-line-break: =
after-white-space;" class=3D""><div style=3D"color: rgb(0, 0, 0); =
letter-spacing: normal; text-align: start; text-indent: 0px; =
text-transform: none; white-space: normal; word-spacing: 0px; =
-webkit-text-stroke-width: 0px; word-wrap: break-word; =
-webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" =
class=3D""><div style=3D"margin: 0px; font-size: 13px;" =
class=3D"">--</div><div style=3D"margin: 0px; font-size: 13px; =
min-height: 16px;" class=3D"">Max Heim</div><div style=3D"margin: 0px; =
font-size: 13px;" class=3D"">'66 MGB</div></div></div>
</div>
<br class=3D""><div><blockquote type=3D"cite" class=3D""><div =
class=3D"">On Apr 1, 2020, at 8:17 PM, Barney Gaylord via Mgs <<a =
href=3D"mailto:mgs@autox.team.net"; class=3D"">mgs@autox.team.net</a>> =
wrote:</div><br class=3D"Apple-interchange-newline"><div class=3D""><div =
class=3D"">Okay, let's see if anyone knows the answer to this one. =
Why is it that "You do eventually get to a point where no more =
advance will help". Why do we stop spark advance at ~32d BTDC at =
road speed, and keep the advance the same for all higher engine =
speeds?<br class=3D""><br class=3D""><br class=3D"">At 07:56 PM =
4/1/2020, Richard Ewald via Mgs wrote:<br class=3D""><blockquote =
type=3D"cite" class=3D"">It takes about 3ms to burn the fuel in a =
cylinder from when the spark ignites it.=C3=82 The engine =
designers=C3=82 want the peak pressure to occur at between 5-10 ATDC.<br =
class=3D"">At idle speed 3ms is about 15 degrees.=C3=82 =C3=82<br =
class=3D"">So if you want your peak pressure at 5 ATDC, your base =
timing=C3=82 is 10 BTDC, if=C3=82 you want=C3=82 your peak =
pressure at 10 ATDC your timing is 5 BTDC.<br class=3D"">If you look at =
tune up specs for non smog controlled engines, most have base timing in =
the 5-10 BTDC range.<br class=3D"">Now if you double the engine speed, =
it still take 3ms to burn, but to keep the max pressure at 5-10 ATDC, =
you have to light the mixture sooner.<br class=3D"">You do eventually=C3=82=
get to a point where no more advance will help.=C3=82 That point =
varies with the engine=C3=82 design,=C3=82 cam profile, combustion =
chamber design, headers etc.=C3=82 =C3=82<br class=3D"">Rick<br =
class=3D"">....<br class=3D""></blockquote><br =
class=3D"">_______________________________________________<br =
class=3D""><br class=3D""><a href=3D"mailto:Mgs@autox.team.net"; =
class=3D"">Mgs@autox.team.net</a><br class=3D"">Donate: =
$12.75<br class=3D""><br class=3D"">Archive: =
http://www.team.net/pipermail/mgs http://autox.team.net/archive<br =
class=3D""><br class=3D"">Unsubscribe: =
http://autox.team.net/mailman/options/mgs/mvheim@sonic.net<br =
class=3D""><br class=3D""></div></div></blockquote></div><br =
class=3D""></div></body></html>=
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