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Max that might be true if the ignition pressure from the combustion didn't
build so long after the spark...within reasonable limits.
On Thu, Apr 2, 2020, 12:11 PM Max Heim <mvheim@sonic.net> wrote:
> 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 reduc=
e
> 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:
>
> 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 th=
e
> same for all higher engine speeds?
>
>
> 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 o=
ccur 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 kee=
p
> 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
> ....
>
>
> _______________________________________________
>
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<div dir=3D"auto"><div>Max that might be true if the ignition pressure from=
the combustion didn't build so long after the spark...within reasonabl=
e limits.<br><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail=
_attr">On Thu, Apr 2, 2020, 12:11 PM Max Heim <<a href=3D"mailto:mvheim@=
sonic.net">mvheim@sonic.net</a>> wrote:<br></div><blockquote class=3D"gm=
ail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-le=
ft:1ex"><div style=3D"word-wrap:break-word">Well, it seems to me that if yo=
u advance the spark to a point on the compression stroke where the piston i=
s still coming up, the expanding ignited mixture would be fighting the pist=
on, and it would actually reduce power. I=E2=80=99m not sure if that is wha=
t you are talking about, though.<div><br><div>
<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;word-wra=
p:break-word"><div style=3D"color:rgb(0,0,0);letter-spacing:normal;text-ali=
gn:start;text-indent:0px;text-transform:none;white-space:normal;word-spacin=
g:0px;word-wrap:break-word"><div style=3D"margin:0px;font-size:13px">--</di=
v><div style=3D"margin:0px;font-size:13px;min-height:16px">Max Heim</div><d=
iv style=3D"margin:0px;font-size:13px">'66 MGB</div></div></div>
</div>
<br><div><blockquote type=3D"cite"><div>On Apr 1, 2020, at 8:17 PM, Barney =
Gaylord via Mgs <<a href=3D"mailto:mgs@autox.team.net"; target=3D"_blank"=
rel=3D"noreferrer">mgs@autox.team.net</a>> wrote:</div><br><div><div>Ok=
ay, let's see if anyone knows the answer to this one.=C2=A0 Why is it t=
hat "You do eventually get to a point where no more advance will help&=
quot;.=C2=A0 Why do we stop spark advance at ~32d BTDC at road speed, and k=
eep the advance the same for all higher engine speeds?<br><br><br>At 07:56 =
PM 4/1/2020, Richard Ewald via Mgs wrote:<br><blockquote type=3D"cite">It t=
akes about 3ms to burn the fuel in a cylinder from when the spark ignites i=
t.=C3=82 =C2=A0The engine designers=C3=82 want the peak pressure to occur a=
t between 5-10 ATDC.<br>At idle speed 3ms is about 15 degrees.=C3=82 =C3=82=
<br>So if you want your peak pressure at 5 ATDC, your base timing=C3=82 is =
10 BTDC, if=C3=82 =C2=A0you want=C3=82 =C2=A0your peak pressure at 10 ATDC =
your timing is 5 BTDC.<br>If you look at tune up specs for non smog control=
led engines, most have base timing in the 5-10 BTDC range.<br>Now if you do=
uble the engine speed, it still take 3ms to burn, but to keep the max press=
ure at 5-10 ATDC, you have to light the mixture sooner.<br>You do eventuall=
y=C3=82 get to a point where no more advance will help.=C3=82 =C2=A0That po=
int varies with the engine=C3=82 design,=C3=82 =C2=A0cam profile, combustio=
n chamber design, headers etc.=C3=82 =C2=A0=C3=82<br>Rick<br>....<br></bloc=
kquote><br>_______________________________________________<br><br><a href=
=3D"mailto:Mgs@autox.team.net"; target=3D"_blank" rel=3D"noreferrer">Mgs@aut=
ox.team.net</a><br>Donate: <a href=3D"http://www.team.net/donate.html"; targ=
et=3D"_blank" rel=3D"noreferrer">http://www.team.net/donate.html</a><br>Sug=
gested annual donation =C2=A0$12.75<br><br>Archive: <a href=3D"http://www.t=
eam.net/pipermail/mgs" target=3D"_blank" rel=3D"noreferrer">http://www.team=
.net/pipermail/mgs</a> <a href=3D"http://autox.team.net/archive"; target=3D"=
_blank" rel=3D"noreferrer">http://autox.team.net/archive</a><br><br>Unsubsc=
ribe: <a href=3D"http://autox.team.net/mailman/options/mgs/mvheim@sonic.net=
" target=3D"_blank" rel=3D"noreferrer">http://autox.team.net/mailman/option=
s/mgs/mvheim@sonic.net</a><br><br></div></div></blockquote></div><br></div>=
</div></blockquote></div></div></div>
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