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Remember the intake valves stay open for a bit past BDC as the piston
starts up, also to get the mixture to go from flammable to flat out
explosive the charge has to be compressed. so those factors tend to limit
how much effective timing you can have.
Going a little deeper when you are using a mechanical distributor with
weights there is only so much refinement you can get in your advance
curve. Once you get into fully electronic ignition systems, there are
systems that have upward to 70 degrees of range from fully retard to full
advance (they may not use it in all applications, but the capability is
there in the electronics)
Rick
On Thu, Apr 2, 2020 at 11:33 AM Max Heim <mvheim@sonic.net> wrote:
> Well, I mean, it would be true at some extreme =E2=80=94 for example, ign=
iting the
> spark at the very beginning of the compression stroke. But I suspect Barn=
ey
> was getting at something else.
>
> --
> Max Heim
> '66 MGB
>
> On Apr 2, 2020, at 11:24 AM, Richard Lindsay <richardolindsay@gmail.com>
> wrote:
>
> 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 redu=
ce
>> power. I=E2=80=99m not sure if that is what you are talking about, thoug=
h.
>>
>> --
>> 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 t=
he
>> 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 =
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 1=
0
>> BTDC, if=C3=82 you want=C3=82 your peak pressure at 10 ATDC your timin=
g 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, combustio=
n chamber
>> design, headers etc.=C3=82 =C3=82
>> Rick
>> ....
>>
>>
>> _______________________________________________
>>
>> Mgs@autox.team.net
>> Donate: http://www.team.net/donate.html
>> Suggested annual donation $12.75
>>
>> Archive: http://www.team.net/pipermail/mgs http://autox.team.net/archive
>>
>> Unsubscribe: http://autox.team.net/mailman/options/mgs/mvheim@sonic.net
>>
>>
>>
>
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<div dir=3D"ltr">Remember the intake valves stay open for a bit past BDC as=
the piston starts up, also to get the mixture to go from flammable=C2=A0to=
flat out explosive the charge has to be compressed.=C2=A0 so those factors=
tend to limit how much effective timing you can have.=C2=A0=C2=A0<div>Goin=
g a little deeper when you are using a mechanical distributor=C2=A0with wei=
ghts there is only so much refinement you can get in your advance curve.=C2=
=A0 Once you get into fully electronic ignition systems, there are systems =
that have upward to 70 degrees of range from fully retard to full advance (=
they may not use it in all applications, but the capability is there in the=
electronics)</div><div>Rick</div></div><br><div class=3D"gmail_quote"><div=
dir=3D"ltr" class=3D"gmail_attr">On Thu, Apr 2, 2020 at 11:33 AM Max Heim =
<<a href=3D"mailto:mvheim@sonic.net";>mvheim@sonic.net</a>> wrote:<br>=
</div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;b=
order-left:1px solid rgb(204,204,204);padding-left:1ex"><div style=3D"overf=
low-wrap: break-word;">Well, I mean, it would be true at some extreme =E2=
=80=94 for example, igniting the spark at the very beginning of the compres=
sion stroke. But I suspect Barney was getting at something else.<div><br><d=
iv>
<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"><div st=
yle=3D"color:rgb(0,0,0);letter-spacing:normal;text-align:start;text-indent:=
0px;text-transform:none;white-space:normal;word-spacing:0px"><div style=3D"=
margin:0px;font-size:13px">--</div><div style=3D"margin:0px;font-size:13px;=
min-height:16px">Max Heim</div><div style=3D"margin:0px;font-size:13px">=
9;66 MGB</div></div></div>
</div>
<br><div><blockquote type=3D"cite"><div>On Apr 2, 2020, at 11:24 AM, Richar=
d Lindsay <<a href=3D"mailto:richardolindsay@gmail.com"; target=3D"_blank=
">richardolindsay@gmail.com</a>> wrote:</div><br><div><div dir=3D"auto">=
<div>Max that might be true if the ignition pressure from the combustion di=
dn't build so long after the spark...within reasonable 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"; target=
=3D"_blank">mvheim@sonic.net</a>> wrote:<br></div><blockquote class=3D"g=
mail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204=
,204,204);padding-left:1ex"><div style=3D"overflow-wrap: break-word;">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><br><div>
<div style=3D"letter-spacing:normal;text-align:start;text-indent:0px;text-t=
ransform:none;white-space:normal;word-spacing:0px"><div style=3D"letter-spa=
cing:normal;text-align:start;text-indent:0px;text-transform:none;white-spac=
e:normal;word-spacing:0px"><div style=3D"margin:0px;font-size:13px">--</div=
><div style=3D"margin:0px;font-size:13px;min-height:16px">Max Heim</div><di=
v 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"; rel=3D"noreferrer=
" target=3D"_blank">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"; rel=3D"noreferrer" target=3D"_blank">Mgs@aut=
ox.team.net</a><br>Donate: <a href=3D"http://www.team.net/donate.html"; rel=
=3D"noreferrer" target=3D"_blank">http://www.team.net/donate.html</a><br>Su=
ggested annual donation =C2=A0$12.75<br><br>Archive: <a href=3D"http://www.=
team.net/pipermail/mgs" rel=3D"noreferrer" target=3D"_blank">http://www.tea=
m.net/pipermail/mgs</a> <a href=3D"http://autox.team.net/archive"; rel=3D"no=
referrer" target=3D"_blank">http://autox.team.net/archive</a><br><br>Unsubs=
cribe: <a href=3D"http://autox.team.net/mailman/options/mgs/mvheim@sonic.ne=
t" rel=3D"noreferrer" target=3D"_blank">http://autox.team.net/mailman/optio=
ns/mgs/mvheim@sonic.net</a><br><br></div></div></blockquote></div><br></div=
></div></blockquote></div></div></div>
</div></blockquote></div><br></div></div></blockquote></div>
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