Subject: [Mgs] Engine maths...and spare time

From: Richard Lindsay via Mgs <[email-masked]>
Date: Sat, 28 Mar 2020 11:57:02 -0500

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Paul shared these sage words,

> Most of what the engine is doing at idle is as a vacuum pump,
> generating about 16 in Hg. or so in the intake manifold, and is
> why when you introduce an intake vacuum leak the idle speed
> goes up.  This may be included in your 'other forces' above.
>
> Whist there may well be a most efficient point to start combustion
> and the flame front, the prime consideration has to be avoiding
> spontaneous combustion at any point, i.e. pinking or detonation.
> As the flame front travels pressure inside the engine is rising,
> but after TDC the volume available is reducing, which tends to
> counteract the pressure increase.  There is also the effect of
> leverage i.e. the angle the con rod makes relative to the piston.
>
> I'm certainly not going to check your maths, a specific engine is
> what it is, and the timing has to be set taking those specifics
> into account plus other factors like fuel grade and type.
>
> PaulH.

   In a follow-on note I recommended Colin Campbell's landmark engine
manual for LBC owners, 'The Sports Car Engine: Its Tuning and
Modification'. Here's a page from that book addressing the combustion
process. All credit to Dr. Campbell.

http://aubard.us/53_MG/20200328_113435.jpg

   I feel that this discussion is of little interest to the average LBC
owner but its been quiet here and we all have delete keys, so hopefully,
minimum foul.

Rick

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<div dir=3D"ltr">Paul shared these sage words,<br><br>&gt; Most of what the=
 engine is doing at idle is as a vacuum pump,<div>&gt; generating about 16 =
in Hg. or so in the intake manifold, and is</div><div>&gt; why when you int=
roduce an intake vacuum leak the idle speed</div><div>&gt; goes up.=C2=A0 T=
his may be included in your &#39;other forces&#39; above.<br>&gt;=C2=A0<br>=
&gt; Whist there may well be a most efficient point to start combustion</di=
v><div>&gt; and the flame front, the prime consideration has to be avoiding=
</div><div>&gt; spontaneous combustion at any point, i.e. pinking or detona=
tion.</div><div>&gt; As the flame front travels pressure inside the engine =
is rising,</div><div>&gt; but after TDC the volume available is reducing, w=
hich tends to</div><div>&gt; counteract the pressure increase.=C2=A0 There =
is also the effect of</div><div>&gt; leverage i.e. the angle the con rod ma=
kes relative to the piston.<br>&gt;=C2=A0<br>&gt; I&#39;m certainly not goi=
ng to check your maths, a specific engine is</div><div>&gt; what it is, and=
 the timing has to be set taking those specifics</div><div>&gt; into accoun=
t plus other factors like fuel grade and type.<br>&gt;=C2=A0<br>&gt; PaulH.=
</div><div><br></div><div>=C2=A0 =C2=A0In a follow-on note I recommended Co=
lin Campbell&#39;s landmark engine manual for LBC owners, &#39;The Sports C=
ar Engine: Its Tuning and Modification&#39;. Here&#39;s a page from that bo=
ok addressing the combustion process. All credit to Dr. Campbell.</div><div=
><br></div><div><a href=3D"http://aubard.us/53_MG/20200328_113435.jpg">http=
://aubard.us/53_MG/20200328_113435.jpg</a></div><div><br></div><div>=C2=A0 =
=C2=A0I feel that this discussion is of little interest to the average LBC =
owner but its been quiet here and we all have delete keys, so hopefully, mi=
nimum foul.</div><div><br></div><div>Rick=C2=A0<br></div></div>

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