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Hello again,
I write to open a discussion about ignition timing. Over on the Triumph
forum ( Apologies for using profanity. ) a member wrote, "Just advance the
timing until you notice preignition." Now I recognize that Triumphs use
farm implement engines, but that advice seems a bit sketchy. So let me
write a preface then ask a question.
The whole reason for ignition advance is to allow time for the flame
front to progress across the combustion chamber and by doing so, to create
the MEP (Mean Effective Pressure) where it does the most work. That is,
where that maximum cylinder pressure occurs with respect to the piston
position and therefore the connecting rod's angular relation to the
crankshaft. Simple geometry says that will happen when the piston is
halfway down, where the rod is at a right angle to the crank pin. But of
course, the MEP is just a single number and theoretical. The pressure
builds as the flame front progresses and continues to build as the piston
moves down.
The whole process is of course dynamic, and I like dynamic systems.
Consider this; Before TDC the volume of the combustion space is decreasing,
reaching a minimum at TDC. After TDC, the combustion volume is increasing
thereby decreasing the pressure, right where the burning charge is
increasing the pressure. The dynamics of that pressure increase-decrease
becomes obvious, if complex.
So I wonder if the best solution isn't to just trust the engine
designers' knowledge, experience and testing? And by doing so, to set the
timing to their recommended position - at least as a starting point?
Okay, to counter that thought; We know engines wear and that their
characteristics change. We also know that fuels are different today. My old
TD's XPAG was designed when fuel's typical octane value was in the low 70s!
Back to the discussion. We know that the progression of the flame across
the combustion chamber is roughly constant. So the occurance of the MEP is
constant, relative to the spark - but as engine speed increases, so does
piston speed. So the centrifugal advance is there to alight the charge
earlier to keep the MEP where it might do the most good.
The vacuum advance is a whole other idea. Its basically a system to
compensate for load. And in later cars, the advance and sometimes retard
capsules are emissions related. One last comment on this. My Maserati
Biturbo has capsules on both sides of the distributor but BOTH provide
advance! One is vacuum operated, as with our LBCs. The other is pressure
operated, offering advance when the twin turbos raise the plenum pressure
above ambient. And we thing our LBCs are complex!
Where and how do YOU set your cars' ignition timing? My TR6 wants 4=C2=
=B0
BTDC. I can't remember what the designers specify for my TD.
Rick, long winded on a Wednesday
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<div dir=3D"auto">Hello again,<div dir=3D"auto"><br></div><div dir=3D"auto"=
>=C2=A0 =C2=A0I write to open a discussion about ignition timing. Over on t=
he Triumph forum ( Apologies for using profanity. ) a member wrote, "J=
ust advance the timing until you notice preignition." Now I recognize =
that Triumphs use farm implement engines, but that advice seems a bit sketc=
hy. So let me write a preface then ask a question.</div><div dir=3D"auto">=
=C2=A0 =C2=A0The whole reason for ignition advance is to allow time for the=
flame front to progress across the combustion chamber and by doing so, to =
create the MEP (Mean Effective Pressure) where it does the most work. That =
is, where that maximum cylinder pressure occurs with respect to the piston =
position and therefore the connecting rod's angular relation to the cra=
nkshaft. Simple geometry says that will happen when the piston is halfway d=
own, where the rod is at a right angle to the crank pin. But of course, the=
MEP is just a single number and theoretical. The pressure builds as the fl=
ame front progresses and continues to build as the piston moves down.</div>=
<div dir=3D"auto">=C2=A0 =C2=A0The whole process is of course dynamic, and =
I like dynamic systems. Consider this; Before TDC the volume of the combust=
ion space is decreasing, reaching a minimum at TDC. After TDC, the combusti=
on volume is increasing thereby decreasing the pressure, right where the bu=
rning charge is increasing the pressure. The dynamics of that pressure incr=
ease-decrease becomes obvious, if complex.</div><div dir=3D"auto">=C2=A0 =
=C2=A0So I wonder if the best solution isn't to just trust the engine d=
esigners' knowledge, experience and testing? And by doing so, to set th=
e timing to their recommended position - at least as a starting point?</div=
><div dir=3D"auto">=C2=A0 =C2=A0Okay, to counter that thought; We know engi=
nes wear and that their characteristics change. We also know that fuels are=
different today. My old TD's XPAG was designed when fuel's typical=
octane value was in the low 70s!</div><div dir=3D"auto">=C2=A0 =C2=A0Back =
to the discussion. We know that the progression of the flame across the com=
bustion chamber is roughly constant. So the occurance of the MEP is constan=
t, relative to the spark - but as engine speed increases, so does piston sp=
eed. So the centrifugal advance is there to alight the charge earlier to ke=
ep the MEP where it might do the most good.</div><div dir=3D"auto">=C2=A0 =
=C2=A0The vacuum advance is a whole other idea. Its basically a system to c=
ompensate for load. And in later cars, the advance and sometimes retard cap=
sules are emissions related. One last comment on this. My Maserati Biturbo =
has capsules on both sides of the distributor but BOTH provide advance! One=
is vacuum operated, as with our LBCs. The other is pressure operated, offe=
ring advance when the twin turbos raise the plenum pressure above ambient. =
And we thing our LBCs are complex!</div><div dir=3D"auto"><br></div><div di=
r=3D"auto">=C2=A0 =C2=A0Where and how do YOU set your cars' ignition ti=
ming? My TR6 wants 4=C2=B0 BTDC. I can't remember what the designers sp=
ecify for my TD.</div><div dir=3D"auto"><br></div><div dir=3D"auto">Rick, l=
ong winded on a Wednesday</div><div dir=3D"auto">=C2=A0 =C2=A0</div></div>
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