[Mgs] Timing discussion
richardolindsay at gmail.com
Wed Apr 24 06:18:26 MDT 2019
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
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°
BTDC. I can't remember what the designers specify for my TD.
Rick, long winded on a Wednesday
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