Les -
You're right, of course, that's why I said >relatively< constant. The
flame propagation time does vary under certain circumstances (like when you
mash the throttle at low RPMs). Remember though, that the pinging goes
away as the RPMs increase (the rotational speed catches up with the
increased flame propagation speed).
The point of my parenthetical comment was why timing had to change with
increased RPMs, so I simplified for the sake of discussion. Relative to
the change in rotational speed between 1000 and 4000 RPM (a 300% increase),
flame propagation doesn't very by much, hence the need for ignition timing
advance as RPMs increase.
Chris Kotting
ckotting@iwaynet.net
On Thursday, June 25, 1998 9:08 PM, Les Myer [SMTP:lmyer@probe.net] wrote:
> At 12:21 PM 6/25/98 -0400, you wrote:
> >Actually, the major factor in having the "bang" happen closer to the
> >"spark" isn't the exhaust valve opening. It's that the "bang" has to
> >happen when the piston is just about at TDC (actually a tiny bit after).
> > Much later than that and you lose power (you don't get the maximum
"push"
> >out of the "bang", making more of the "bang" result in waste heat), much
> >before that and you get knocking and engine damage (the "bang" tries to
> >turn things the other way). (The reason that you need to advance the
> >ignition timing at higher RPMs is that the time lag between "spark" and
> >"bang" is relatively constant, but the faster the engine turns, the more
> >degrees of rotation happen in that time interval.)
> >
> I think the flame travel varies upon a number of factors, instead of
being
> constant. That is why engines ping when you step on the gas, even though
> the rpm is the same. If this was constant, I think there wouldn't need
to
> be a vacuum advance unit.
>
> Les
> >It seems to me that if you can minimize that time lag, you stand a
better
> >chance of being able to make the "bang" happen consistently at the
optimal
> >moment. Can anybody confirm/correct this supposition?
> >
> >Chris Kotting
> >ckotting@iwaynet.net
> >
> >On Thursday, June 25, 1998 12:50 PM, Editors, Molecular Vision
> >[SMTP:jboatri@emory.edu] wrote:
> >> True and I agree with you and Les: throwing in a hodgepodge of smoke
and
> >> mirrors may do a lot more harm than good. OTOH, like Chris Kotting, if
I
> >> can avoid gapping points by using a Hall cell, I'll use it if I can
> >afford
> >> it. Having sold a couple of parts lately, I could afford it. A higher
> >> voltage coil allows one to increase spark plug gap, but as Chris
points
> >> out, the coil and other downstream components (be they Hall cells or
> >> points) must operate at higher temps and so could fail, or rather,
have a
> >> lower mean time between failures. The issue of whether increased spark
> >plug
> >> gap does _anything_ beneficial or harmful appears to be the largest
point
> >> of discussion.
> >>
> >> >From a practical standpoint, if 0.025 plug gap ignites a real-world
> >range
> >> of mixtures that vary with winter/summer grade fuel or other fuel
quality
> >> issues, density altitude variations, and acceleration demands, then
there
> >> is little reason to go to a larger gap. But two questions remain: (1)
> >does
> >> a 0.025 gap always ignite real-world ranges of mixtures? and (2) can
> >> further performance (whether economy or power) be realized with an
> >> increased gap leading to a shorter lag time between spark and bang (to
> >> quote Chris). Would this allow for more of the mixture to be burned
> >before
> >> the exhaust port is opened? Is a faster burn better inherently, or
must
> >> many other variables (mixture quality, cylinder and piston design,
> >timing,
> >> etc) be matched?
> >>
> >> Finally, let's face it, the 500 lb gorilla here is that those gold
Sport
> >> coils look really cool...:)
> >>
> >> At 9:00 AM -0500 6/25/98, Shawn J. Tobin wrote:
> >>
> >>
> >> > I used to think that throwing a kluge of performance parts together
> >would
> >> > result in a really wicked ride. This kind of thinking was due to a
> >vivid
> >> > imagination and the naive assumption that advertising claims were
> >> > guaranteed to be true because the government kept them to their
word.
> >> >
> >> ...
> >>
> >>
> >>
> >> Jeffrey H. Boatright, PhD
> >> Senior Editor, Molecular Vision
> >> http://www.molvis.org/molvis
> >> Mailto:jboatri@emory.edu
> >> 404-778-4113
> >>
> >>
> >
> >
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