Someone asked about detonation. It has been a while since I have have been
in school but I will try to remember something other than how many glasses
of beer are in a pitcher.
When people talk about combustion problems, three are bundled together that
deal with having too much of a good thing. Pinging or pinking,
preignition, and detonation. They are not neccesarily all the same thing.
Pinging is from a rapid rise in pressure inside the combustion chamber.
This can be from preignition- the fuel igniting before the spark- or it
could be from timing too advanced for conditions. It could also be just
the "nature of the beast" in certain situations. Light pinging is not
destructive. In fact, many engines run best when tuned to have a light
audible ping under acceleration.
Preignition- the fuel igniting before the spark gets there- can be
identified by pinging in some cases but usually not. Preignition is
usually caused by hot spots in the combustion chamber- either deposits that
get warm or sharp edges such as the widow's peak in an MGA combustion
chamber. Preignition almost always reduces power. In fact, I have been in
a car where all cylinders go into preignition and the engine runs but
produces no power. This is a classic situation and is described in most
engine books as a "textbook" example. Another classic example is run on.
Detonation is an entirely different beast than preignition even though many
people use the terms interchangably. Detonation occurs when the flame
front reaches a critical speed and pressure. At that point the rest of the
mixture all ignites almost simultaneously while pressures and temperatures
skyrocket. A classic example used in combustion textbooks of detonation is
a coal mine where gasses ignite. At first the fire moves in an organized
fashion up the mineshaft then the speed and pressure reach critical and
kaploowy- bodies fly out of the tunnel like popcorn. There are equations
for this which I was forced to solve between trips to the bar. I don't
remember why, however. Maybe if you made the tunnel big enough for its
length you could avoid detonation. Personally, I always found that cheap
beer and Led Zeppelin went best with math problems while imported bear and
Jethro Tull went best with history.
Anyway, back to the point of the story. Preignition and pinging are not
good but are not destructive unless detonation follows. This is a
temperature/speed/pressure thing so the key is to avoid long, narrow, hot,
high pressure situations. One of the reasons that four valve heads are so
populare now is that they have the shortest flame propogation distances
possible in a cylinder- the plug is in the middle so the flame front is
short and always expanding. Flatheads are just the opposite since the far
reaches of the combustion chamber are hot and narrow. That's why small
four valve engines run 11:1 compression ratios while a flathead chokes
around 7:1. Another engine that doesn't like high compression ratio is the
Jag XK. That deep hemi head requires a pop up pistion to make compression
and the plug is off to one side so 9:1 is it. Thers are lot's of other
variables like quench, squish, and such but the theory is the same.
Regards,
Bill Eastman
61 MGA whose widow's peaks have been shaved to avoid preignition.
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