Comet badly tuned or just underpowered?

[gwild]

Dear all,
Thanks to many helpful people on and off the forum my basketcase Comet goes, and even stops nicely.
I wondered if I could call upon the collective wisdom of the forum to help me with a little question of tuning:-

Riding last Sunday though it was having problems. With my little experience I worry whether it was just bad luck or something more drastic. Here are the symptoms:

Riding two-up from Oxford to Henley (not particularly far nor hilly) the bike seemed unwilling to travel faster than 50 mph in fourth gear. If I opened the throttle further a noise like a small hammer striking metal at quite high frequency was emitted.
As the hills got longer I was forced to decrease speed and gears further until the bike would not go without the noise. At the top of Gangsdown Hill I cut the engine and let it cool for 20 minutes whereupon it was difficult to start (starts first kick when cold, always harder when warm).
Worried about missing an appointment I swapped plugs and proceeded the remainder of the journey under 40 mph, and returned alone.

Further observations:
- The day was the hottest this year, but this is still March in the UK so not above 20 degrees C
- I've noticed when accelerating (solo) from standstill if I open the throttle too far too fast I get a similar hammering noise. This stops if I open the throttle at a slower pace to match engine speed.

As this is my only motorised conveyance
1. Are all Comets this slow when two- up? (I'll think about buying a twin some time after I get my first proper job!)
2. If not a) is this something I should be worried about or to b) ignore and get on with it

If 2. a) as I am fearing, what would be a logical set of tests to find and cure the problem? There are so many variables I don't really know where to start.

thanks very much in anticipation,

gwild

1952ish C Comet with stock carb, magneto and dynamo, 12v electrics and camouflaged modern rectifier

 

[davidd]

Dear Gwild,

I think that it would further the discussion to give some of the specifics of the bike. What is the compression, what plug, what rear sproket, what silencer, etc. If you are going to work the bike hard, I do not think you should be running the piston clearances too tight. With the Comet, you are shedding all the heat through one cylinder and the temperatures can fluctuate much more quickly than on the twin.

It does sound a bit weak. I think that unless the physical characteristics of you and your passenger yeilds astoundingly high traction enhancement, I would have thought 60 mph more reasonable, 75-80mph solo.

David

 

[Peter Stokes]

Hello gwild

I think I would check the ignition timing first - do others think this is reasonable? I am not up to date with what is the correct timing (taking account of modern fuels and modern thinking) but there are experts out there...

It is some years since I had my Comet, but I remember that it went quite well - I think yours should go better than it does.

Good luck - Pete

I have re-read your post and see that the Comet is your only transport. Good for you, mine was too, and I learned a lot!

 

[roy the mechanic]

from your description it sounds like detonation[pinking] first check ignition timing, second look a the plug colour, if it's white it is most likely weak mixture. weather this is down to jetting, or fuel starvation , i cannot tell from this distance. certainly this needs to get sorted before you damage your motor.

 

[Tom Gaynor]

Comets are overgeared. Taking a tooth off the gearbox makes them slightly sprightlier. I do know someone who thinks the best way to tune a Comet is to sell it and buy a Velo Venom. There is however a good reason for there being so many 570, 600, and 650 cc Comets about, which I'm sure you can figure out. And rebuilding a Comet as 90 mm bore, giving 572 cc, is not actually much more expensive than building one standard. And those 90 bores GO, even uphill, two up, with tall Dutch people called Nobby and Ate riding them.
It sounds to me as though you have at least one fixable problem, maybe two. I think your magneto is going to glory, because it starts easy cold, but not hot. These are classic symptoms. Dave Lindsley is the man you need (see MPH). If the magneto IS going west, most other tests are misleading. If you haven't got a decent spark, then the mixture won't burn properly, and your plug will look rich. I know someone who seized a Manx Norton (ex-pen-sive...) having failed to realise it was the mag, not the main jet that gave the excess of unburnt mixture.
The second problem sounds like pinking: this suggests your spark is too far advanced. Try knocking it back to 34 deg BTDC. Time it on full advance (ATD wedged) to find out. The ATD has a nominal range of about 30 degrees, but wear increases this. So if you timed on fully retarded, i.e. not wedged, and the range was more than 30 degrees...
One last tip: if the ATD is rusted and worn, THROW THE BLOODY THING AWAY and 'phone Roy Price (MPH) to buy a new one. IMHO there are things that are simply not satisfactorily repairable, and Lucas ATD's are one of them.

 

[Hugo Myatt]

Hi Gwild,

As everyone says it definitely ain't right. Join the club and get an expert from your local section to assist. A standard Comet should be good for the mid eighties solo and not much different two up.

Hugo

 

[clevtrev]

from your description it sounds like detonation[pinking] first check ignition timing, second look a the plug colour, if it's white it is most likely weak mixture. weather this is down to jetting, or fuel starvation , i cannot tell from this distance. certainly this needs to get sorted before you damage your motor.

Detonation is not pinking, it`s pre-ignition. Detonation occurs after TDC.
Copied from Jtan.
There are many articles on the difference between the two at your Google
fingertips. Here is one:

Reprinted from Issue 54 of CONTACT! Magazine, published in January, 2000
All high output engines are prone to destructive tendencies as a result of
over boost, misfueling, mis-tuning and inadequate cooling. The engine
community pushes ever nearer to the limits of power output. As they often
learn cylinder chamber combustion processes can quickly gravitate to engine
failure. This article defines two types of engine failures, detonation and
pre-ignition, that are as insidious in nature to users as they are hard to
recognize and detect. This discussion is intended only as a primer about
these combustion processes since whole books have been devoted to the
subject.
First, let us review normal combustion. It is the burning of a fuel and air
mixture charge in the combustion chamber. It should burn in a steady, even
fashion across the chamber, originating at the spark plug and progressing
across the chamber in a three dimensional fashion. Similar to a pebble in a
glass smooth pond with the ripples spreading out, the flame front should
progress in an orderly fashion. The burn moves all the way across the
chamber and , quenches (cools) against the walls and the piston crown. The
burn should be complete with no remaining fuel-air mixture. Note that the
mixture does not "explode" but burns in an orderly fashion.
There is another factor that engineers look for to quantify combustion. It
is called "location of peak pressure (LPP)." It is measured by an
in-cylinder pressure transducer. Ideally, the LPP should occur at 14 degrees
after top dead center. Depending on the chamber design and the burn rate, if
one would initiate the spark at its optimum timing (20 degrees BTDC, for
example) the burn would progress through the chamber and reach LPP, or peak
pressure at 14 degrees after top dead center. LPP is a mechanical factor
just as an engine is a mechanical device. The piston can only go up and down
so fast. If you peak the pressure too soon or too late in the cycle, you
won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any
engine.
I introduce LPP now to illustrate the idea that there is a characteristic
pressure buildup (compression and combustion) and decay (piston downward
movement and exhaust valve opening) during the combustion process that can
be considered "normal" if it is smooth, controlled and its peak occurs at 14
degrees ATDC.
Our enlarged definition of normal combustion now says that the charge/bum is
initiated with the spark plug, a nice even burn moves across the chamber,
combustion is completed and peak pressure occurs at at 14 ATDC.
Confusion and a lot of questions exist as to detonation and pre-ignition.
Sometimes you hear mistaken terms like "pre-detonation". Detonation is one
phenomenon that is abnormal combustion. Pre-ignition is another phenomenon
that is abnormal combustion. The two, as we will talk about, are somewhat
related but are two distinctly different phenomenon and can induce
distinctly different failure modes.
KEY DEFINITIONS
Detonation: Detonation is the spontaneous combustion of the end-gas
(remaining fuel/air mixture) in the chamber. It always occurs after normal
combustion is initiated by the spark plug. The initial combustion at the
spark plug is followed by a normal combustion burn. For some reason, likely
heat and pressure, the end gas in the chamber spontaneously combusts. The
key point here is that detonation occurs after you have initiated the normal
combustion with the spark plug.
Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior
to the spark plug firing. Anytime something causes the mixture in the
chamber to ignite prior to the spark plug event it is classified as
pre-ignition. The two are completely different and abnormal phenomenon.

 

[clevtrev]

Carried on from previous.

DETONATION
Unburned end gas, under increasing pressure and heat (from the normal
progressive burning process and hot combustion chamber metals) spontaneously
combusts, ignited solely by the intense heat and pressure. The remaining
fuel in the end gas simply lacks sufficient octane rating to withstand this
combination of heat and pressure.
Detonation causes a very high, very sharp pressure spike in the combustion
chamber but it is of a very short duration. If you look at a pressure trace
of the combustion chamber process, you would see the normal burn as a normal
pressure rise, then all of a sudden you would see a very sharp spike when
the detonation occurred. That spike always occurs after the spark plug
fires. The sharp spike in pressure creates a force in the combustion
chamber. It causes the structure of the engine to ring, or resonate, much as
if it were hit by a hammer. Resonance, which is characteristic of combustion
detonation, occurs at about 6400 Hertz. So the pinging you hear is actually
the structure of the engine reacting to the pressure spikes. This noise of
detonation is commonly called spark knock. This noise changes only slightly
between iron and aluminum. This noise or vibration is what a knock sensor
picks up. The knock sensors are tuned to 6400 hertz and they will pick up
that spark knock. Incidentally, the knocking or pinging sound is not the
result of "two flame fronts meeting" as is often stated. Although this clash
does generate a spike the noise you sense comes from the vibration of the
engine structure reacting to the pressure spike.
One thing to understand is that detonation is not necessarily destructive.
Many engines run under light levels of detonation, even moderate levels.
Some engines can sustain very long periods of heavy detonation without
incurring any damage. If you've driven a car that has a lot of spark advance
on the freeway, you'll hear it pinging. It can run that way for thousands
and thousands of miles. Detonation is not necessarily destructive. It's not
an optimum situation but it is not a guaranteed instant failure. The higher
the specific output (HP/in3) of the engine, the greater the sensitivity to
detonation. An engine that is making 0.5 HP/in3 or less can sustain moderate
levels of detonation without any damage; but an engine that is making 1.5
HP/in3, if it detonates, it will probably be damaged fairly quickly, here I
mean within minutes.
Detonation causes three types of failure:
1. Mechanical damage (broken ring lands)
2. Abrasion (pitting of the piston crown)
3. Overheating (scuffed piston skirts due to excess heat input or high
coolant
temperatures)
The high impact nature of the spike can cause fractures; it can break the
spark plug electrodes, the porcelain around the plug, cause a clean fracture
of the ring land and can actually cause fracture of valves-intake or
exhaust. The piston ring land, either top or second depending on the piston
design, is susceptible to fracture type failures. If I were to look at a
piston with a second broken ring land, my immediate suspicion would be
detonation.
Another thing detonation can cause is a sandblasted appearance to the top of
the piston. The piston near the perimeter will typically have that kind of
look if detonation occurs. It is a swiss-cheesy look on a microscopic basis.
The detonation, the mechanical pounding, actually mechanically erodes or
fatigues material out of the piston. You can typically expect to see that
sanded look in the part of the chamber most distant from the spark plug,
because if you think about it, you would ignite the flame front at the plug,
it would travel across the chamber before it got to the farthest reaches of
the chamber where the end gas spontaneously combusted. That's where you will
see the effects of the detonation; you might see it at the hottest part of
the chamber in some engines, possibly by the exhaust valves. In that case
the end gas was heated to detonation by the residual heat in the valve.
In a four valve engine with a pent roof chamber with a spark plug in the
center, the chamber is fairly uniform in distance around the spark plug. But
one may still may see detonation by the exhaust valves because that area is
usually the hottest part of the chamber. Where the end gas is going to be
hottest is where the damage, if any, will occur.
Because this pressure spike is very severe and of very short duration, it
can actually shock the boundary layer of gas that surrounds the piston.
Combustion temperatures exceed 1800 degrees. If you subjected an aluminum
piston to that temperature, it would just melt. The reason it doesn't melt
is because of thermal inertia and because there is a boundary layer of a few
molecules thick next to the piston top. This thin layer isolates the flame
and causes it to be quenched as the flame approaches this relatively cold
material. That combination of actions normally protects the piston and
chamber from absorbing that much heat. However, under extreme conditions the
shock wave from the detonation spike can cause that boundary layer to
breakdown which then lets a lot of heat transfer into those surfaces.
Engines that are detonating will tend to overheat, because the boundary
layer of gas gets interrupted against the cylinder head and heat gets
transferred from the combustion chamber into the cylinder head and into the
coolant. So it starts to overheat. The more it overheats, the hotter the
engine, the hotter the end gas, the more it wants to detonate, the more it
wants to overheat. It's a snowball effect. That's why an overheating engine
wants to detonate and that's why engine detonation tends to cause
overheating.
Many times you will see a piston that is scuffed at the "four corners". If
you look at the bottom side of a piston you see the piston pin boss. If you
look across each pin boss it's solid aluminum with no flexibility. It
expands directly into the cylinder wall. However, the skirt of a piston is
relatively flexible. If it gets hot, it can deflect. The crown of the piston
is actually slightly smaller in diameter on purpose so it doesn't contact
the cylinder walls. So if the piston soaks up a lot of heat, because of
detonation for instance, the piston expands and drives the piston structure
into the cylinder wall causing it to scuff in four places directly across
each boss. It's another dead give-a-way sign of detonation. Many times
detonation damage is just limited to this.
Some engines, such as liquid cooled 2-stroke engines found in snowmobiles,
watercraft and motorcycles, have a very common detonation failure mode. What
typically happens is that when detonation occurs the piston expands
excessively, scurfs in the bore along those four spots and wipes material
into the ring grooves. The rings seize so that they can't conform to the
cylinder walls. Engine compression is lost and the engine either stops
running, or you start getting blow-by past the rings. That torches out an
area. Then the engine quits.
In the shop someone looks at the melted result and says, "pre-ignition
damage". No, it's detonation damage. Detonation caused the piston to scuff
and this snowballed into loss of compression and hot gas escaping by the
rings that caused the melting. Once again, detonation is a source of
confusion and it is very difficult, sometimes, to pin down what happened,
but in terms of damage caused by detonation, this is another typical sign.
While some of these examples may seem rather tedious I mention them because
a "scuffed piston" is often blamed on other factors and detonation as the
problem is overlooked. A scuffed piston may be an indicator of a much more
serious problem which may manifest itself the next time with more serious
results.

 

[clevtrev]

In the same vein, an engine running at full throttle may be happy due to a
rich WOT air/fuel ratio. Throttling back to part throttle the mixture may be
leaner and detonation may now occur. Bingo, the piston overheats and scuffs,
the engine fails but the postmortem doesn't consider detonation because the
the failure didn't happen at WOT.
I want to reinforce the fact that the detonation pressure spike is very
brief and that it occurs after the spark plug normally fires. In most cases
that will be well after ATDC, when the piston is moving down. You have high
pressure in the chamber anyway with the burn. The pressure is pushing the
piston like it's supposed to, and superimposed on that you get a brief spike
that rings the engine.
CAUSES
Detonation is influenced by chamber design (shape, size, geometry, plug
location), compression ratio, engine timing, mixture temperature, cylinder
pressure and fuel octane rating. Too much spark advance ignites the burn too
soon so that it increases the pressure too greatly and the end gas
spontaneously combusts. Backing off the spark timing will stop the
detonation. The octane rating of the fuel is really nothing magic. Octane is
the ability to resist detonation. It is determined empirically in a special
running test engine where you run the fuel, determine the compression ratio
that it detonates at and compare that to a standard fuel, That's the octane
rating of the fuel. A fuel can have a variety of additives or have higher
octane quality. For instance, alcohol as fuel has a much better octane
rating just because it cools the mixture significantly due to the extra
amount of liquid being used. If the fuel you got was of a lower octane
rating than that demanded by the engine's compression ratio and spark
advance detonation could result and cause the types of failures previously
discussed.
Production engines are optimized for the type or grade of fuel that the
marketplace desires or offers. Engine designers use the term called MBT (
Minimum spark for Best Torque) for efficiency and maximum power; it is
desirable to operate at MBT at all times. For example, let's pick a specific
engine operating point, 4000 RPM, WOT, 98 kPa MAP. At that operating point
with the engine on the dynamometer and using non-knocking fuel, we adjust
the spark advance. There is going to be a point where the power is the
greatest. Less spark than that, the power falls off, more spark advance than
that, you don't get any additional power.
Now our engine was initially designed for premium fuel and was calibrated
for 20 degrees of spark advance. Suppose we put regular fuel in the engine
and it spark knocks at 20 degrees? We back off the timing down to 10 degrees
to get the detonation to stop. It doesn't detonate any more, but with 10
degrees of spark retard, the engine is not optimized anymore. The engine now
suffers about a 5-6 percent loss in torque output. That's an unacceptable
situation. To optimize for regular fuel engine designers will lower the
compression ratio to allow an increase in the spark advance to MBT. The
result, typically, is only a 1-2 percent torque loss by lowering the
compression. This is a better trade-off. Engine test data determines how
much compression an engine can have and run at the optimum spark advance.
For emphasis, the design compression ratio is adjusted to maximize
efficiency/power on the available fuel. Many times in the aftermarket the
opposite occurs. A compression ratio is "picked" and the end user tries to
find good enough fuel and/or retards the spark to live with the
situation...or suffers engine damage due to detonation.
Another thing you can do is increase the burn rate of the combustion
chamber. That is why with modem engines you hear about fast burn chambers or
quick burn chambers. The goal is the faster you can make the chamber burn,
the more tolerant to detonation it is. It is a very simple phenomenon, the
faster it burns, the quicker the burn is completed, the less time the end
gas has to detonate. If it can't sit there and soak up heat and have the
pressure act upon it, it can't detonate.
If, however, you have a chamber design that burns very slowly, like a
mid-60s engine, you need to advance the spark and fire at 38 degrees BTDC.
Because the optimum 14 degrees after top dead center (LPP) hasn't changed
the chamber has far more opportunity to detonate as it is being acted upon
by heat and pressure. If we have a fast burn chamber, with 15 degrees of
spark advance, we've reduced our window for detonation to occur
considerably. It's a mechanical phenomenon. That's one of the goals of
having a fast burn chamber because it is resistant to detonation.
There are other advantages too, because the faster the chamber burns, the
less spark advance you need. The less time pistons have to act against the
pressure build up, the air pump becomes more efficient. Pumping losses are
minimized. In other words, as the piston moves towards top dead center
compression of the fuel/air mixture increases. If you light the fire at 38
degrees before top dead center, the piston acts against that pressure for 38
degrees. If you light the spark 20 degrees before top dead center, it's only
acting against it for 20. The engine becomes more mechanically efficient.
There are a lot of reasons forfast burn chambers but one nice thing about
them is that they become more resistant to detonation. A real world example
is the Northstar engine from 1999 to 2000. The 1999 engine was a 10.3:1
compression ratio. It was a premium fuel engine. For the 2000 model year, we
revised the combustion chamber, achieved faster bum. We designed it to
operate on regular fuel and we only had to lower the compression ratio .3 to
only 10:1 to make it work. Normally, on a given engine (if you didn't change
the combustion chamber design) to go from premium to regular fuel, it will
typically drop one point in compression ratio: With our example, you would
expect a Northstar engine at 10.3:1 compression ratio, dropped down to 9.3:1
in order to work on regular. Because of the faster burn chamber, we only had
to drop to 10:1. The 10:1 compression ratio still has very high compression
with attendant high mechanical efficiency and yet we can operate it at
optimum spark advance on regular fuel. That is one example of spark advance
in terms of technology. A lot of that was achieved through computational
fluid dynamics analysis of the combustion chamber to improve the swirl and
tumble and the mixture motion in the chamber to enhance the bum rate.

 

[clevtrev]

CHAMBER DESIGN
One of the characteristic chambers that people are familiar with is the
Chrysler Hemi. The engine had a chamber that was like a half of a baseball.
Hemispherical in nature and in nomenclature, too. The two valves were on
either side of the chamber with the spark plug at the very top. The charge
burned downward across the chamber. That approach worked fairly well in
passenger car engines but racing versions of the Hemi had problems. Because
the chamber was so big and the bores were so large, the chamber volume also
was large; it was difficult to get the compression ratio high. Racers put a
dome on the piston to increase the compression ratio. If you were to take
that solution to the extreme and had a 13:1 or 14:1 compression ratio in the
engine pistons had a very tall dome. The piston dome almost mimicked the
shape of the head's combustion chamber with the piston at top dead center.
One could call the remaining volume "the skin of the orange." When ignited
the charge burned very slowly, like the ripples in a pond,, covering the
distance to the block cylinder wall. Thus, those engines, as a result of the
chamber design, required a tremendous amount of spark advance, about 40-45
degrees. With that much spark advance detonation was a serious possibility
if not fed high octane fuel. Hemis tended to be very sensitive to tuning. As
often happened, one would keep advancing the spark, get more power and all
of a sudden the engine would detonate, Because they were high output
engines, turning at high RPM, things would happen suddenly.
Hemi racing engines would typically knock the ring land off, get blow by,
torch the piston and fall apart. No one then understood why. We now know
that the Hemi design is at the worst end of the spectrum for a combustion
chamber. A nice compact chamber is best; that's why the four valve pent roof
style chambers are so popular. The flatter the chamber, the smaller the
closed volume of the chamber, the less dome you need in the piston. We can
get inherently high compression ratios with a flat top piston with a very
nice bum pattern right in the combustion chamber, with very short distances,
with very good mixture motion - a very efficient chamber.
Look at a Northstar or most of the 4 valve type engines - all with flat top
pistons, very compact combustion chambers, very narrow valve angles and
there is no need for a dome that impedes the burn to raise the compression
ratio to 10:1.
DETONATION INDICATORS
The best indication of detonation is the pinging sound that cars,
particularly old models, make at low speeds and under load. It is very
difficult to hear the sound in well insulated luxury interiors of today's
cars. An unmuffled engine running straight pipes or a propeller turning can
easily mask the characteristic ping. The point is that you honestly don't
know that detonation is going on. In some cases, the engine may smoke but
not as a rule. Broken piston ring lands are the most typical result of
detonation but are usually not spotted. If the engine has detonated visual
signs like broken spark plug porcelains or broken ground electrodes are dead
giveaways and call for further examination or engine disassembly.
It is also very difficult to sense detonation while an engine is running in
an remote and insulated dyno test cell. One technique seems almost
elementary but, believe it or not, it is employed in some of the highest
priced dyno cells in the world. We refer to it as the "Tin Ear". You might
think of it as a simple stethoscope applied to the engine block. We run a
ordinary rubber hose from the dyno operator area next to the engine. To
amplify the engine sounds we just stick the end of the hose through the
bottom of a Styrofoam cup and listen in! It is common for ride test
engineers to use this method on development cars particularly if there is a
suspicion out on the road borderline detonation is occurring. Try it on your
engine; you will be amazed at how well you can hear the different engine
noises.
The other technique is a little more subtle but usable if attention is paid
to EGT (Exhaust Gas Temperature). Detonation will actually cause EGTs to
drop. This behavior has fooled a lot of people because they will watch the
EGT and think that it is in a low enough range to be safe, the only reason
it is low is because the engine is detonating.
The only way you know what is actually happening is to be very familiar with
your specific engine EGT readings as calibrations and probe locations vary.
If, for example, you normally run 1500 degrees at a given MAP setting and
you suddenly see 1125 after picking up a fresh load of fuel you should be
alert to possible or incipient detonation. Any drop from normal EGT should
be reason for concern. Using the "Tin Ear" during the early test stage and
watching the EGT very carefully, other than just plain listening with your
ear without any augmentation, is the only way to identify detonation. The
good thing is, most engines will live with a fairly high level of detonation
for some period of time. It is not an instantaneous type failure.
PRE-IGNITION
The definition of pre-ignition is the ignition of the fuel/air charge prior
to the spark plug firing. Pre-ignition caused by some other ignition source
such as an overheated spark plug tip, carbon deposits in the combustion
chamber and, rarely, a burned exhaust valve; all act as a glow plug to
ignite the charge.
Keep in mind the following sequence when analyzing pre-ignition. The charge
enters the combustion chamber as the piston reaches BDC for intake; the
piston next reverses direction and starts to compress the charge. Since the
spark voltage requirements to light the charge increase in proportion with
the amount of charge compression; almost anything can ignite the proper
fuel/air mixture at BDC!! BDC or before is the easiest time to light that
mixture. It becomes progressively more difficult as the pressure starts to
build.
A glowing spot somewhere in the chamber is the most likely point for
pre-ignition to occur. It is very conceivable that if you have something
glowing, like a spark plug tip or a carbon ember, it could ignite the charge
while the piston is very early in the compression stoke. The result is
understandable; for the entire compression stroke, or a great portion of it,
the engine is trying to compress a hot mass of expanded gas. That obviously
puts tremendous load on the engine and adds tremendous heat into its parts.
Substantial damage occurs very quickly. You can't hear it because there is
no rapid pressure rise. This all occurs well before the spark plug fires.
Remember, the spark plug ignites the mixture and a sharp pressure spike
occurs after that, when the detonation occurs. That's what you hear. With
pre-ignition, the ignition of the charge happens far ahead of the spark plug
firing, in my example, very, very far ahead of it when the compression
stroke just starts. There is no very rapid pressure spike like with
detonation. Instead, it is a tremendous amount of pressure which is present
for a very long dwell time, i.e., the entire compression stroke. That's what
puts such large loads on the parts. There is no sharp pressure spike to
resonate the block and the head to cause any noise. So you never hear it,
the engine just blows up! That's why pre-ignition is so insidious. It is
hardly detectable before it occurs. When it occurs you only know about it
after the fact. It causes a catastrophic failure very quickly because the
heat and pressures are so intense.
An engine can live with detonation occurring for considerable periods of
time, relatively speaking. There are no engines that will live for any
period of time when pre-ignition occurs. When people see broken ring lands
they mistakenly blame it on pre-ignition and overlook the hammering from
detonation that caused the problem. A hole in the middle of the piston,
particularly a melt ed hole in the middle of a piston, is due to the extreme
heat and pressure of pre-ignition.
Other signs of pre-ignition are melted spark plugs showing splattered,
melted, fused looking porcelain. Many times a "pre-ignited plug" will melt
away the ground electrode. What's left will look all spattered and fuzzy
looking. The center electrode will be melted and gone and its porcelain will
be spattered and melted. This is a typical sign of incipient pre-ignition.
The plug may be getting hot, melting and "getting ready" to act as a
pre-ignition source. The plug can actually melt without pre-ignition
occurring. However, the melted plug can cause pre-ignition the next time
around.
Thetypical pre-ignition indicator, of course, would be the hole in the
piston. This occurs because in trying to compress the already burned mixture
the parts soak up a tremendous amount of heat very quickly. The only ones
that survive are the ones that have a high thermal inertia, like the
cylinder head or cylinder wall. The piston, being aluminum, has a low
thermal inertia (aluminum soaks up the heat very rapidly). The crown of the
piston is relatively thin, it gets very hot, it can't reject the heat, it
has tremendous pressure loads against it and the result is a hole in the
middle of the piston where it is weakest.
I want to emphasis that when most people think of pre-ignition they
generally accept the fact that the charge was ignited before the spark plug
fires. However, I believe they limit their thinking to 5-10 degrees before
the spark plug fires. You have to really accept that the most likely point
for pre-ignition to occur is 180 degrees BTDC, some 160 degrees before the
spark plug would have fired because that's the point (if there is a glowing
ember in the chamber) when it's most likely to be ignited. We are talking
some 160-180 degrees of bum being compressed that would normally be
relatively cool. A piston will only take a few revolutions of that distress
before it fails. As for detonation, it can get hammered on for seconds,
minutes, or hours depending on the output of the engine and the load, before
any damage occurs. Pre-ignition damage is almost instantaneous.
When the piston crown temperature rises rapidly it never has time to get to
the skirt and expand and cause it to scuff. It just melts the center right
out of the piston. That's the biggest difference between detonation and
pre-ignition when looking at piston failures. Without a high pressure spike
to resonate the chamber and block, you would never hear pre-ignition. The
only sign of pre-ignition is white smoke pouring out the tailpipe and the
engine quits running.