Odd Ping - Power Loss -

[Don Coatney]

Wow James. Sounds as if you have a catastrophic failure in #5. But that does not explain the low compression in the other cylinders. When my original P-15 engine failed I first had an overheating issue. Then when sitting at a stop light I heard a pronounced ticking noise much like pre-detonation. The noise only lasted a brief time but engine power was greatly reduced as only 5 cylinders were still working. I drove home, did not do a compression check but pulled the head. This was a high mileage engine and it was unknown to me if it had ever been pulled apart. Pictured below is what I found. 

 

It sounds as if is possible that something entered your combustion chamber and destroyed the electrode. The noise you heard may very well have been something bouncing around on top of the piston. But as I said this does not explain the low compression in the other cylinders.

 

Looks like your thoughts on re-powering your Desoto may happen sooner than planned.

 

[James_Douglas]

Hey Don,

 

The engine has been slowly loosing compression for 10 years since the rebuild.  It started at about 140 and now is down to 100 except for #5 at 75.

 

There is an 80 year old machinist that does work around here for Brizio Street Rods, the very high end stuff. Bruno told me that the MOPAR inline engines had a reputation back in the day of having the cylinder bores tapper quite a bit after about 50K miles.  A combination of the long stroke and the block material.

 

As you know, I run the car hard in both city stop and go and freeway running.  It will be interesting to see how it looks when I open it up later today.  I suspect that #5 piston will have to come out.  If that is the case, then I will probable pull all of them and check the tapper.  I will then hone it in place and put in a new set of rings.

 

I did a valve adjust a couple of years ago, although they seem a little quiet like they are too tight.  I will see with the head off what if anything is going on with them.

 

Best, James

[Lumpy]

A destroyed spark plug is exactly what detonation will do, and it will destroy a piston in short order. The spark plug did not fail, the detonation/ping destroyed it. That's a sure sign that the engine has been "allowed" to detonate. No offense. I still believe that your engine just reached a point of too much carbon on the head, for what it's worth.

 

Never let an engine ping, ever, at all. If it won't drive without pinging, don't drive it. De-carbon the head AT LEAST every 10,000 miles.

 

ken.

[James_Douglas]

Well,

 

The head is off. No evidence of any damage to the cylinder wall, the piston top, or the valves. 

 

The carbon build up is very little.  That said, it is worse on cylinders 4 and 5. 

 

I now have to decide if I want to just clean it up, stick a new gasket on and run it and see what happens...

 

OR

 

Do I want to jack the car up and pull the pan and take out all the pistons and valves and do a ring and valve job.

 

I will sleep on it and decide on Monday.

 

James

 

 

 

[James_Douglas]

A destroyed spark plug is exactly what detonation will do, and it will destroy a piston in short order. The spark plug did not fail, the detonation/ping destroyed it. That's a sure sign that the engine has been "allowed" to detonate. No offense. I still believe that your engine just reached a point of too much carbon on the head, for what it's worth.

 

Never let an engine ping, ever, at all. If it won't drive without pinging, don't drive it. De-carbon the head AT LEAST every 10,000 miles.

 

ken.

 

Ken, Normally I would agree with you. However, in my racing days when we had detonation issues, the ground strap (the part one adjusts) usually get deformed AND the tip of the electrode gets messed up as well. 

 

In this case the ground strap is perfect as these are plugs installed a couple of weeks ago.  The porcelain is not damaged.  Only the tip seems to have gone away.

 

I am well aware of pinging and the issues around it.  In this case the sound of "the ping" was nothing like I have ever heard before in any of my engines.  Flat Head Six, Inline 4, V8, slant Six, and others.

 

The sound was much higher in frequency.  It could not be altered by timing.  It also could not be made to happen unless the head got very hot going up a long long hill and then only after going up that hill for at lease 2-4 minutes.

 

Since I have been driving this car for well over 10 years and in all kinds of conditions...something fundamental changed.  If you look at the photos in my other post you will see that the carbon build up is not that great for a 50K mile engine. 

 

That said, in #4 and #5 there is some "deck" carbon about the size of a nickel.  It could get hot enough to cause the gas to ignite before TDC and the plug firing.  HOWEVER, I put in a 1/2 a tank of premium gas and that did not affect it.  I am at about 8 to 1.  With premium in it, that should have held the fuel from firing with hot carbon on an 8 to 1 engine.

 

I am still wondering if I ran into a fuel issues with the fuel vaporing in the carburetor and therefore leaning out the mixture which would cause the cylinder temperatures to go way up.  If that happened even at 8 to 1 the carbon, what little there is, may have caused the "ping" and the problem.   

 

James

[Lumpy]

Now the plot is so think, I can hardly move. Pulling/replacing the head is so easy, if there is no other damage, why not de-carbon the head, polish it up, and see what happens. ?? Seems like whatever is going on, is going on the combustion chamber(s), be it fuel, hot spots...??

 

And the exhaust valves looked good? Seems I've seen a lot of burnt and cracked ones in flatheads.

 

You may be better at reading damaged plugs than I. I do know that usually, when parts of a plug go "away", it's from detonation. ??

 

ken.

[JDaniel64]

Looks pretty normal inside, can still see cross-hatching on the cylinder walls. I think that any more disassembling of the engine at this point is probably fruitless, unless you observe something else that I don't. Valves lash (at operating temp), new plugs, and checking fuel sys. from tank-to-carb would seem like more likely culprits to me, regardless of the electric fuel pump. Replacing a gasket and troubleshooting newer or seemingly more reliable parts might cure the problem, and be easier imo. Just advice, i don't like tearing into engines if i can help it  Best of luck!

[greg g]

Defective sparkplug???

[Young Ed]

Defective sparkplug???

 

It does happen. We chased a rough running at speed in Dads GMC for most of a summer because of one. Even had it into the dealer once and they didn't find anything. We had rechecked the plugs at first and didnt see anything. After it went on for a while longer you could see the carbon down the outside of the plug.

[_shel_ny]

Pinging last couple of weeks. Spark plugs 2 weeks.  Which came first, the plugs, or the ping?

[James_Douglas]

Plugs first.  A week or two goes by, then pings.

[Adam H P15 D30]

I know this was mentioned but I didn't see an answer. Did you check your heat riser? Possibly stuck heating the manifold? Scold me if I'm wrong and it's been answered

[55 Fargo]

I know this was mentioned but I didn't see an answer. Did you check your heat riser? Possibly stuck heating the manifold? Scold me if I'm wrong and it's been answered

thats not a bad theory...

[Jeff Balazs]

Sure sounds as if you might have had a bad plug. I have run into that problem before.......especially with Champion brand.

Quality control at the manufacturer could definitely be at fault. Two of the last set of new plugs I fitted had the gap completely closed right out of the box. Never seen that before and as I had a few fresh spares I fitted them instead.

 

Jeff

[Dave72dt]

As long as the head is off, why not have it magnafluxed and checked for warpage?   At least at that point something is eliminated.  You should be able to get an indication of cylinder taper on the cylinders with the  piston down.

[Ulu]

If there's nothing physically wrong with the cylinders/pistons/valves, then perhaps a new head gasket will bring up the compression in #5.

 

But I'm still thinking about the cooling system. If cyl #5 is normal, it shouldn't be the only one running lean. They should all be lean and burning electrodes like matchsticks.

 

If the distribution tube is plugged/rusty/collapsed/missing, then maybe cyl 5 & 6 aren't cooling right, but because 6 is on the end it can't get as hot as  #5.

 

I don't really know how much effect this could have, but everything I read says the distribution tube is critical to keeping the engine temp balanced and uniform.

[James_Douglas]

Stainless steel tube.  Engine is very clean in the cooling system.  When this engine was rebuilt the block was acid dipped. We also drilled out the back of the block so we could run brushes through the entire tube passage after it was out and the block dipped.

 

This is one of those things that I think I should think on for a day or two before tearing apart an otherwise good short block.

 

James

[James_Douglas]

I found this article and thought people may like to read it:

*****************

Engine Basics: Detonation and Pre-Ignition

Written by Allen W. Cline

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.

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.
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.

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.
The engine will not run more than a few seconds with pre-ignition. The only way to control pre-ignition is just keep any pre-ignition sources at bay. Spark

plugs should be carefully matched to the recommended heat range. Racers use cold spark plugs and relatively rich mixtures. Spark plug heat range is also

affected by coolant temperatures. A marginal heat range plug can induce pre-ignition because of an overheated head (high coolant temperature or inadequate

flow). Also, a loose plug can't reject sufficient heat through its seat. A marginal heat range plug running lean (suddenly?) can cause pre-ignition.
Passenger car engine designers face a dilemma. Spark plugs must cold start at -40 degrees F. (which calls for hot plugs that resist fouling) yet be capable of

extended WOT operation (which calls for cold plugs and maximum heat transfer to the cylinder head).

Here is how spark plug effectiveness or "pre-ignition" testing is done at WOT. Plug tip/gap temperature is measured with a blocking diode and a small battery

supplying current through a milliamp meter to the spark plug terminal. The secondary voltage cannot come backwards up the wire because the large blocking diode

prevents it.

As the spark plug tip heats up, it tends to ionize the gap and small levels of current will flow from the battery as indicated by the milliamp gauge. The

engine is run under load and the gauges are closely watched. Through experience techni-cians learn what to expect from the gauges. Typically, very light

activity, just a few milliamps of current, is observed across the spark plug gap. In instances where the spark plug tip/gap gets hot enough to act as an

ignition source the mil-liamp current flow suddenly jumps off scale. When that hap-pens, instant power reduction is necessary to avoid major en-gine damage.
Back in the 80s, running engines that made half a horsepower per cubic inch, we could artificially and safely induce pre-ignition by using too hot of a plug

and leaning out the mixture. We could determine how close we were by watching the gauges and had plenty of time (seconds) to power down, before any damage

occurred.

With the Northstar making over 1 HP per cubic inch, at 6000 RPM, if the needles move from nominal, you just failed the engine. It's that quick! When you

disassemble the engine, you'll find definite evidence of damage. It might be just melted spark plugs. But pre-ignition happens that quick in high output

engines. There is very little time to react.

If cold starts and plug fouling are not a major worry run very cold spark plugs. A typical case of very cold plug application is a NASCAR type engine. Because

the prime pre-ignition source is eliminated engine tuners can lean out the mixture (some) for maximum fuel economy and add a lot of spark advance for power and

even risk some levels of detonation. Those plugs are terrible for cold starting and emissions and they would foul up while you were idling around town but for

running at full throttle at 8000 RPM, they function fine. They eliminate a variable that could induce pre-ignition.

Engine developers run very cold spark plugs to avoid the risk of getting into pre-ignition during engine mapping of air/fuel and spark advance, Production

engine calibration requires that we have much hotter spark plugs for cold startability and fouling resistance. To avoid pre-ignition we then compensate by

making sure the fuel/air calibration is rich enough to keep the spark plugs cool at high loads and at high temperatures, so that they don't induce pre-

ignition.

Consider the Northstar engine. If you do a full throttle 0-60 blast, the engine will likely run up to 6000 RPM at a 11.5:1 or 12:1 air fuel ratio. But under

sustained load, at about 20 seconds, that air fuel ratio is richened up by the PCM to about 10:1. That is done to keep the spark plugs cool, as well as the

piston crowns cool. That richness is necessary if you are running under continuous WOT load. A slight penalty in horsepower and fuel economy is the result. To

get the maximum acceleration out of the engine, you can actually lean it out, but under full load, it has to go back to rich. Higher specific output engines

are much more sensitive to pre-ignition damage because they are turning more RPM, they are generating a lot more heat and they are burning more fuel. Plugs

have a tendency to get hot at that high specific output and reaction time to damage is minimal.

A carburetor set up for a drag racer would never work on a NASCAR or stock car engine because it would overheat and cause pre-ignition. But on the drag strip

for 8 or 10 seconds, pre-ignition never has time to occur, so dragsters can get away with it. Differences in tuning for those two different types of engine

applications are dramatic. That's why a drag race engine would make a poor choice for an aircraft engine.

MUDDY WATER

There is a situation called detonation induced pre-ignition. I don't want to sound like double speak here but it does happen. Imagine an engine under heavy

load starting to detonate. Detonation continues for a long period of time. The plug heats up because the pressure spikes break down the protective boundary

layer of gas surrounding the electrodes. The plug temperature suddenly starts to elevate unnaturally, to the point when it becomes a glow plug and induces

pre-ignition. When the engine fails, I categorize that result as "detonation induced pre-ignition." There would not have been any danger of pre-ignition if the

detonation had not occurred. Damage attributed to both detonation and pre-ignition would be evident.

Typically, that is what we see in passenger car engines. The engines will typically live for long periods of time under detonation. In fact, we actually run a

lot of piston tests where we run the engine at the torque peak, induce moderate levels of detonation deliberately. Based on our resulting production design,

the piston should pass those tests without any problem; the pistons should be robust enough to survive. If, however, under circumstances due to overheating or

poor fuel, the spark plug tip overheats and induces pre-ignition, it's obviously not going to survive. If we see a failure, it probably is a detonation induced

pre-ignition situation.

I would urge any experimenter to be cautious using automotive based engines in other applications. In general, engines producing .5 HP/in3 (typical air-cooled

aircraft engines) can be forgiving (as leaning to peak EGT, etc.). But at 1.0 HP/in3 (very typical of many high performance automotive conversions) the window

for calibration induced engine damage is much less forgiving. Start out rich, retarded and with cold plugs and watch the EGTs!

Hopefully this discussion will serve as a thought starter. I welcome any communication on this subject. Every application is unique so beware of blanket

statements as many variables affect these processes.

AWC

[Don Coatney]

Excellent reading. Thanks for posting.

[Lumpy]

Can't believe I read the whole thing. Would be interesting to hear him address flatheads specifically.

 

k.

[Ulu]

Too long to read at work. how about a condensed version of whatever clue this offered?

 

 

 

EDIT:

 

Ok I skimmed the article, and it's true that if you're too lean at cruise it'll overheat on a good long straightaway. It can accelerate well and yet not cruise well at speed.

 

A cracked carb top cover gasket (or cracked carb) can do this to ya if it leaks air into the wrong passage.

 

It can do the opposite as well. If you have a gasket leak around the economizer, a carb might never lean out enough at cruise. The plugs foul on the freeway.

Edited April 28, 2014 by Ulu

[Ulu]

Stainless steel tube . . . 

 

I never knew they were available stainless. Wish I had as mine were so dang rusty. 

 

In fact, the inside of my block was a bit crusty and yet that 218 never overheated even after the head & block were milled (and I did pick up some power when that was done.)

[55 Fargo]

I never knew they were available stainless. Wish I had as mine were so dang rusty. 

 

In fact, the inside of my block was a bit crusty and yet that 218 never overheated even after the head & block were milled (and I did pick up some power when that was done.)

Every tube I ever pulled or saw in a Mopar flathead 6 in Canada, had a brass tube, last forever...

[Grdpa's 50 Dodge]

Yes to spark plug woes.  What brand plug???,,,agree on Champion probs

[Don Coatney]

If I recall James made or had made a stainless water distribution tube.