modpx
1st Class Ticket
Posts: 87
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Post by modpx on Jan 6, 2013 22:50:18 GMT
Just skimming through here, so I may have missed something... You're using sealants? DO NOT USE SEALANTS!!!! It'll blow out eventually, then the engine will run lean, then you'll be back here again. Hmm..... 1.4 squish is deliberately on the safe side on mine. I don't want to break anything unnecessarily! Nik what type of set you have? For what purpose did u measure the squish if i can ask? thanks
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Post by joey on Jan 6, 2013 23:22:25 GMT
Just like to point out pinking IS NOT pre ignition, pinking is DETONATION both completely different. Pinking is caused by incorrect timing/carburation/compression or too large a squish, the other by the fuel/air mixture being ignited PRIOR it's pre determined ignition point. Pre ignition is usually indicated by a nice round hole in the top of your piston, whereas detonation usually produces a pockmarked piston most noticeable around the outer edge close to the exhaust port. A standard Pinasco kit generally has a very generous squish (over 2mm) reducing the chance of pinking as the compression is so low but it sounds as though you've fitted a 60mm crank so it may have raised your compression and reduced your squish. As has been mentioned check your squish.... solder down through the plug hole at right angles to the crank (to achieve a fairly accurate measurement,) and check the thickness...... between 1.2 and 1.6 is a safe measurement. next thing to do if it is detonation is to further retard your timing, you may have to file out the stator to achieve this......... retard your timing as far as it will go and then advance a little at a time until you reintroduce the pinking then knock it back again to the last safe point. Don't be fooled by the IT mark on the casings, it's a guide only. I've strobed scoots on the IT mark only to find that some have been running 20 deg BTDC and some running as low as 14...... so have it checked properly, or just keep retarding until the pinking goes away. Last point, Nik is right, never use a sealant (especially hylomar) on your head.......... it looks as though it's blown out from your pic which could actually be the cause of your woes.
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Post by joey on Jan 6, 2013 23:29:22 GMT
A quick read if you're interested (copied from SCOOTEROTICA.)
Detonation A destructive tendency!
All high output engines are prone to destructive tendencies as a result of over boost, mis-fueling, 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 dropping 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.
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 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. 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, rod bearing failure)
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 sand blasted 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 giveaway sign of detonation. Many times detonation damage is just limited to this.
Some engines, such as air cooled 2-stroke engines found in snowmobile, watercraft and motorcycles, have a very common detonation failure mode. What typically happens is that when detonation occurs the piston expands excessively, scuffs 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 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, air/fuel mixture, fuel 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 combust. 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 tradeoff. 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 after market 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 for fast 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 and was designed to use premium fuel. For the 2000 model year, with a revised the combustion chamber and faster burn it is designed to operate on regular fuel and the compression ratio only had to be lowered .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, it only had to drop to 10:1. The 10:1 compression ratio still has very high compression with attendant high mechanical efficiency and yet it 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 burn 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 melted 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.
The typical 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 technicians learn what to expect from the gauges. Typically, very light activity, just a few milliamp 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 milliamp current flow suddenly jumps off scale. When that happens, instant power reduction is necessary to avoid major engine damage.
Back in the 80s, running engines that made half a horsepower per cubic inch, you could artificially and safely induce pre-ignition by using too hot of a plug and leaning out the mixture. You could determine how close you were by watching the gauges and had plenty of time (seconds) to power down, before any damage occurred.
With the engine 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 you have much hotter spark plugs for cold start ability and fouling resistance. To avoid pre-ignition you 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
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modpx
1st Class Ticket
Posts: 87
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Post by modpx on Jan 7, 2013 0:12:45 GMT
Just like to point out pinking IS NOT pre ignition, pinking is DETONATION both completely different. Pinking is caused by incorrect timing/carburation/compression or too large a squish, the other by the fuel/air mixture being ignited PRIOR it's pre determined ignition point. Pre ignition is usually indicated by a nice round hole in the top of your piston, whereas detonation usually produces a pockmarked piston most noticeable around the outer edge close to the exhaust port. A standard Pinasco kit generally has a very generous squish (over 2mm) reducing the chance of pinking as the compression is so low but it sounds as though you've fitted a 60mm crank so it may have raised your compression and reduced your squish. As has been mentioned check your squish.... solder down through the plug hole at right angles to the crank (to achieve a fairly accurate measurement,) and check the thickness...... between 1.2 and 1.6 is a safe measurement. next thing to do if it is detonation is to further retard your timing, you may have to file out the stator to achieve this......... retard your timing as far as it will go and then advance a little at a time until you reintroduce the pinking then knock it back again to the last safe point. Don't be fooled by the IT mark on the casings, it's a guide only. I've strobed scoots on the IT mark only to find that some have been running 20 deg BTDC and some running as low as 14...... so have it checked properly, or just keep retarding until the pinking goes away. Last point, Nik is right, never use a sealant (especially hylomar) on your head.......... it looks as though it's blown out from your pic which could actually be the cause of your woes. Quote: At right angles to the crank. [glow=red,2,300][/glow] Hi joey, could u please tell me exactly how?? Cause like i said before i put the solder wire into the spark hole but i cant see how the wire can be squeezed as theres no friction, compression, (in other words they don't touch each other )between the head of the piston, the wire and the cylinder head...
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Post by pxguru on Jan 7, 2013 6:20:52 GMT
Joey, This info might confuse. This article is written for 4 stroke engines by an American NASCAR tuner! Some parts are relevant to 2 strokes some are not.
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Post by pxguru on Jan 7, 2013 6:53:23 GMT
The thing with the squish is that i don't know if is so necessary to measure it, i start to think that Pinasco set the kit (that includes also the head and the crankshaft 60) already with the perfect squish... probably if i start to thicken the base in order to make it rev higher then the squish is important..but for a torquey kit like Pinasco maybe not? hmmm about the sealants... i was suggested to put the red silicon at the cylinder base together with the gasket...Pxguru suggested wd40 for the cylinder head but is it ok also for the cylinder base? Sorry, you are having some trouble understanding. I'll try to be clearer. WD40 is to put on the 1200 wet and dry paper, ontop of a glass sheet, to polish the head gasket surface. Your Pinasco kit does not need any sealant anywhere. Pinasco/Malossi/Polini make the all kits to fit all scooters. If you want yours to be fast as possible you need to adjust it to fit your engine. The trick is knowing what to adjust where. That spark plug picture is not so clear but confirms the overheating anyway.
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Post by joey on Jan 7, 2013 8:27:50 GMT
1st the good news. It isn't pinking (pre ignition). . I was just pointing out that pinking is detonation and not pre ignition, and the info is relevant whether 2 or 4 sroke as both cause and effect are the same......... knackered motor. Personally, having re read the initial post, I'm more inclined to think it's a head leak, and not pinking, that's caused this problem. Looking at the pic of the head you can see blow by which is quite bad for a motor that's only done 20km. Modpx.......... you need to use a twist of solder, 1, 2, 3, possibly even 4 strands of solder twisted together, and then bend into a right angle.......... poke it into the cylinder head with the aim being to get the end of the solder touching the edge of the barrel so that when you turn the motor over by hand the piston crushes the solder. Do this at either 2 or 4 o'clock when looking at the cylinder head as this will give a slightly more accurate reading. If you try it at the top or bottom of head the piston will rock in the bore giving a bad measurement.. Personal opinion is that with the Pinasco being a relatively low revving kit with generous squish and low compression that pinking isn't really going to be an issue but with todays crap fuels it's always better to err on the side of caution and retard your timing
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modpx
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Post by modpx on Jan 7, 2013 8:53:20 GMT
The thing with the squish is that i don't know if is so necessary to measure it, i start to think that Pinasco set the kit (that includes also the head and the crankshaft 60) already with the perfect squish... probably if i start to thicken the base in order to make it rev higher then the squish is important..but for a torquey kit like Pinasco maybe not? hmmm about the sealants... i was suggested to put the red silicon at the cylinder base together with the gasket...Pxguru suggested wd40 for the cylinder head but is it ok also for the cylinder base? Sorry, you are having some trouble understanding. I'll try to be clearer. WD40 is to put on the 1200 wet and dry paper, ontop of a glass sheet, to polish the head gasket surface. Your Pinasco kit does not need any sealant anywhere. Pinasco/Malossi/Polini make the all kits to fit all scooters. If you want yours to be fast as possible you need to adjust it to fit your engine. The trick is knowing what to adjust where. That spark plug picture is not so clear but confirms the overheating anyway. yeah i understood that wd 40 was just for polishing, it's it only purpose..i was wandering if there was any other way of sealing required other than polishing the cylinder from the blue stuff, but you just explain it doesnt.i made also other questions about squish which i cant meausre. There i maybe have not understood something since the wire doesnt get squashed anywhere.
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modpx
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Post by modpx on Jan 7, 2013 9:16:46 GMT
1st the good news. It isn't pinking (pre ignition). . I was just pointing out that pinking is detonation and not pre ignition, and the info is relevant whether 2 or 4 sroke as both cause and effect are the same......... knackered motor. Personally, having re read the initial post, I'm more inclined to think it's a head leak, and not pinking, that's caused this problem. Looking at the pic of the head you can see blow by which is quite bad for a motor that's only done 20km. Modpx.......... you need to use a twist of solder, 1, 2, 3, possibly even 4 strands of solder twisted together, and then bend into a right angle.......... poke it into the cylinder head with the aim being to get the end of the solder touching the edge of the barrel so that when you turn the motor over by hand the piston crushes the solder. Do this at either 2 or 4 o'clock when looking at the cylinder head as this will give a slightly more accurate reading. If you try it at the top or bottom of head the piston will rock in the bore giving a bad measurement.. Personal opinion is that with the Pinasco being a relatively low revving kit with generous squish and low compression that pinking isn't really going to be an issue but with todays crap fuels it's always better to err on the side of caution and retard your timing Hi i just read your explanation. It was very accurate. Yes i have the same opinion on Pinasco kits. Thanks.
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Post by pxguru on Jan 7, 2013 9:39:04 GMT
I was just pointing out that pinking is detonation and not pre ignition, and the info is relevant whether 2 or 4 sroke as both cause and effect are the same......... knackered motor. Pinking is an English slang word for pre-ignition ( Pre Ignition k Noc KING). Which we in England call anything to do with all types of early igniting of the mixture. Pinging is an American slang word for pre-Ignition detonation in a 4 stroke. Listen to those yanks too much and you won't be able to tell an ass from a fanny either ;D Any type of pinking; pinging, detonation or pre-ignition in a 2 stroke scooter will kill it dead......possibly in seconds. This is not helping Modpx in anyway.
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Post by pxguru on Jan 7, 2013 9:48:47 GMT
Modpx, Did you read this article Kru posted and not understand it?
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Post by joey on Jan 7, 2013 12:44:10 GMT
Sorry completely disagree, pinking/pinging/knock/detonation are one and the same, pre ignition is a completely different beast. I do agree that they'll both goose an engine but they are completely different problems with completely different causes and solutions.
pre ignition can be solved by regular decoking (especially important on a 2 stroke,) or selecting a hotter plug.
Pinking is a completely different beast and a lot harder to cure.
And trust me I do know where I'm coming from here, got a fair few pistons that'll bear witness to my past pinking problems, funny though never had a problem with pre ignition.
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Post by sbwnik on Jan 7, 2013 13:50:39 GMT
Pre-ignition and pinking are definately the same, but to try and claim this
as the root of the name is laughable. It's a fakeronym.
It just refers to the noise an engine can make (not all are the same, but similar enough).
Someone asked further back why I set mine to 1.4mm. Apart from me having the utmost faith in my tuner (mad inventor type - 320cc Lambretta on standard-ish 200 cases anyone? It'll run 'one day' LOL), I didn't want to push the tuning too far, but on the other hand, I didn't want to lose too much power. As above, take a piece of thick solder, put a Z bend into it (so you can use one end as a handle) and insert through the plug hole, ensuring it reaches the edge of the barrel at 90 degrees to the crank (On most engines, towards the flywheel). Wind the engine slowly round so that it passes TDC. Gently remove the solder and measure the thickness. If it hasn't been squashed, then your squish is way too high, get the engine looked at again. If you're running sealant at the bottom end, that could lift it too.
Either way, drop it back to 1.6 (safe) and 1.2 (I really wouldn't want to run this on the road) and get back to us then.
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Post by pxguru on Jan 7, 2013 16:05:37 GMT
I'm running exactly 1.2mm on my Polini and now is as far tuned as a road scooter could possibly be! No issues with any kind of irregular firing. Almost all scooter pinking is the fine balance between jetting and timing.
Finally we agree, the only reason you would get true pre ignition (firing before the spark arrives) on a 2 stroke scooter is excessive carbon build up or exposed plug thread! Almost never happens.
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Post by modpx on Jan 7, 2013 16:28:46 GMT
im writin from the garage, im using a 2mm solder wire with standard 0.25 mm base gasket and nothing happens which means pinasco has over 2mm squish. end of the squish case.
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Post by modpx on Jan 7, 2013 17:05:08 GMT
so now i assume i have to decide if and how much i want to thicken the base right? any suggestions?
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Post by pxguru on Jan 7, 2013 17:08:47 GMT
Are you sure? twist two strands of the solder together and try again. just to see how much it measures.
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Post by pxguru on Jan 7, 2013 17:11:35 GMT
measure the height from the transfer ports to the top of the barrel.
And measure the small distance from the piston to the top of the barrel.
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Post by modpx on Jan 7, 2013 17:30:48 GMT
from the upper edge of the exhaust port to the top of the barrel is 37,15mm 3,7 cm approx
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Post by pxguru on Jan 7, 2013 17:33:07 GMT
And from the main transfers to the top?
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Post by modpx on Jan 7, 2013 17:35:37 GMT
the piston at his tdc is higher than the top of the barrel, approx 5,5mm
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Post by pxguru on Jan 7, 2013 17:38:22 GMT
can you take a picture of that? i think we would all like to see
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Post by modpx on Jan 7, 2013 17:42:29 GMT
pic of what exactlY? the piston at is tic i took it yesterday, its there before the spark plug
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Post by pxguru on Jan 7, 2013 17:46:54 GMT
The picture of the piston at the top yesterday looked like about 0.5mm below the barrel. agree?
can you measure the transfer ports? should be something like 48mm down
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Post by modpx on Jan 7, 2013 17:58:02 GMT
The picture of the piston at the top yesterday looked like about 0.5mm below the barrel. agree? can you measure the transfer ports? should be something like 48mm down here are 2 pics one from the piston at the top from the side you see is slightly above not below, the second is the distance from the ports (from the exhaust is 37,15 mm approx) Attachments:
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Post by modpx on Jan 7, 2013 17:58:41 GMT
The picture of the piston at the top yesterday looked like about 0.5mm below the barrel. agree? can you measure the transfer ports? should be something like 48mm down Attachments:
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Post by modpx on Jan 7, 2013 18:01:14 GMT
in the second pic the piston is at his lowest
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Post by pxguru on Jan 7, 2013 18:02:54 GMT
can you measure the transfer port (smaller one next to exhaust) to the top? should be something like 48mm down
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Post by modpx on Jan 7, 2013 18:06:26 GMT
yes its exactly 48mm
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Post by pxguru on Jan 7, 2013 18:10:04 GMT
I would like you to try to measure the squish again. With the piston higher than the barrel at the top and the head you showed a picture of yesterday the squish will be very small. Under 1mm. This would certainly explain why it overheated. It also explains why it doesnt pull the standard 4th gear I just made some rough calculations for you. The numbers are even worse than I thought but the good news is it's very easy to put right.
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