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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet badly tuned or just underpowered?
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<blockquote data-quote="clevtrev" data-source="post: 22651" data-attributes="member: 187"><p>CHAMBER DESIGN</p><p>One of the characteristic chambers that people are familiar with is the </p><p>Chrysler Hemi. The engine had a chamber that was like a half of a baseball. </p><p>Hemispherical in nature and in nomenclature, too. The two valves were on </p><p>either side of the chamber with the spark plug at the very top. The charge </p><p>burned downward across the chamber. That approach worked fairly well in </p><p>passenger car engines but racing versions of the Hemi had problems. Because </p><p>the chamber was so big and the bores were so large, the chamber volume also </p><p>was large; it was difficult to get the compression ratio high. Racers put a </p><p>dome on the piston to increase the compression ratio. If you were to take </p><p>that solution to the extreme and had a 13:1 or 14:1 compression ratio in the </p><p>engine pistons had a very tall dome. The piston dome almost mimicked the </p><p>shape of the head's combustion chamber with the piston at top dead center. </p><p>One could call the remaining volume "the skin of the orange." When ignited </p><p>the charge burned very slowly, like the ripples in a pond,, covering the </p><p>distance to the block cylinder wall. Thus, those engines, as a result of the </p><p>chamber design, required a tremendous amount of spark advance, about 40-45 </p><p>degrees. With that much spark advance detonation was a serious possibility </p><p>if not fed high octane fuel. Hemis tended to be very sensitive to tuning. As </p><p>often happened, one would keep advancing the spark, get more power and all </p><p>of a sudden the engine would detonate, Because they were high output </p><p>engines, turning at high RPM, things would happen suddenly.</p><p>Hemi racing engines would typically knock the ring land off, get blow by, </p><p>torch the piston and fall apart. No one then understood why. We now know </p><p>that the Hemi design is at the worst end of the spectrum for a combustion </p><p>chamber. A nice compact chamber is best; that's why the four valve pent roof </p><p>style chambers are so popular. The flatter the chamber, the smaller the </p><p>closed volume of the chamber, the less dome you need in the piston. We can </p><p>get inherently high compression ratios with a flat top piston with a very </p><p>nice bum pattern right in the combustion chamber, with very short distances, </p><p>with very good mixture motion - a very efficient chamber.</p><p>Look at a Northstar or most of the 4 valve type engines - all with flat top </p><p>pistons, very compact combustion chambers, very narrow valve angles and </p><p>there is no need for a dome that impedes the burn to raise the compression </p><p>ratio to 10:1.</p><p>DETONATION INDICATORS</p><p>The best indication of detonation is the pinging sound that cars, </p><p>particularly old models, make at low speeds and under load. It is very </p><p>difficult to hear the sound in well insulated luxury interiors of today's </p><p>cars. An unmuffled engine running straight pipes or a propeller turning can </p><p>easily mask the characteristic ping. The point is that you honestly don't </p><p>know that detonation is going on. In some cases, the engine may smoke but </p><p>not as a rule. Broken piston ring lands are the most typical result of </p><p>detonation but are usually not spotted. If the engine has detonated visual </p><p>signs like broken spark plug porcelains or broken ground electrodes are dead </p><p>giveaways and call for further examination or engine disassembly.</p><p>It is also very difficult to sense detonation while an engine is running in </p><p>an remote and insulated dyno test cell. One technique seems almost </p><p>elementary but, believe it or not, it is employed in some of the highest </p><p>priced dyno cells in the world. We refer to it as the "Tin Ear". You might </p><p>think of it as a simple stethoscope applied to the engine block. We run a </p><p>ordinary rubber hose from the dyno operator area next to the engine. To </p><p>amplify the engine sounds we just stick the end of the hose through the </p><p>bottom of a Styrofoam cup and listen in! It is common for ride test </p><p>engineers to use this method on development cars particularly if there is a </p><p>suspicion out on the road borderline detonation is occurring. Try it on your </p><p>engine; you will be amazed at how well you can hear the different engine </p><p>noises.</p><p>The other technique is a little more subtle but usable if attention is paid </p><p>to EGT (Exhaust Gas Temperature). Detonation will actually cause EGTs to </p><p>drop. This behavior has fooled a lot of people because they will watch the </p><p>EGT and think that it is in a low enough range to be safe, the only reason </p><p>it is low is because the engine is detonating.</p><p>The only way you know what is actually happening is to be very familiar with </p><p>your specific engine EGT readings as calibrations and probe locations vary. </p><p>If, for example, you normally run 1500 degrees at a given MAP setting and </p><p>you suddenly see 1125 after picking up a fresh load of fuel you should be </p><p>alert to possible or incipient detonation. Any drop from normal EGT should </p><p>be reason for concern. Using the "Tin Ear" during the early test stage and </p><p>watching the EGT very carefully, other than just plain listening with your </p><p>ear without any augmentation, is the only way to identify detonation. The </p><p>good thing is, most engines will live with a fairly high level of detonation </p><p>for some period of time. It is not an instantaneous type failure.</p><p>PRE-IGNITION</p><p>The definition of pre-ignition is the ignition of the fuel/air charge prior </p><p>to the spark plug firing. Pre-ignition caused by some other ignition source </p><p>such as an overheated spark plug tip, carbon deposits in the combustion </p><p>chamber and, rarely, a burned exhaust valve; all act as a glow plug to </p><p>ignite the charge.</p><p>Keep in mind the following sequence when analyzing pre-ignition. The charge </p><p>enters the combustion chamber as the piston reaches BDC for intake; the </p><p>piston next reverses direction and starts to compress the charge. Since the </p><p>spark voltage requirements to light the charge increase in proportion with </p><p>the amount of charge compression; almost anything can ignite the proper </p><p>fuel/air mixture at BDC!! BDC or before is the easiest time to light that </p><p>mixture. It becomes progressively more difficult as the pressure starts to </p><p>build.</p><p>A glowing spot somewhere in the chamber is the most likely point for </p><p>pre-ignition to occur. It is very conceivable that if you have something </p><p>glowing, like a spark plug tip or a carbon ember, it could ignite the charge </p><p>while the piston is very early in the compression stoke. The result is </p><p>understandable; for the entire compression stroke, or a great portion of it, </p><p>the engine is trying to compress a hot mass of expanded gas. That obviously </p><p>puts tremendous load on the engine and adds tremendous heat into its parts. </p><p>Substantial damage occurs very quickly. You can't hear it because there is </p><p>no rapid pressure rise. This all occurs well before the spark plug fires.</p><p>Remember, the spark plug ignites the mixture and a sharp pressure spike </p><p>occurs after that, when the detonation occurs. That's what you hear. With </p><p>pre-ignition, the ignition of the charge happens far ahead of the spark plug </p><p>firing, in my example, very, very far ahead of it when the compression </p><p>stroke just starts. There is no very rapid pressure spike like with </p><p>detonation. Instead, it is a tremendous amount of pressure which is present </p><p>for a very long dwell time, i.e., the entire compression stroke. That's what </p><p>puts such large loads on the parts. There is no sharp pressure spike to </p><p>resonate the block and the head to cause any noise. So you never hear it, </p><p>the engine just blows up! That's why pre-ignition is so insidious. It is </p><p>hardly detectable before it occurs. When it occurs you only know about it </p><p>after the fact. It causes a catastrophic failure very quickly because the </p><p>heat and pressures are so intense.</p><p>An engine can live with detonation occurring for considerable periods of </p><p>time, relatively speaking. There are no engines that will live for any </p><p>period of time when pre-ignition occurs. When people see broken ring lands </p><p>they mistakenly blame it on pre-ignition and overlook the hammering from </p><p>detonation that caused the problem. A hole in the middle of the piston, </p><p>particularly a melt ed hole in the middle of a piston, is due to the extreme </p><p>heat and pressure of pre-ignition.</p><p>Other signs of pre-ignition are melted spark plugs showing splattered, </p><p>melted, fused looking porcelain. Many times a "pre-ignited plug" will melt </p><p>away the ground electrode. What's left will look all spattered and fuzzy </p><p>looking. The center electrode will be melted and gone and its porcelain will </p><p>be spattered and melted. This is a typical sign of incipient pre-ignition.</p><p>The plug may be getting hot, melting and "getting ready" to act as a </p><p>pre-ignition source. The plug can actually melt without pre-ignition </p><p>occurring. However, the melted plug can cause pre-ignition the next time </p><p>around.</p><p>Thetypical pre-ignition indicator, of course, would be the hole in the </p><p>piston. This occurs because in trying to compress the already burned mixture </p><p>the parts soak up a tremendous amount of heat very quickly. The only ones </p><p>that survive are the ones that have a high thermal inertia, like the </p><p>cylinder head or cylinder wall. The piston, being aluminum, has a low </p><p>thermal inertia (aluminum soaks up the heat very rapidly). The crown of the </p><p>piston is relatively thin, it gets very hot, it can't reject the heat, it </p><p>has tremendous pressure loads against it and the result is a hole in the </p><p>middle of the piston where it is weakest.</p><p>I want to emphasis that when most people think of pre-ignition they </p><p>generally accept the fact that the charge was ignited before the spark plug </p><p>fires. However, I believe they limit their thinking to 5-10 degrees before </p><p>the spark plug fires. You have to really accept that the most likely point </p><p>for pre-ignition to occur is 180 degrees BTDC, some 160 degrees before the </p><p>spark plug would have fired because that's the point (if there is a glowing </p><p>ember in the chamber) when it's most likely to be ignited. We are talking </p><p>some 160-180 degrees of bum being compressed that would normally be </p><p>relatively cool. A piston will only take a few revolutions of that distress </p><p>before it fails. As for detonation, it can get hammered on for seconds, </p><p>minutes, or hours depending on the output of the engine and the load, before </p><p>any damage occurs. Pre-ignition damage is almost instantaneous.</p><p>When the piston crown temperature rises rapidly it never has time to get to </p><p>the skirt and expand and cause it to scuff. It just melts the center right </p><p>out of the piston. That's the biggest difference between detonation and </p><p>pre-ignition when looking at piston failures. Without a high pressure spike </p><p>to resonate the chamber and block, you would never hear pre-ignition. The </p><p>only sign of pre-ignition is white smoke pouring out the tailpipe and the </p><p>engine quits running.</p></blockquote><p></p>
[QUOTE="clevtrev, post: 22651, member: 187"] 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. [/QUOTE]
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