The Spares Company
Club Shop/Regalia
Parent Website
Contact Officials
Machine Registrar
Club Secretary
Membership Secretaries
MPH Editor and Forum Administrator.
Section Newsletters
Technical Databases
Photos
Home
What's new
Latest activity
Forums
New posts
What's new
New posts
Latest activity
Information
Bike Modifications
Machine Data Services
Manufacturers Manuals
Spare Parts Listings
Technical Diagrams
Whitakerpedia (Vincent Wiki)
The Club
MPH Material Archive
Flogger's Corner
Obituaries
VOC Sections
Local Sections
Local Section Newsletters
Miscellaneous
Club Assets
Club History
Club Rules
Machine Data Services
Meeting Documents
Miscellaneous
Essential Reading
Magazine/Newspaper Articles/Letters
Adverts and Sales Brochures
The Mighty Garage Videos
Bikes For Sale (Spares Company)
Log in
Register
What's new
New posts
Menu
Log in
Register
Install the app
Install
Home
Forums
Forums: Public Access
Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet badly tuned or just underpowered?
JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an
alternative browser
.
Reply to thread
Message
<blockquote data-quote="clevtrev" data-source="post: 22652" data-attributes="member: 187"><p>The engine will not run more than a few seconds with pre-ignition. The only </p><p>way to control pre-ignition is just keep any pre-ignition sources at bay. </p><p>Spark plugs should be carefully matched to the recommended heat range. </p><p>Racers use cold spark plugs and relatively rich mixtures. Spark plug heat </p><p>range is also affected by coolant temperatures. A marginal heat range plug </p><p>can induce pre-ignition because of an overheated head (high coolant </p><p>temperature or inadequate flow). Also, a loose plug can't reject sufficient </p><p>heat through its seat. A marginal heat range plug running lean (suddenly?) </p><p>can cause pre-ignition.</p><p>Passenger car engine designers face a dilemma. Spark plugs must cold start </p><p>at -40 degrees F. (which calls for hot plugs that resist fouling) yet be </p><p>capable of extended WOT operation (which calls for cold plugs and maximum </p><p>heat transfer to the cylinder head).</p><p>Here is how spark plug effectiveness or "pre-ignition" testing is done at </p><p>WOT. Plug tip/gap temperature is measured with a blocking diode and a small </p><p>battery supplying current through a milliamp meter to the spark plug </p><p>terminal. The secondary voltage cannot come backwards up the wire because </p><p>the large blocking diode prevents it.</p><p>As the spark plug tip heats up, it tends to ionize the gap and small levels </p><p>of current will flow from the battery as indicated by the milliamp gauge. </p><p>The engine is run under load and the gauges are closely watched. Through </p><p>experience techni-cians learn what to expect from the gauges. Typically, </p><p>very light activity, just a few milliamps of current, is observed across the </p><p>spark plug gap. In instances where the spark plug tip/gap gets hot enough to </p><p>act as an ignition source the mil-liamp current flow suddenly jumps off </p><p>scale. When that hap-pens, instant power reduction is necessary to avoid </p><p>major en-gine damage.</p><p>Back in the 80s, running engines that made half a horsepower per cubic inch, </p><p>we could artificially and safely induce pre-ignition by using too hot of a </p><p>plug and leaning out the mixture. We could determine how close we were by </p><p>watching the gauges and had plenty of time (seconds) to power down, before </p><p>any damage occurred.</p><p>With the Northstar making over 1 HP per cubic inch, at 6000 RPM, if the </p><p>needles move from nominal, you just failed the engine. It's that quick! When </p><p>you disassemble the engine, you'll find definite evidence of damage. It </p><p>might be just melted spark plugs. But pre-ignition happens that quick in </p><p>high output engines. There is very little time to react.</p><p>If cold starts and plug fouling are not a major worry run very cold spark </p><p>plugs. A typical case of very cold plug application is a NASCAR type engine. </p><p>Because the prime pre-ignition source is eliminated engine tuners can lean </p><p>out the mixture (some) for maximum fuel economy and add a lot of spark </p><p>advance for power and even risk some levels of detonation. Those plugs are </p><p>terrible for cold starting and emissions and they would foul up while you </p><p>were idling around town but for running at full throttle at 8000 RPM, they </p><p>function fine. They eliminate a variable that could induce pre-ignition.</p><p>Engine developers run very cold spark plugs to avoid the risk of getting </p><p>into pre-ignition during engine mapping of air/fuel and spark advance, </p><p>Production engine calibration requires that we have much hotter spark plugs </p><p>for cold startability and fouling resistance. To avoid pre-ignition we then </p><p>compensate by making sure the fuel/air calibration is rich enough to keep </p><p>the spark plugs cool at high loads and at high temperatures, so that they </p><p>don't induce pre-ignition.</p><p>Consider the Northstar engine. If you do a full throttle 0-60 blast, the </p><p>engine will likely run up to 6000 RPM at a 11.5:1 or 12:1 air fuel ratio. </p><p>But under sustained load, at about 20 seconds, that air fuel ratio is </p><p>richened up by the PCM to about 10:1. That is done to keep the spark plugs </p><p>cool, as well as the piston crowns cool. That richness is necessary if you </p><p>are running under continuous WOT load. A slight penalty in horsepower and </p><p>fuel economy is the result. To get the maximum acceleration out of the </p><p>engine, you can actually lean it out, but under full load, it has to go back </p><p>to rich. Higher specific output engines are much more sensitive to </p><p>pre-ignition damage because they are turning more RPM, they are generating a </p><p>lot more heat and they are burning more fuel. Plugs have a tendency to get </p><p>hot at that high specific output and reaction time to damage is minimal.</p><p>A carburetor set up for a drag racer would never work on a NASCAR or stock </p><p>car engine because it would overheat and cause pre-ignition. But on the drag </p><p>strip for 8 or 10 seconds, pre-ignition never has time to occur, so </p><p>dragsters can get away with it. Differences in tuning for those two </p><p>different types of engine applications are dramatic. That's why a drag race </p><p>engine would make a poor choice for an aircraft engine.</p><p>MUDDY WATER</p><p>There is a situation called detonation induced pre-ignition. I don't want to </p><p>sound like double speak here but it does happen. Imagine an engine under </p><p>heavy load starting to detonate. Detonation continues for a long period of </p><p>time. The plug heats up because the pressure spikes break down the </p><p>protective boundary layer of gas surrounding the electrodes. The plug </p><p>temperature suddenly starts to elevate unnaturally, to the point when it </p><p>becomes a glow plug and induces pre-ignition. When the engine fails, I </p><p>categorize that result as "detonation induced pre-ignition." There would not </p><p>have been any danger of pre-ignition if the detonation had not occurred. </p><p>Damage attributed to both detonation and pre-ignition would be evident.</p><p>Typically, that is what we see in passenger car engines. The engines will </p><p>typically live for long periods of time under detonation. In fact, we </p><p>actually run a lot of piston tests where we run the engine at the torque </p><p>peak, induce moderate levels of detonation deliberately. Based on our </p><p>resulting production design, the piston should pass those tests without any </p><p>problem; the pistons should be robust enough to survive. If, however, under </p><p>circumstances due to overheating or poor fuel, the spark plug tip overheats </p><p>and induces pre-ignition, it's obviously not going to survive. If we see a </p><p>failure, it probably is a detonation induced pre-ignition situation.</p><p>I would urge any experimenter to be cautious using automotive based engines </p><p>in other applications. In general, engines producing .5 HP/in3 (typical </p><p>air-cooled aircraft engines) can be forgiving (as leaning to peak EGT, </p><p>etc.). But at 1.0 HP/in3 (very typical of many high performance automotive </p><p>conversions) the window for calibration induced engine damage is much less </p><p>forgiving. Start out rich, retarded and with cold plugs and watch the EGTs!</p><p>Hopefully this discussion will serve as a thought starter. I welcome any </p><p>communication on this subject. Every application is unique so beware of </p><p>blanket statements as many variables affect these processes.</p></blockquote><p></p>
[QUOTE="clevtrev, post: 22652, member: 187"] 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. [/QUOTE]
Insert quotes…
Verification
What was Mr Vincent's Christian Name?
Post reply
Home
Forums
Forums: Public Access
Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet badly tuned or just underpowered?
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
Accept
Learn more…
Top