{"id":5089,"date":"2019-06-22T01:53:15","date_gmt":"2019-06-22T05:53:15","guid":{"rendered":"http:\/\/maxtorqueperformance.com\/?p=5089"},"modified":"2019-06-22T02:09:13","modified_gmt":"2019-06-22T06:09:13","slug":"misunderstood-causes-of-detonation-in-high-performance-applications","status":"publish","type":"post","link":"https:\/\/maxtorqueperformance.com\/staging\/index.php\/2019\/06\/22\/misunderstood-causes-of-detonation-in-high-performance-applications\/","title":{"rendered":"Misunderstood Causes of Detonation in High Performance Applications"},"content":{"rendered":"
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By Jennifer Szabo<\/a> <\/span> <\/span> June 20, 2019<\/span> <\/div>\n<\/header>\n

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In the engine tuning world, detonation is defined as one of the following: combustion that causes engine damage; combustion that causes banging or pinging noises; or combustion that causes power loss, bucking, or kicking. Detonation is not controlled and often unwanted. It occurs when fuel in the cylinder auto-ignites outside the intended flame front of the spark ignition.<\/p>\n

Detonation does not always cause damage. At lower engine loadings during part throttle or low RPM, detonation may be warranted. For example, during the late \u201970s and \u201980s, pinging during normal operation was common with carbureted engines. Certain intake manifold design compromises combined with smog equipment caused lean fuel mixtures that burned outside of the controlled flame front from the spark plug.<\/p>\n

Minor detonation sometimes occurs that is not heard through mufflers at low load or even with loud open exhaust. Major detonation causes a more severe noise during engine loading where the throttle is open and the engine is twisting hard against a heavy load.<\/p>\n

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Detonation and Preignition<\/strong><\/h4>\n

Preignition is auto-ignition of the air\/fuel mixture before the spark plug fires. The auto-ignition occurs at a location in the cylinder outside of the controlled flame front from spark ignition.<\/p>\n

Similarly, detonation is auto-ignition of fuel, usually after the spark plug fires. Like preignition, detonation occurs outside of a controlled flame front from the spark plug. The term detonation is often used by racers as both preignition (before the spark), as well as uncontrolled burning after the spark. The same convention is used in this article.<\/p>\n

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This illustration is from 5000 Horsepower on Methanol<\/em> (Bob Szabo, Szabo Publishing, 2006<\/a>) showing auto-ignition temperatures for various racing fuels.<\/p>\n<\/p><\/div>\n

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Both preignition and detonation are from auto-ignition of fuel. They share characteristics \u2014 like a very high burn speed \u2014 which are comparable to explosive flame speeds. These include firearms muzzle velocities or explosives combustion velocities \u2014 typically well over 1,000 feet per second. The high velocity causes noise due to pressure fronts that collide within the cylinder.<\/p>\n

Detonation and RPM<\/strong><\/h4>\n

Detonation can be masked at higher RPM by high frequency noise, like that of an exhaust valve opening. It can be such a brief occurrence that it doesn\u2019t cause damage before the exhaust valve opens, relieving the cylinder pressure and ending the detonation.<\/p>\n

At lower engine speeds the time between detonation and opening of the exhaust valve is a longer interval, so detonation is more noticeable. As the RPM increases, it may sound like the detonation goes away because of shorter intervals between detonation and opening of the exhaust valve.<\/p>\n

Racing engines in the \u201930s and \u201940s ran on lower octane gasoline, as higher octane gasoline was not yet developed. The lower octane fuels were susceptible to detonation as racers raised engine compression ratio for more power. Detonation was especially detectable at low engine speeds. To combat the low-speed detonation, those early racing engines were revved continuously at higher engine speeds to inhibit the effects of detonation.<\/p>\n

If the engine was lugged by mistake, detonation could cause poor performance and possible engine damage. As a result, drivers coming into the pits for service would free-rev their engines continuously. Slipping the clutch to launch from the pits became an art form for many successful racing drivers. When starting in the pits, there was a great risk of engine stalling from a combination of inadequate clutch slipping, low engine torque at low-RPM, and detonation at low-RPM.<\/p>\n

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The best performance for modern day gasoline-fueled engines is achieved with a racing gasoline blend with an octane rating just high enough to avoid detonation. A gasoline blend with a higher octane rating does not usually increase performance by itself. Instead, the slower burn rate of high octane gasoline will often actually reduce an engine\u2019s performance without other changes made to take advantage of the higher octane.<\/p>\n

Gasoline\u2019s octane need is characteristic of a specific RPM operating range. If that range is changed, a racing gasoline with a different octane may be needed. For example, if the engine spends more time under load at a lower engine speed, the engine may encounter detonation, whereas it would not detonate at the same load higher in the RPM range. A racing gasoline with a higher octane may be needed to combat the potential for detonation operating in the lower RPM range.<\/p>\n

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