Whats the purpose of the 2nd plug in the Hemi?
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Whats the purpose of the 2nd plug in the Hemi?
I just found out my Hemi has 16 spark plugs...2 per cylinder. So as far as I can see, I have just 8 coil packs, and when the coil fires, it also lights a plug on the other side of the engine via a wire.
Assuming the firing order mandates 8 different fires per 2 revolutions, what is the purpose of the second spark plug?
I know this is something simple, but Im just not getting it.
Regards,
-Chris
Assuming the firing order mandates 8 different fires per 2 revolutions, what is the purpose of the second spark plug?
I know this is something simple, but Im just not getting it.
Regards,
-Chris
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From:
From: High-Performance NEW HEMI Builder's Guide 2003-Present by Barry Kluczyk
"The use of a pair of spark plugs per cylinder effectively doubles the fire power (pun intended) of the ignition system, which is needed for a cleaner, more complete burn of the large air/fuel charge-particularly at higher RPM, as the valves open and close more rapidly. Without the twin-plug design, it would have been almost impossible to burn the mixture and clear the chambers sufficiently to meet fuel economy targets and federally mandated emissions guidelines. In fact, the system works so well, that the 5.7L Hemi delivers approximately 10 percent better fuel economy than the less powerful 5.9L V-8 it replaced in trucks and SUVs."
So by this reading, the second spark plug is used as a power spark, in the combustion cycle, and not as a wasted spark, to burn unspent fuel, in the exhaust cycle.
"The use of a pair of spark plugs per cylinder effectively doubles the fire power (pun intended) of the ignition system, which is needed for a cleaner, more complete burn of the large air/fuel charge-particularly at higher RPM, as the valves open and close more rapidly. Without the twin-plug design, it would have been almost impossible to burn the mixture and clear the chambers sufficiently to meet fuel economy targets and federally mandated emissions guidelines. In fact, the system works so well, that the 5.7L Hemi delivers approximately 10 percent better fuel economy than the less powerful 5.9L V-8 it replaced in trucks and SUVs."
So by this reading, the second spark plug is used as a power spark, in the combustion cycle, and not as a wasted spark, to burn unspent fuel, in the exhaust cycle.
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Unfortunately, this is impossible, as no two pistons are at TDC on the compression stoke at the same time.
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My bike has two spark plugs -- has to do with the volume of displacement.
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In most dual plug applications, one would start the burning process, and the other would finish it...firing milliseconds after the first.
Still others simply fire both at the same time, but Chrysler isnt doing any of that with the Hemi. Strangely enough, they are simultaneously firing another pug on the other side of the engine.
Scavenging unburnt fuel on the exhaust cycle makes some sense, but why would there be a surplus? Simply lean out a bit, and get even better mileage!
Oh well...I thought it was going to be simple, but the more I look around, methinks the answer is going to be much more complex.
Regards.
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06-up do fire both plugs at the same time via one coil per cylinder and no plug wires. The older system did fire a plug on the opposite side during the exhaust stroke to guarantee all gases were burnt.
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Since this has gone back-forth a few times with various answers, I figured I'd do a very quick search and put this to rest.
Taken directly from ALLPAR
This guy had it correct from the beginning!
lxman, I seen nothing in my findings that state the 06+ has changed from the prior years???
Taken directly from ALLPAR
Michael E. Gemmel wrote: “Each cylinder has an ignition coil pack over one spark plug, and a regular plug wire connected to the other spark plug. Further, the coil pack also has a plug wire attached to it that extends to the opposite cylinder bank. Each cylinder shares a coil pack with another cylinder. Each of the two plugs on a given cylinder is fired by a separate coil. One plug has a coil directly attached, and the other is fired via an ignition wire connected to a coil located on another cylinder on the opposite bank. The benefits would be one-half the number of coils (8 vs. 16) compared to each plug having its own coil, and of course less weight.”
“Cryptojoe” wrote: “The extra plug fires during the power stroke to more fully burn the hydrocarbons. ... the second ignition allows additional power in the down stroke while lowering the need for restrictive catalyst plates in the converter.
“In the 1980s Japanese manufacturers reduced unburned hydrocarbons by placing spark plugs either in the exhaust pipe (which fired with every piston ignition) or in the exhaust manifold (which fired each time their corresponding cylinder fired). Chrysler morphed this idea to include dual fired plugs on each cylinder, which allows the firing to take place closer to top dead center, and then again when the piston is on the back side of the power stroke.”
Patrick added: “This [also reduces] NOx and ozone. Full combustion results in heat, water, and carbon dioxide. NOx emissions are only significant during incomplete or partial combustion, due to the lack of available oxygen, high temperatures, and various chemical reactions. That's why catalytic converters have been standard on cars for the past 3 decades. The extra set of spark plugs on the HEMI and on previous engines are designed to reduce emissions before a catalyst is needed. They add some horsepower, but not very much.”
“Cryptojoe” wrote: “The extra plug fires during the power stroke to more fully burn the hydrocarbons. ... the second ignition allows additional power in the down stroke while lowering the need for restrictive catalyst plates in the converter.
“In the 1980s Japanese manufacturers reduced unburned hydrocarbons by placing spark plugs either in the exhaust pipe (which fired with every piston ignition) or in the exhaust manifold (which fired each time their corresponding cylinder fired). Chrysler morphed this idea to include dual fired plugs on each cylinder, which allows the firing to take place closer to top dead center, and then again when the piston is on the back side of the power stroke.”
Patrick added: “This [also reduces] NOx and ozone. Full combustion results in heat, water, and carbon dioxide. NOx emissions are only significant during incomplete or partial combustion, due to the lack of available oxygen, high temperatures, and various chemical reactions. That's why catalytic converters have been standard on cars for the past 3 decades. The extra set of spark plugs on the HEMI and on previous engines are designed to reduce emissions before a catalyst is needed. They add some horsepower, but not very much.”
Originally Posted by Powerubi Wacon
So by this reading, the second spark plug is used as a power spark, in the combustion cycle, and not as a wasted spark, to burn unspent fuel, in the exhaust cycle.
lxman, I seen nothing in my findings that state the 06+ has changed from the prior years???
Last edited by dirtydog; Oct 27, 2010 at 04:33 PM.
