Why Do Hemi Engines Have 16 Spark Plugs?

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I found a very good explanation, on allpar, as to why our Hemi engines have 16 spark plugs. Here it is: "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 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. 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." #1 · Jul 24, 2012 I found a very good explanation, on allpar, as to why our Hemi engines have 16 spark plugs. Here it is: "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 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. 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." See less See more Sort by Oldest first Oldest first Newest first Most reactions #2 · Jul 24, 2012 Some of that description ^^^ pertains to the earlier 5.7s (pre-MDS?) that had the 1 of the 2 coil wires cross over to the opposite bank. The current engines have a different coil-on-plug setup that don't have spark plug wires at all now. Mainly, the dual plug setups will promote more complete combustion, especially with the leaner mixtures. With a hemispherical (or flattened hemispherical like we have now), the quench areas are problematic for unburned fuel / high HC content in the exhaust. The opposed valve layout with the hemi chamber breathes well and makes good power, but requires more measure to keep emissions low. With the dual plug setup, it assures more complete combustion. What's unique with the VCT '09+ engines, is they run clean enough (low NOx) that EGR systems are not needed. #3 · Dec 22, 2014 Dual plugs shorten the flame front propagation and increase power for displacements of 175cc and greater. #4 · Dec 23, 2014 Also, I think the "classic EGR" has been consolidated as "internal EGR" in the way the vct allows some reversion of exhaust gas along with the intake charge at lower rpm and low engine load. #5 · Dec 26, 2016 So would you not expect to see any advantage in a 35 cc engine (four stroke)? #6 · Dec 26, 2016 Interesting since EGR was the first thing to go on old cars, unless they had CATs and air pumps (injecting air into the exhaust) then those were the first to go. The Top Fuel engines have 2 because if one misfires the other should fire (odds of both not firing). With a 1" fuel line to each cylinder (Nitro methane) if the plugs did not fire it would send the super charger into orbit.

Those engines are awesome with 2 distributors and 2 complete ignition systems. Our Hemis are a product of lots of racing research. #11 · Dec 29, 2016 Hey Frank, no disrespect, but as a guy who grew up as a " pit rat" back in the match racing days, you missed the dual magneto thing by a bit. Now, bear in mind that NHRA does not allow electronic engine management in the Nitro classes, it's still a bottle of air and solenoids >

. The reason behind the dual mags has to do with putting the power down. Mag "A" has curve set to launch the car and get it to roughly 330 feet, which is the point you want to try and have the clutch locked. Mag "B" takes over at that point with a different curve to keep the engine under the correct load to keep all the candles lit. Two scenarios to consider...car drops a hole at the hit (white smoke vs flame) or car drops hole after 330. If your scenario was correct, you wouldn't drop cylinders, but because only one mag is being used, you can easily see why these cars can drop cylinders, either at the hit or down track. As far as popping the supercharger, that has more to do with crank twist and cam profile, which is way too much for this thread. Bottom line, is that our Gen III Hemis use dual plugs for a completely different reason than the Nitro cars. EPA, anyone????? :grin2: Show more replies 0 Reply #7 · Dec 27, 2016 Spark Plug CEO at meeting, "If we can get cars to run 16 plugs instead of 8 our profits will go up." Thats how I always seen it. #8 · Dec 27, 2016 Emissions is a big factor, Ducati does this as well on their air cooled motors. Vehicle manufacturers aren't going to add additional plugs coils and wires unless they have to... #9 · Dec 27, 2016 ^^^ this is the main case. In some instances (there can also be phased spark for dual plug engines). My other car has this and in diagnostic mode (with the appropriate diagnostic system), the "A" bank of "B" bank plugs can be switched on/ off to diagnose it. The ignition can be closer or further apart between the two banks, depending on load, knock sensor readings, O2 readings etc. I can tell the smoothness of the idle changes, if one bank or the other is switched out. Its remarkable with the technology we have engines that can run as well at below sea level to 14K' elevations, -20 to 130*F and start reliably every time. I don't miss the days of carburetion - having to adjust for changes in altitude / trying to get the 'average' where everything would run satisfactorily, cold start issues, hot start issues, potential for vapor lock / fuel boil out, etc. You often hear gripes about the "old ways" but frankly, you had to mess with it regularly and many didn't have the skills to do it right. I see the older carbed vehicles on the road today (pre '85 American makes) - and the majority of them aren't tuned right. So much for the "easier to work on" I hear about. Show more replies 0 Reply #12 · Dec 29, 2016 On the newer HEMI's do both spark plugs fire at same time in the cylinder? Or maybe 1°-2° different for more total spark? #15 · Dec 29, 2016 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 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 other words, using dual fired plugs on each cylinder 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. Show more replies 0 Reply #13 · Dec 29, 2016 A Guy

#19 · Mar 8, 2020 #20 · Mar 8, 2020 The ignition coils changed in ‘09 (I believe) to fire two coils in the same cylinder at once instead of two plugs in difference cylinders. Pull up an ‘06 ignition coil on a parts website and you will see where it plugs into one plug on the cylinder head and has a place to plug in an ignition wire that leads to a different cylinder. Look at an ‘12 and you will see two “fingers” and no place for a plug wire. Pretty sure MDS was a feature on the Gen3 Hemi from the beginning, so this change was unrelated. In the end, the motor doesn’t care much. You can swap the valve covers and coils and fire both plugs on one cylinder on an earlier motor without changing anything programming wise. Might make a little more power, but I only looked at doing it because it got rid of the spaghetti over the top of the intake manifold and cleaned up the motor significantly (in my opinion). Insert Quotes Post Reply

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