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Here’s Why the 5.7 Hemi Has 16 Spark Plugs

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Did you know that the 5.7 Hemi has 16 spark plugs?

That’s right, the engine has two spark plugs for each cylinder. It’s a pretty weird design, but it does serve a few interesting purposes.

But why did Chrysler design the 5.7 Hemi to have 16 spark plugs? Is it bad design? An attempt to save on manufacturing costs? In this article, we’ll explore the reasons behind this design choice and see if it actually makes sense.

Why the 5.7 Hemi Has 16 Spark Plugs

Here’s the short answer:

The 5.7 Hemi has 16 spark plugs because there’s no space in the center of the combustion chamber, and two plugs per cylinder improve combustion efficiency and reduce emissions.

But why don’t other engines use two plugs per cylinder? If it really does help, why don’t engineers follow Crysler and use two spark plugs per cylinder?

Well, the answer to that lies in the design and shape of the 5.7 Hemi combustion chamber and valves. Take a look at the picture of the heads on a 5.7 Hemi below.

5.7 hemi combustion chamber shape

The valves on the 5.7 Hemi take up so much space, that there’s hardly any room in the center, let alone enough space to fit a spark plug. Keep in mind that the center is the most optimal place for a spark plug.

Obviously, the only logical way to go around that is by placing a spark plug in a corner, but that creates another problem.

If Crysler put a single spark plug in a corner, then the engine would have two problems – the mixture wouldn’t fully combust and the flame would have to travel from one side to the other side. This is incredibly inefficient.

To fix that, the engineers simply decided to put another spark plug on the opposite side. While this is not ideal, it does minimize the potential issues caused by the shape of the combustion chamber and the size of the two valves.

So, is this design choice lazy or flawed? Let’s just say that the most reliable and most powerful engines use a single spark plug, and that two spark plugs per cylinder are extremely rare in modern cars.

What Are the Benefits of Having 16 Spark Plugs (2 Spark Plugs per Cylinder)?

For the 5.7 Hemi specifically, the benefits of 16 spark plugs were slightly more power, better fuel economy, and significantly fewer NOx emissions.

Now, two spark plugs per cylinder are not exclusive to the 5.7 Hemi only – there are quite a few aero engines with dual ignition systems and spark plugs, but they are used for redundancy. No pilot could stop and change a spark plug mid-flight.

But apart from redundancy on aero engines, 16 spark plugs or two spark plugs per cylinder were used in the 30s. Back in the good old days, when fuel quality was highly questionable at best, placing two spark plugs per cylinder was nothing special. Even Le Mans endurance race cars used two spark plugs per cylinder way back in the 60s.

Back then, two sparks per cylinder were used to combat unpredictable and slow-burning fuel. The main benefits of using two spark plugs per cylinder were smoother running, more power, and fewer chances of fouling the spark plugs.

Once fuel quality improved, some engineers still utilized the two spark plug per cylinder design in order to meet emissions regulations in the 80s. Two spark plugs per cylinder became a rare design in the 90s and was hardly ever used in the 00s.

How Much Does It Cost to Change Spark Plugs on 5.7 Hemi?

Since the 5.7 Hemi has 16 spark plugs, you might be interested in how much it costs to change them. If you own a 5.7 Hemi, I recommend you to sit down before reading further.

The cost of changing spark plugs on a 5.7 Hemi can range from $400 to $600, depending on your location and how much the shop or dealership charges for labor.

According to the shop manual, spark plug replacement for the 5.7 Hemi takes 4 hours. On average, independent shops charge around $90 for 1 hour of labor, while dealerships charge $130 and more.

Related: 5.7 Hemi Ticking Noise on Startup – How To Diagnose & Fix

Final Thoughts

The 5.7 Hemi is a great engine, but it’s not perfect. One of its main drawbacks is the number of spark plugs – 16 spark plugs is a lot, and it’s not uncommon for people to have issues with changing them.

They’re difficult to reach and replacing them takes at least 3 hours for a very experienced mechanic.

Now, while having 16 spark plugs on an engine is not a great design, it’s the best that Chrysler could do given the shape of the combustion chamber. If you change the shape of the combustion chamber in order to fit a single spark plug in the center, then the engine is no longer a Hemi then, is it?

I hope this article cleared up some confusion and helped you understand why the 5.7 Hemi has 16 spark plugs. If you have any questions, feel free to leave a comment below.

Thanks for reading!

1 thought on “Here’s Why the 5.7 Hemi Has 16 Spark Plugs”

  1. Enlarging the combustion chamber size to accommodate a single spark plug in the center does not disqualify an engine as a hemi. The early 426 Chrysler, the Mercedes Benz, and the Ford 429 hemis only had one spark plug per cylinder and had good volumetric efficiency due to the fact that the center of a sphere creates the shortest distance for ignition to travel to all edges. The same is true for a hemisphere. These engines created tremendous power, but they were gas guzzlers.

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