Why does Hemi use 2 spark plugs per cylinder?

Two spark plugs per cylinder are used to improve ignition reliability and combustion efficiency in Hemi engines, helping performance and emissions. In modern Hemi designs, dual ignition promotes faster, more complete combustion across a broad operating range and provides redundancy to reduce misfires.

How it works in a hemispherical combustion chamber

The Hemi’s hemispherical combustion chamber geometry favors rapid flame spread and high compression. Placing two spark plugs at opposite sides of the chamber creates two ignition points, which generate twin flame kernels that merge quickly. This arrangement helps the air-fuel mix burn more evenly and completely, even under challenging conditions such as cold starts, high loads, or lean-burn scenarios.

In practice, the two plugs can fire nearly simultaneously or with a carefully timed stagger that optimizes flame front development. The result is a faster burn, reduced misfires, and the possibility of more efficient combustion without resorting to excessively rich mixtures.

Benefits of dual ignition in the Hemi

Two ignition points per cylinder offer several practical advantages that have driven widespread adoption in modern Hemi engines. The list below outlines the key benefits engineers seek when implementing dual ignition.

• Faster flame propagation and more complete combustion, improving power delivery and fuel efficiency.


• Better idle quality and startup reliability, especially in cold weather or under low-load conditions.


• Enhanced tolerance for lean-burn operation, which can lower fuel consumption and reduce certain emissions.


• Reduced risk of misfires because two independent ignition events provide redundancy.


• Improved emissions performance through a more consistent burn and reduced unburned hydrocarbons.

Overall, dual ignition helps the Hemi achieve a balance of performance, efficiency, and emissions compliance across a wide operating envelope.

Design considerations and tradeoffs

Implementing two spark plugs per cylinder adds complexity and cost, so automakers weigh several practical factors when deciding whether to use dual ignition in a Hemi engine.

• Increased parts count and wiring or coil-pack complexity, which can raise manufacturing and service costs.


• Higher maintenance requirements, since two plugs per cylinder must be serviced or replaced over the engine’s life (though design aims to optimize plug life and maintenance intervals).


• Potential packaging and reliability considerations, such as ensuring both plugs operate in harmony and that ignition electronics can drive two plugs per cylinder reliably.


• Tradeoffs with calibration: ignition timing and ignition coil strategies must be tuned to maximize the benefits of dual plugs without compromising reliability.

Engineers typically justify the added complexity by citing gains in efficiency, responsiveness, and emissions that align with stricter regulatory standards and consumer expectations for modern performance cars.

Bottom line for drivers and the market

The dual-spark approach in Hemi engines reflects a broader industry focus on improving combustion efficiency without sacrificing performance. By lighting the air-fuel mix at two well-placed points, the Hemi can achieve quicker, more stable ignition, better response, and cleaner burning—benefits that matter at both the pump and on the road.

Summary

In short, two spark plugs per cylinder in Hemi engines are a deliberate design choice to promote faster, more complete combustion, bolster ignition reliability, and help meet modern emissions and efficiency targets. While it adds complexity and cost, the performance and environmental benefits observed across various driving conditions have made dual ignition a common feature in contemporary Hemi applications.

Why do Hemis have 16 spark plugs?

Modern HEMI V8 engines have 16 spark plugs (two per cylinder) to create a faster, more complete combustion, which improves performance, fuel economy, and emissions. The hemispherical combustion chamber of a HEMI is efficient for airflow, but the deep dome shape makes it difficult for a single spark plug to ignite the entire fuel-air mixture quickly. Placing a second spark plug on the opposite side of the cylinder allows the two flame fronts to meet, ensuring a more complete burn and smoother operation, especially at high RPMs. 
This video explains why some HEMI engines use 16 spark plugs: 58sThe Enthusiast MechanicYouTube · Jul 26, 2024
Key reasons for dual spark plugs

• More complete combustion: The two plugs ensure the entire fuel-air mixture is burned more efficiently, reducing wasted fuel. 
• Improved performance: A more complete burn translates to a slight increase in horsepower and better throttle response. 
• Better emissions: By burning fuel more completely, the engine produces fewer harmful emissions. 
• Smoother operation: Dual ignition points provide a more stable and consistent power stroke, which can improve cold starts and idle quality. 
• Address the hemispherical design: The unique shape of the HEMI's combustion chamber requires two ignition points for optimal efficiency, as a single plug cannot effectively ignite the entire volume of the chamber. 

What is the bad year for the 5.7 Hemi?

The "bad" years for the 5.7 Hemi include 2003-2008 due to a known issue with valve seats dropping, which can cause significant engine damage. Additionally, 2011-2012 model years, particularly in Ram trucks, were notorious for the "Hemi tick," often linked to lifter or camshaft problems.
 
This video explains the common issues with the 5.7L Hemi engine: 58s8020 AutomotiveYouTube · Oct 13, 2022
2003–2008 (Pre-Eagle) 

• Valve seat drop: This is the most significant issue for this period, where valve seats could fall out of the aluminum cylinder heads, leading to engine damage. This is due to the different expansion rates of the steel valve seats and aluminum heads. 

You can watch this video to learn more about how to prevent the valve seat drop issue: 56sMarspeedYouTube · Nov 20, 2020
2009 and later (Eagle) 

• Hemi tick: The 2009 "Eagle" Hemi introduced revisions that brought a new problem: an intermittent ticking noise that can indicate worn lifters and camshafts. 
• Lifter and camshaft failure: The ticking noise is often a precursor to lifter failure, which can seize and cause the camshaft to wear down. This is a costly repair, often running thousands of dollars. 
• Multi-Displacement System (MDS): The MDS system, introduced in 2005, can contribute to engine tick. It deactivates four cylinders during cruising, creating temperature differences that may cause oil flow problems. 
• Specific problematic years: While the issue became more widespread in later years, the 2011 and 2012 models are particularly noted for the "Hemi tick". 

This video shows how to diagnose and address the Hemi tick issue: 57sJack FletcherYouTube · Dec 10, 2022
Other considerations

• Exhaust manifold leaks: This can occur in many model years and may be mistaken for the engine tick. It often sounds like a tick, especially on a cold start, and is caused by broken manifold bolts. 
• Transmission issues: Many 5.7L Hemi vehicles have also had issues with their transmissions, which can lead to costly repairs. 

How many spark plugs per cylinder 5.7 Hemi?

two spark plugs
All modern Chrysler Hemi engines use two spark plugs per cylinder for a total of 16 spark plugs. This includes engines built in 2003 and later, ranging from the 5.7 liter to the 6.2 liter Hellcat engine.

Why use two spark plugs per cylinder?

Engines use two spark plugs per cylinder for redundancy, improved efficiency, and higher performance. Redundancy ensures the engine continues to run if one plug fails, which is critical for applications like aircraft. The two sparks also create a faster and more complete burn of the fuel-air mixture, leading to greater power, better fuel economy, and lower emissions. 
This video explains why aircraft engines have two spark plugs per cylinder: 27sSporty's Pilot ShopYouTube · Aug 31, 2023
Benefits of dual spark plugs

• Redundancy: If one spark plug fails, the other can continue firing, preventing engine failure and allowing for safe operation or landing. This is a primary reason for using dual ignition in aircraft engines. 
• Improved efficiency: Having two points of ignition allows the fuel-air mixture to burn more quickly and completely. This reduces wasted fuel and results in better fuel economy. 
• Higher performance: The faster and more even burn of the fuel-air mixture can lead to increased power output. 
• Even combustion: Two sparks ignite the mixture from opposite sides, which creates a more even flame front. This reduces hot spots in the cylinder, leading to smoother operation and potentially increasing engine longevity. 
• Lower emissions: More complete combustion means fewer unburnt hydrocarbons are released, which helps an engine meet emissions standards. 
• Design flexibility: In some engine designs where the valves make it hard to place a single spark plug in the center, two plugs on opposite sides ensure more efficient ignition.