How many spark plugs you’ll find in a car? In almost every case, you’ll find one spark plug per cylinder.
There are few uncommon cases like Hemi and rotary engines which have two spark plugs per cylinder/rotor. However, almost all cars, except for these, have only one.
Diesel engines have no spark plugs at all. They use glow plugs and compression ignition instead. EVs also don’t have any spark plugs, of course – they’re powered by batteries or hydrogen.
This guide will explain how a spark plug works and why there’s usually only one per cylinder.
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How Many Spark Plugs Do I Need?
To respond to that inquiry, you need to first understand the functioning of an engine. Watch the video shown above to gain a clearer grasp of the fundamentals. It presents a basic four-cylinder in-line motor, which is the easiest model to comprehend and study.
In short, a car’s internal combustion engine is known as a four-stroke. Within each cylinder (and there could be anything from three to 16 cylinders!), a piston moves up and down (reciprocally). It goes through the below four “strokes” constantly until the engine switches off.
- Intake – within the cylinder, the piston travels down. The air intake valve opens, allowing air into the cylinder. At the same time, a fuel injector releases pressurized fuel into the combustion chamber (the space created by the piston’s movement).
- In some cars, the air and fuel get mixed before the combination is sent into the cylinder.
- Compression – the piston rises from BDC to TDC (bottom dead center to top dead center), squashing the air/fuel mixture into a tiny space between the piston head and the top of the cylinder.
- Ignition – also known as the power stroke, this is the crucial point. The car gets its power in each cylinder, one by one, from this stroke. The spark plug, poking through the top of the cylinder into the chamber, sparks, igniting the mixture and driving the piston down again.
- Exhaust – the piston then rises under its own momentum and the power from the other cylinders. The exhaust valve then opens, allowing all the burnt gases to be expelled through the exhaust manifold, heading to the tailpipe.
The spark plug inserts through the top of the cylinder. It is connected to the ignition coil, which brings the electrical charge from the battery to allow it to fire.
How Many Cylinders Does My Car Have?
Although cars could have anywhere from three to 16 cylinders, most American cars have either four, six, or eight. These generally come in an in-line (also known as “straight”) or V format.
In-line engines are arranged in a straight line (you won’t be surprised to hear). V engines form a “V” shape when looked at from the front.
If you live in the US, your car is likely an I4, I6, V6, or V8.
- I4 – in-line engine with four cylinders.
- I6 – in-line engine with six cylinders, often referred to as a “Straight-6”.
- V6 – V-shaped engine with six cylinders.
- V8 – V-shaped engine with eight cylinders.
Note: you’ll also find Flat/Boxer engines with four or six cylinders. The pistons in these are horizontally opposed and perfectly balanced. However, it’s fiddly to change the spark plugs. You’ll find these in some Porsches and Subarus.
Note: some cars, particularly Mazdas, use rotary engines. These are based on the same principles, but the entire process is accomplished differently.
How Many Spark Plugs Are There Per Cylinder?
For simplicity, there’s almost always just one spark plug per cylinder. As mentioned, it might be slightly different if you have a Hemi or rotary engine. Read the following section to learn more about these formats.
- How many spark plugs are in a four-cylinder (I4) engine? 4
- How many spark plugs are in a V6? 6
- How many spark plugs are in a V8? 8
And so on and so forth.
Why is there only one spark plug per cylinder? Quite simply because that’s all you need to burn the fuel. Hemi engines have two because there’s more space within the chamber and, therefore, more fuel to burn.
How Do Hemi Engines Work?
Things can vary slightly when it comes to Hemi engines.
Hemi engines get their name from the Hemispherical cylinder head (where the spark plug goes into the chamber). This allows for an optimal ratio between volume and surface area.
The lower the surface area, the less heat the metal absorbs away from the piston. This means more power.
Hemi engines had only one spark plug at first. But, they didn’t fully burn the fuel, causing higher emissions.
To meet US emissions standards, Chrysler engineers inserted an additional spark plug. This means the air/fuel mixture ignites at two different points, making it much more likely for all the fuel to combust.
Today, Hemi is usually a marketing term that doesn’t really refer to the cylinder head shape.
If you own a Hemi I6 or V8 engine, you will require 12 or 16 spark plugs, respectively – a pair for every cylinder.
How Do Rotary Engines Work?
A rotary (or “Wankel”) engine works very differently from a standard internal combustion engine, although it still produces the same four “strokes”: intake, compression, ignition, and exhaust.
A rotor sits in a rotor housing with an intake and exhaust port connected to an eccentric shaft (a little like a camshaft). The rotor itself is triangular.
As the rotor moves past the intake port, it creates a vacuum, sucking pre-mixed air and fuel into the chamber. The rotor then turns, with the next node cutting the intake port off.
The rotor then compresses the air/fuel mixture against the cylinder wall, where two spark plugs ignite it.
This drives the rotor’s rotational movements, revealing the exhaust port where the air is expelled.
This process happens continuously. While it’s very effective at producing power, manufacturers struggled to keep fuel economy and emissions down to acceptable levels. Even though it has only two moving parts, rotary engines burn a lot of oil and are known for blown apex seals.
In a rotary engine, you’ll almost always need either two or four spark plugs (two spark plugs per rotor).
How Many Spark Plugs Should I Get For My Car?
Check your owner’s manual. It will tell you how many spark plugs you need and recommend the best brand. Please note that all spark plugs are different. You must buy parts that fit your make, model, and engine.
Head on to a hardware store or to your automotive parts website of choice. Entering your license plate should filter out everything else, leaving only the spark plugs that fit your car. Budget spark plugs are rarely a good idea – mid-range options are probably best for most people.
Spark plugs should be sold in the correct quantities in most cases. Don’t hesitate to ask a mechanic or store attendant if you’re ever in doubt.
Spark plugs are relatively easy to change by yourself in in-line engines. V-engines are a little trickier, and flat models will leave you cursing the ground you stand on.
Most mechanics won’t charge much labor to do it for you, so if you’re unsure, simply drive over to someone you trust and pay them to swap the spark plugs for you.