What Hemi Means in an Engine: Generation 1, 2 and 3 Hemis

The 426 Hemi in the 1960s put the word Hemi into a different category. The Hemi engine earns respect from all car enthusiasts. Although not everyone knows the exact meaning of the word. Let’s answer, what does Hemi mean in an engine?

Hemi, short for hemispherical, refers to the shape and design of the combustion chamber. The Hemi combustion chamber is similar to half a sphere and allows the intake and exhaust valves placed opposite each other instead of next to each other. The Hemi design allows for optimum air flow, spark and combustion.

This article will discuss in more detail how the combustion chambers are designed including the spark plug location and ports. I’ll examine the differences between a Hemi and regular engine and the benefits.

The following specifications and facts were learned from my personal experience and research, Mopar brochures and engine manuals, videos, webinars, books and articles.

What Does Hemi Mean in an Engine?

The shape of the Hemi combustion chamber is like cutting a globe in half and using a little less than one half as the chamber. The domed chamber design allows for the intake and exhaust valves to face opposite each other perpendicular to the engine bank.

The valves are angled in the domed chamber allowing them to be larger in size and avoid hitting each other or the cylinder wall.

The larger valves allow for more air flow through the heads. Also, due to the location of the valves, the intake and exhaust air are able to flow in a straighter path.

The spark plug tip in the combustion chamber is located in the middle of the chamber making the spark more efficient.

The hemispherical designed combustion chamber produces a quicker rise in combustion chamber pressure.

What is the benefits of a Hemi Engine? The benefits of a Hemi engine include the following:

• Larger intake and exhaust valves.
• Optimal spark plug tip location.
• Increased air flow into and out of the combustion chamber.
• Faster combustion chamber pressure.
• Straighter intake and exhaust ports for better air flow.
• Burns fuel more efficiently.
• Increased horsepower.

Technically the Hemi head is not truly hemispherical. This is so because less than half of a sphere is used for the combustion chamber.

The Difference Between a Hemi and a Regular V8 Engine

Knowing the benefits of the engine is one thing, but how it compares to a regular engine is another.

Other than having the ability to overtake another car on the top end of the drag strip, what is the difference between a Hemi and a regular engine?

The difference between a Hemi and a regular engine is the cylinder head designs. A Hemi engine has a domed combustion chamber while a regular engine has a flatter chamber.

A Hemi engine has the intake and exhaust valves opposite each other. A regular engine has the valves placed next to each other in a straight line.

A Hemi engine design has larger intake and exhaust valves which face down at an angle. A regular engine has smaller valves facing straight down instead of angled.

A Hemi engine has the tip of the spark plug located in the center of the chamber while a regular engine has the spark plug tip to the side.

A Hemi head intake and exhaust ports are in a straighter path allowing the air to pass with less disturbance. A regular engine’s ports are opposite each other with more bends.

The valvetrain on a Hemi is different than on a regular V8. Since the Hemi intake and exhaust valves are opposite each other, the rocker arms pivot on two rocker shafts instead of one.

A regular V8 has the intake and exhaust rocker arms on one shaft because the intake and exhaust valves are in a straight line parallel with the cylinder heads.

There’s more to the head design that makes a 426 so special. Find out more in my article, What Makes the 426 Hemi So Special.

The Difference Between an Older and a Newer Hemi

The newer Hemi head design differs a little from the generation ll engines which made the four letter word famous.

The first generation engine (oldest) was placed into production cars between 1951 and 1958. In 1959 it was used in trucks and phased out.

The generation ll engine (older Hemi) was produced from 1964 to 1971 and the street version from 1966 to 1971.

The generation lll engine (newer Hemi) started in 2003 and is still in production. Let’s examine the differences between the Hemi engines.

The newer Hemis are fast. If you ever wondered how fast the original Street Hemi was check out my article, How Fast is a 426 Hemi? 1/4 Mile & 0-60 Results.

Combustion Chamber Shape

The generation 1 and generation 2 Hemi heads are similar in design. The generation 1 has a small combustion chamber and valves. The valves are angled the same.

The main difference between the two older Hemi engines and the generation 3 is the shape of the combustion chamber. The older Hemi is dome shaped at the top of the chamber while the newer Hemi’s chamber is flatter at the top and shallower.

The new design encourages the air-fuel mixture to stir itself prior to ignition. Due to the flatter top of the combustion chamber, the piston can be flatter and not domed like the older pistons.

According to Chrysler the flatter design aids the flow of intake and exhaust gasses into and out of the head.

Another difference on the newer combustion chamber is the oval shape compared to the rounder shape of the older generation heads.

Spark Plugs

The next largest difference between the old and new is the spark plugs. The generation 1 and 2 Hemi has one spark plug per cylinder while the generation 3 Hemi has two spark plugs per cylinder.

Valves

Although the newer heads have the intake and exhaust valves opposite each other like the original engine, the angles are slightly different.

The generation 1 and 2 Hemi valves were angled at 58.5 degrees and the generation lll Hemi valves are angled at 34.5 degrees.

The less domed chamber doesn’t allow for more angle like in the older engines.

Learn more about the carburetors in my article, The Carburetors on a 426 Hemi.

Is a Hemi and V8 the Same?

A Hemi engine is a V8 engine but with a different cylinder head design.

The remaining parts of the engine like the block, crankshaft, rods, camshaft and intake are similar between a Hemi and a regular engine.

A V8 engine without Hemispherical heads is not a Hemi engine.

Find out what cars had the Hemi in its first year by checking out my article, The First Year of the 426 Hemi.

Faqs

Does a Hemi need premium gas?

A generation ll 426 Race Hemi Engine should run on racing fuel. The 426 street Hemi Engine should run on Premium gas due to its 10.25:1 compression ratio.

Generation lll HEMI Engines have different compression ratios and gas requirements. Dodge states the 6.1L engine and the newer 6.4L/392 engine requires premium fuel for best performance.

Always check your owner’s manual or dealership to inquire if your particular engine requires premium gas or another octane.

Find out why the Hemi was banned twice in my article, Why the 426 Hemi was Banned From Nascar.

How many spark plugs does a V8 Hemi have?

Generation l and ll V8 Hemi engines have eight sparks, one per cylinder. The generation lll Hemi V8 engines have 16 spark plugs, two per cylinder.

Find out about the mythical 426 Hemi DOHC in my article, The 426 Hemi Dual Overhead Cam Engine – The “Doomsday Hemi”

Check Out More 426 Hemi Articles!

The Last Year of the 426 Hemi

How Much An Original 426 Hemi Car is Worth: Recent Prices

What Cars Had the 426 Hemi: Street and Race Hemis

426 Hemi Horsepower and Torque – Rated and Real HP

426 Hemi Specifications – Complete Engine Specs

The Worth of a 426 Hemi Engine

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