Cast Iron vs Aluminum Engine Blocks: Which is Better？

The internal combustion engine is the heart of any gas-powered vehicle. Normally, any manufacturer of automobiles wants such essential parts of its products to be well-protected and fully functional at the same time. 

Nowadays, iron and aluminum are the two common materials used to make engine blocks. A comprehensive analysis is due to determine which one is better. 

Main Materials to Craft Engine Blocks

Well, there is not much to discuss. The automotive engine progressed over decades, but two main materials to craft engine blocks are still in wide use. Let’s take a look at what these materials are in detail. 

Cast Iron 

Cast iron is one of the oldest ferrous metals in commercial use. To be exact, it is a group of metal alloys that has a similar composition you can familiarise yourself with from the image below.

But we are jumping ahead. Here’s the complete list of all the cast iron types that currently exist and are widespread.

•  Gray cast iron.
•  Ductile cast iron.
•  White cast iron.
•  Malleable cast iron.
•  Compacted graphite iron.
•  Alloy cast iron (Mottled cast iron).

All of them are significantly different in their physical properties. As such, compacted graphite iron has 80%more tensile strength and double the fatigue strength of regular (grey) cast iron. but is not deprived of its own drawbacks.

To not confuse you, just take it that the most widely used casting is gray cast iron with containment of carbon varying between 2.5% and 4.0%, and containment of silicon varying in the range of 1-3%. 

As for aluminum, there are a lot of them. So let’s stick to commonly used, for engine blocks craft, ones:

• 319 Alloy. Its composition is: 5.5% to 6.5% silicon, 3% to 4% copper, nickel, titanium, manganese, iron, zinc, and magnesium in small quantities (0.1%-1%), and the rest – aluminum. It receives T5 treatment to harden and strengthen the metal. 
• A356 Alloy and A357 Alloy. their compositions are: 6.5% to 7.5% silicon, only 0.2% copper, less iron, more magnesium than in 319 alloys, and the rest – aluminum. It receives T6 treatment to harden and strengthen the metal even more. 

What is a Cast Iron Engine Block?

For starters, let’s define what an engine block actually is. In simple words, an engine block or cylinder block is the metal structure that contains all the parts of an internal combustion engine. It can be regarded as a protective shell for components that are sensitive to external physical stresses.

Typically, cast iron engine blocks are fabricated from metal workpieces using CNC machining. This manufacturing method is considered superior to alternatives in terms of precision and efficiency of cast iron machining.

If you wonder how essential an engine block is but to hold cylinders and other components together, here’s the list of reasons it is so vital:

• Engine block is a foundation for engine components, including cylinders and pistons.

• It provides oil and coolant passages and flat surfaces for the engine’s heads.

• It transfers frictional heat to the air and the engine’s coolant.

• It has an oil pan integrated into the bottom.

No vehicle ran without a fully operating engine block. If it is malfunctioning, it basically means that the entire internal engine does not function well.

Pros and Cons

With respect to the benefits of cast iron engine blocks, they are the following:

• Good friction coefficient that benefits a vehicle’s performance. 
• Low-cost.
• Durable.
• Good heat retention.
• High strength.

The limitations and cons of such blocks are the next:

• Extremely heavy-weight affects a vehicle’s performance.
• Low corrosion resistance.
• Low explosion resistance.
• It is not wear-resistant.
• It cannot be repaired or DIY finished.
• It cannot be recycled.

What is an Aluminum Engine Block?

Aluminum is typically used for PCB manufacturing/fabrication performed by a router. But, unsurprisingly, means of CNC machining are able to make engine blocks of such lightweight metal as well. 

Besides the fact that aluminum engine blocks are crafted of this widespread metal, there is not much to add to the similar section above, so refer to it. Engine blocks made of these two materials serve absolutely the same purposes and do not have any such worthwhile difference to discuss, but ones detailed further in the text. 

One thing not everyone knows about aluminum engine blocks – they are superior to cast iron ones. It is so because they can be DIY finished for some construction purposes. 

Pros and Cons

With respect to the benefits of aluminum engine blocks, they are the following:

• Lightweight.
• Good cooling properties.
• Good corrosion and explosion resistance.
• It is wear-resistant.
• It can be repaired or DIY finished.
• It can be recycled.

The limitations and cons of such blocks are the next:

• Do not retain heat well.
• Expensive.
• Less durable.
• Vulnerable to warping.
• Lower strength.

Differences Between Cast Iron and Aluminum Engine Blocks

Regarding the essential differences between engine blocks made of these two metals, all existing ones are detailed in the table below.

Factor	Cast Iron Blocks	Aluminum Blocks
Cost	Mostly affordable because the material is up to 4 times cheaper than aluminum.	Considered expensive because of the costly alloys used.
Durability	Is highly durable and rigid.	Is not that durable, and somewhat prone to warping.
Resistance	The major drawback is cast iron is not corrosion-resistant. It is also vulnerable to overheating.	Aluminum is less susceptible to rust and handles excessive heat better than iron.
Weight	Extremely heavy.	Lightweight.
Ease of repairment	Typically, they are not worth the effort.	Can be repaired multiple times, depending on the damage.
Engine’s life	Low to medium length of engine’s life.	Medium to a high length of engine life.
Power	Can handle a significant power – suitable for vehicles related to cargo duty.	Can handle less power – suitable for regular and race cars.

As a bonus, here is the visual comparison of these two types of engine blocks.

Final Word

Grey cast iron makes engine blocks durable and strong and facilitates heat retention while compromising weight limitations. Aluminum is lightweight, resistant to corrosion, and is a great coolant. Although it does not sustain high volumes of energy. 

So, in most cases, but when a vehicle is intended for cargo duties, an all-aluminum engine block is likely to be a better decision. But it significantly depends on the performance you expect from your car.