ENGINE BLOCKS: Aluminum vs. Iron

What is up engine heads, welcome to another episode of engine boot camp, and today we're talking about the backbone of every engine, the engine block!

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First let's start with the basics. As you probably know the engine block is the backbone of every engine...if the engine were a human, the engine block would be the skeleton. And just like a human would be nothing but a blob of meat and skin without it's skeleton, so too would an engine just be a mess of parts on the floor without it's block.
The engine block is the largest and most intricate single piece of metal of every engine. Everything on the engine, the crankshaft, the cylinder head, the exhaust, the intake, and even the transmission, gets bolted onto the engine block. And as you're probably already the engine block is as old as the internal combustion engine itself, it was there from day one and it will be there until the end.
Although the first ever airplane engine to fly, the one in the Wright flyer of 1903 had an aluminum engine block for weight saving purposes, aluminum blocks were rare throughout much of the internal combustion engine's history, where cast iron blocks held a dominance for a very long time. Aluminum engine blocks started could be found in mass production passenger cars as early as the 60s, but they were far less common than cast iron blocks. Throughout the 60's and 70's aluminum engine blocks accounted for less than 2% of newly manufactured engine blocks. But this percentage would keep increasing through the decades, with aluminum engine blocks reaching almost one third of all new engine blocks in the late 90's. Beyond this point Ever tighter emissions and fuel consumption regulations pushed manufactures to find ways of building ever lighter cars and vehicles and ever more efficient engines, this tipped the scales in favor of aluminum and by 2005 aluminum engine blocks caught up with iron ones and shared an equal 50/50 percentage in newly manufactured engine blocks. Today, aluminum engine blocks account for more than 2 two thirds of all newly manufactured blocks, a percentage that will likely keep increasing.

But newly manufactured engines aside, you will still find many tuners, enthusiasts and race engine builders preferring and sticking to cast iron engine blocks by re-machining and rebuilding these blocks into very serious and capable engines.

Before we proceed there's something we first have to make clear. The term aluminum or aluminum and iron is a bit misleading , because within the term aluminum there are hundreds of different aluminum alloys and there are dozens of different grades and classes of gray cast iron.
So to be more accurate, let's first make it clear what kind of aluminum and what kind of iron are engine blocks actually made from.
As I said iron engine blocks are usually made from gray iron, one of the most common types of iron used for casting.

Now cast grey iron is divided into classes or grades . Engine blocks are typically made from class 20 or 25 grey iron and have a tensile strength in the range of 20.000-25.000 psi.
OEM Aluminum engine blocks are most often made from the one of three alloys: 319, A356 or A357,

Now there's another aluminum alloy that billet aluminum engine blocks are made from, and that alloy is 6061 alloy which is significantly stronger at 60-70.000 psi, however billet engine blocks are an extremely expensive aftermarket only thing reserved only for the most extreme of racing applications

Now Aluminum cylinder blocks aren't just lighter than cast ones, they also run cooler because they are better heat conductors, so they're able to transfer more of their heat onto the coolant and pull more heat away from the combustion chambers. This enables engineers to specify higher compression ratios by keeping combustion chamber temps lower and preventing hot spots and detonation. Higher compression is good for both power and efficiency. This is why the 4g63 was replaced by the 4b11, the rb26 (rb25dett) was replaced by the vr38 (vr38dett) and the 2jz (2jz-gte) was replaced by the BMW B58 I guess. But there's a price to be paid for better heat conductivity, and in case of aluminum engine blocks it's a higher chance of warping if the engine overheats.

To sum it up: Aluminum engine blocks are lighter, their cracks are easier to repair, and they're capable of having higher maximum compression ratios and are more thermally efficient. On the other hand cast iron blocks can take more boost, are cheaper and easier to rebuild, and are better at absorbing noise and vibrations. So who's the winner? Well it really depends on the application, both have significant benefits and the better choice really depends on what you want to do with the engine

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