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What is the advantage that the aluminium engines provide over the cast iron engine apart from the fact of rusting and weight reduction.

Manufacturers like to boast about their all new aluminium engines does this really mean anything to the customer?

Should i buy a slightly more powerful cast iron engine or the less powerful aluminium engine?

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My 1996 Jaguar had an aluminium block. It isn't a new thing. I am not sure there are weight advantages either, at least not in the older ones. Steel blocks had greater strength for the volume of material used. Now days the material design is probably much better, so they can take weight out without knocking the strength. I guess the question is power to weight ratio. If you iron engine car is 10% more powerful but 20% heavier you have not gained anything. – TafT Feb 23 at 16:32
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If the engine makes 10% more power but is 20% heavier you are WINNING in a normal car because the car may only get 5% heavier and therefore will have a faster zero to 100K time . – Autistic Feb 24 at 11:52
    
@Autistic the car with the lighter engine may be more agile, handle better and have a better weight distribution so may actually be quicker through the corners which may mean, on certain roads, it's point to point times are faster. Alfa Romeo's old Twin Spark engines used alloy blocks. This did mean that they used more oil as they would flex under heavy use but the way those cars handled was razor sharp. – Steve Matthews Feb 25 at 9:55
    
@TafT: You are right that it isn't anything new, the 1963 Hillman Imp had an aluminium engine. – RedGrittyBrick Feb 26 at 13:39
    
@Autistic if you were referencing my comment with the 10% I was careful to phrase it as "If your iron engine car is 10% more powerful but 20% heavier" so that I was talking about the weight and power of the whole car not just the engine. Sorry if that was not clear enough. As others have pointed out where the weight is also changed the cars handling so it is not always as simple as heavier means slower acceleration or more grip even with no change in power. – TafT Feb 29 at 10:39
up vote 16 down vote accepted

You've already stated the main reason for the aluminum blocks which is the weight reduction. It is the same reasoning which has led Ford to produce their F-Series trucks with an aluminum body: weight reduction equates to better fuel economy. Most manufacturers today are using aluminum in various places to reduce the overall weight of the vehicle. As far as rusting goes, this usually isn't an issue on cast iron blocks as it's life is spent under cover. Only when it is left sitting in the elements does it suffer this fate.

Since you are reducing weight in the front end of the vehicle, it can in most instances provide a better weight bias as well. For those vehicles which are concerned with centering the weight of the vehicle (ie: Corvette), taking 110 lbs off the front of the car (when they moved to the LS1 engine and variants) helped a lot in this transition (moving the transmission to the rear of the car helped as well). This can provide better cornering attitude and help with over/under steer mannerisms.

There are three main advantages (that I can think of) for a cast iron block over an aluminum one:

  • Dimensional Stability: Aluminum grows more during the heating process than does iron. Extra precautions must therefore be built into aluminum blocks so as counteract this condition and prevent issues.
  • No Cylinder Liners: If you ever want to rebuild an aluminum block where the cylinder liners are toast, they must be replaced. This is a large machining expense in comparison to having to just bore the cylinders in a cast iron block.
  • Cost: Cast iron has been used in industry for many years and is fairly easy to produce. Aluminum, on the other hand, costs a lot more to refine from bauxite ore. Just in materials alone, the cost is greater. Then look at the cost for the cylinder liners and special casting processes which must be employed to get the aluminum block correct and the cost goes even higher. There is some trade-off, however, in the heating process (doesn't take as much energy to melt aluminum v. cast iron), and aluminum is easier to machine (less wear on the machine tooling/fixtures v. cast iron).

I'm not sure why you think any given cast iron block is going to be more powerful than any given aluminum one? This is all dependent upon the engine which you are looking at and what the manufacturer has done in the performance realm. If you are just trying to compare the two, this is a decision you'll have to make on your own.

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Nice write up as usual, I am not saying the cast iron is more powerful, given the choice between the two engines for a particular model for example one car has a 1 liter cast iron engine producing 70hp the other is a full aluminium producing 64 hp, which would be the better choice ? considering both cost the same to the consumer? – Anarach Feb 23 at 12:49
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@Anarach - I don't know that I, nor anyone, can answer the question but you. Look at the other factors involved with the engines, such as fuel mileage. Figure out what means the most to you. Is it that, or weight, performance? Cast iron blocks will usually last longer than aluminum, but that also depends on who is making the engine. It's really a crap shoot. – Pᴀᴜʟsᴛᴇʀ2 Feb 23 at 13:28
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Another benefit of cast iron is that it's easier to design. When I worked for an auto-company supplier, we'd have a few cast-iron blocks come through each year for simple "did we screw anything up?" testing, and a bunch of aluminum blocks for "what went wrong this time?" testing. – Mark Feb 23 at 21:38

In terms of material properties an aluminum casting will tend to be lighter for equivalent strength and should be easier to cool.

A lot of the disadvantages of aluminium come down to manufacturing issues. Aluminium castings are potentially prone to defects and although aluminium is soft it is not as easy to machine as cast iron especially to the sorts of tolerances required for an engine. Similarly its softness means that any bearing surfaces need to be formed with inserts (cylinder liners have already been mentioned in another answer) rather than just machined out of the casting. For the same reason mating surfaces like cylinder heads and manifold connections are more prone to damage during assembly and maintenance and the same applied to threads which are prone to stripping and seizing.

In comparison cast iron has the particular property that you can create hardened surfaces in a casting by putting chill plates in the mould to create wear resistant bearing surfaces. Cast iron also has excellent dimensional stability and good vibration absorbing properties.

Although we think of aluminium as being 'non-rusting', corrosion can actually be a big problem in aluminium blocks. One reason for this is that contact between aluminium and steel can cause galvanic corrosion which is a particular problem when you have things like cylinder liners and studs which have to be made from steel.

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From a practical point,

My 04 VW GTI has an aluminum block and turbo, and one thing to keep in mind a Aluminum is softer than Cast Iron and is easier to strip, or scar. I have first hand experience striping stud holes going into the turbo to the down pipe. Times you wished it was cast iron. Also I'd imagine it would be more susceptible to compression issues due to breaking down faster than cast iron.

Just my points from under the hood.

Also, the extra weight in front of the vehicle hear in New England is probably more of a benefit as it should help with traction on the road in slippery conditions.

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Actually your GTI had an grey cast iron block. The head where the turbo/manifold attaches is AL though. – draksia Feb 24 at 3:05
    
Oh wow, I always thought it was aluminum. Anyway I still believe my answer still stands. – Devin Gleason Lambert Feb 24 at 11:30
    
In slippery conditions you want more weight on the back wheels, not on the front wheel (where the engine usually is). – Dmitry Grigoryev Feb 24 at 14:25
    
If its a rear wheel drive vehicle, yes. if its a front wheel drive vehicle, put the weight in the front. Hence why trucks with 2wd would put sand bags in the back to help with traction in the winter. – Devin Gleason Lambert Feb 24 at 15:45

There is also compacted graphite iron, vehicles have been using this new alternative since 2004. It allows the molecules to form a more dense matrix upon cooling, which provides the benefits of iron (much longer lasting) as well as the benefits of aluminum (much lighter.)

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Welcome to Mechanics.SE! While I agree with your assessment, I'm not sure how this actually answers the question of aluminum v cast iron? – Pᴀᴜʟsᴛᴇʀ2 Feb 23 at 22:48
    
It helps provide context and perhaps inform the poster that there is additional information they might not have had when asking their original question. Like asking about the Democratic - Republican split in the Senate. And then someone posts, saying "well, you might not have known this, but there are actually two senators that are registered Independent." – Jerome Wiley Segovia Feb 24 at 20:16
    
I understand where you are coming from. Please remember, though, Stack Exchange is about answering the OP's question. Since the OP isn't asking about alternatives to aluminum or cast iron, your answer doesn't have a place here ... it isn't pertinent to the answer chain. Please realize I'm not dogging you out because of your answer, I'm just trying to help you understand how Stack Exchange works. Please check out the Help Center for more information about answering questions. – Pᴀᴜʟsᴛᴇʀ2 Feb 24 at 22:47

Aluminum engines also have better heat transfer, which can reduce engine hot spots and tendency to knock.

When the limit on power is engine knock, which is often the case, aluminum engines with the same specs/dimensions would be more knock resistant and tend to be capable of more power. For example, with more knock resistance, one thing commonly done is increasing compression/boost/ignition advance to increase the power output of the engine.

If all the engine settings remain the same, such as when the engine tune is not adjusted for the change in materials, the cast iron would likely make a small amount more horsepower. This happens because the cast iron combustion chamber steals less heat during the combustion event, and more heat would be conserved for actual work.

A stronger cast iron engine block would be an advantage in some special circumstance where the engine is limited by part strength instead of knock, such as a low compression forced induction methanol engine with large amounts of boost and nitrous.

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Can you better qualify your second paragraph? It appears you are saying an aluminum block is capable of making more power than an iron block? – Pᴀᴜʟsᴛᴇʀ2 Feb 23 at 18:31
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I would say this is rather inaccurate, AL cylinder heads definitely do reduce the likelyhood of a hot spot compared to cast iron but in the block this shouldn't really make any difference, almost all the block is covered by the piston when it gets to the hot part. Also AL for the most part have cast iron liners. – draksia Feb 24 at 3:04
    
Of course mixing aluminum and cast iron engine components would have different results. I am not comparing just the engine block or individual components here... by aluminum engine I mean aluminum head and block, and by cast iron engine I am assuming it means iron heads and block. – Netduke Feb 24 at 14:32
    
@draksia: It looks like most German manufacturers are moving towards liner-less engines using Alusil. Not sure why more manufacturers aren't joining that party, it seems it would be a lot simpler than pressing in liners. – TMN Feb 24 at 14:33
    
Historically there have been a lot of issues with nikasil or alusil coatings. They are not generally replaceable either. – draksia Feb 24 at 14:43

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