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?
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:
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.
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.
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.
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.)
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.
As already mentioned before: Aluminium is lighter (sg 2.5 instead of 7.8 kg/l) has better thermal conductivity, but is more expensive to produce, but less expensive to recycle. Cast aluminium can be repaired by welding, even examples of cast and welded engines exits. (Rover Metro amputated V8 to V6 engines for rallies in the eighties) Normal cast iron (lamellar grey cast iron) is cheaper, easier to cast, has lower mechanical properties and better vibration damping capacities. Cast iron is hard to repair by welding. Nodular cast iron is much stronger but has much lower mechanical damping capacity for vibrations. (butt still over cast aluminium). Over the years the move has been made from all gray cast iron engines (with even cast iron pistons) (40HP / liter total cylinder capacity) to cast iron cylinder blocks with aluminum heads (50-60 HP/L, to all aluminum engines (cylinder block plus head) up to 80 HP/L (atmospherical engines) Nowadays there is an tendency to switch to nodular cast iron due to costs and comfort) In high performance cars, aluminium engines (up to 140 HP/l atmospheric) are still favorite, due to the weight savings and cooling capacity, making the costs less relevant. Kind regards, Jurgen Prinsen, Metalurgical and welding engineer, Petrolhead and owner of several Italian cars (Alfa Romeo (boxers, straight 4 Nord-engines and V6 Busso engines) and Ferrari 456 (5.5L V12 116B engine)
From the heat capacity point of view, cast iron engines will take more time to heat, but also more time to cool down. Let's say you stop the engine after the drive, and start it again in a couple of hours, the cast iron block will be much warmer than alloy one, and if you do this multiple times, you will have much less warm up/cool down cycles, which is better for engine lifespan. This is one of the contributors of cast iron engine longevity compared to similar aluminum engines.
