Aluminised steel is mild steel with a thin aluminium-silicon coating applied at the mill. The coating resists oxidation, so the tube survives far better than bare mild steel in the heat and moisture of an exhaust. It bends and welds much like ordinary steel, which is why it remains common on volume production systems.
It is not stainless and it is not titanium. The base metal still corrodes wherever the coating is breached, and the coating itself becomes more fragile after repeated heating and cooling, which is exactly what an exhaust does to it.
The gap comes from the material itself, from the processing and from the labour. Titanium costs more per unit of raw stock, needs dedicated tooling and consumables, and takes longer to form and weld because the atmosphere around every joint has to be controlled. Aluminised steel needs none of that.
So a titanium part is not expensive only because of a markup on the metal. A meaningful share of the cost is the extra time and shielding the material demands, and that is true whether the part is a simple straight section or a complex assembly with several branches.
For a basic replacement section on a car used on the road, both will carry gas and both can be made to fit. In the middle of a straight run, at modest temperatures and with good support, the difference in behaviour is smaller than the difference in cost.
The gap opens up at the extremes, which means the manifold, the collector, the first length behind the engine, and anywhere the part is worked hard and repeatedly. That is where the limits of the cheaper material are reached first.
The real comparison is not the price on the invoice but the cost of owning the part over several years. A coated steel section that fails at a joint means another part, more labour and another alignment job. A titanium section that lasts the life of the car can be cheaper in total even though it costs far more to buy.
That calculation changes completely for a car that will be sold within a year or two, or for a project being built to a tight budget and likely to be revised again before it is finished.
Weight and heat behaviour are rarely priced in, but they should be. A titanium system changes how the car behaves and how much protection nearby components need, and those consequences carry their own cost. Sound is a factor for the same reason, since getting a tone you like from a steel system may mean extra mufflers and extra fabrication work.
Where a build is aiming for a specific character, the cost of iterating on a cheaper material can quietly exceed the cost of choosing the right one the first time.
Decide on how long the car will keep the part, how hot that section runs, and how much the owner values mass and sound. For a daily driver with a mild engine, a good aluminised or stainless section is a sensible answer, and the money is better spent elsewhere.
For a built car running hard, or one being kept long term, a titanium exhaust system is usually the calculation that wins. Either way, compare it honestly against a stainless steel exhaust system, which sits between the two in both cost and capability and is the right answer for a great many builds.
