In short, the highest temperatures that an exhaust manifold or exhaust pipe should ever reach would be approximately 850°C (1,600°F). As a guide, metals will start to turn red at 500°C and be a dark cherry red at around 635°C (1,175°F).
The hottest parts of your exhaust system will be either a bend in an exhaust pipe right next to the cylinder hard or around the catalytic converter.
Temperatures naturally increase as RPM or the engine work load increases. This is when the engine is consuming the maximum amount of fuel and producing the most amount of torque or horsepower.
Tests on the exhaust system temperatures of typical road cars ranged from 120°C (250°F) at 50kph (30mph) up to 550°C (1,020°F) at 112kph (70mph).
There are 3 ways to prevent damage to nearby components under the above conditions:
- Insulate the exhaust pipe to keep the heat inside the pipe
- Place a reflecting barrier with an air gap between the exhaust pipe and the rest of the engine bay and its components
- Add reflective and insulating materials to objects that could be damaged by the radiant heat coming from the exhaust pipes or manifold.
We typically use exhaust wraps or tapes and wrap these in spiral pattern around the exhaust pipe or manifold to keep the heat inside the pipes. Tests have shown that exhaust wrap can reduce engine bay temperatures by as much as 50%. These exhaust wraps can be made from fibreglass (starts to melt at 815°C), silica, basalt and ceramic wraps.
Car manufacturers have also caught on to the importance of heat control and most late model cars have embossed aluminium or steel heat shields. These are usually mounted in fresh air approximately 1-2 cm away from the exhaust or exhaust manifold. This air gap helps to carry away excess heat.
Heat sleeves are also now used in many new car engine bays to reduce the chances of heat damage to cables, wiring, hoses and hard lines. These sleeves are usually a lamination of aluminium foil and an insulating fibreglass backing. These sleeves use the reflective ability of reflective aluminium foil to repel radiant heat. The fibreglass backing gives the sleeve strength but also acts as an insulator.
Some heat sleeves use a mylar foil outer layer. Mylar is made from a microscopic layer of foil laminated to an outer layer of polyester resin. This is usually backed with an insulating layer of fibreglass. The polyester outer layer makes Mylar really tough, but it will burn off at around 200°C (400°F). Kool Wrap uses a thicker outer aluminium foil backed by insulating fibreglass. This material is available as a sleeve or in sheet form so that it can be used to insulate car components such as a starter motors or a firewall. The Kool Wrap foil and fibreglass can withstand temperatures approaching 660°C (1,220°F).
Remember that air is actually a great insulator when trapped in small pockets. Air is excellent at convection (electric fan heater) but is a poor conductor of heat due to its low mass. You can see evidence of this in styrene foam or ceiling insulating batts. These two products are designed to trap air pockets and reduce heat conduction. The material acts as a heat block. The heat cannot be transferred through the material. The same applies to exhaust wraps and fibreglass or silica blankets or wraps. The air trapped between the fibres reduces heat conduction.
A good example of trapped air acting as a heat barrier is double glazed windows.
University of Washington: Underhood Surface Temperature Tests: Summary of Published Results