Integrated Exhaust Manifold
Integrated Exhaust Manifold
On many modern engines, the exhaust manifold is not a separate bolt-on part. It is cast directly into the cylinder head as part of the head casting. The exhaust ports route through passages inside the head before exiting to the turbocharger or catalytic converter. This is called an integrated exhaust manifold or IEM.
Why manufacturers do this
Three big reasons. First, it moves the catalytic converter closer to the combustion chambers, which means the catalyst reaches operating temperature faster after a cold start. Cold-start emissions account for 70 to 80 percent of a vehicle's total tailpipe pollution, so faster catalyst light-off is a huge win for emissions compliance. Second, the coolant passages in the cylinder head cool the exhaust gas before it reaches the turbocharger. Cooler exhaust gas means less thermal stress on the turbo, which improves durability and allows the engine to run at stoichiometric air-fuel ratio under more conditions instead of dumping extra fuel for cooling. Third, it reduces weight and the number of gaskets that can leak.
Who uses it
BMW uses integrated exhaust manifolds on their B48 four-cylinder and B58 six-cylinder engines — which cover most of the BMW and Toyota Supra lineup. Ford uses it on the 2.0 EcoBoost and 2.7 EcoBoost. GM uses it on the 2.0-liter turbo (LTG and LSY). Hyundai and Kia use it on their Smartstream turbo engines. It has become the standard approach for modern turbocharged engines.
What techs need to know
The biggest service impact is cost. If an exhaust manifold cracks on a conventional engine, you replace the manifold — a relatively affordable part. If the integrated exhaust manifold cracks on an IEM engine, you may need to replace the entire cylinder head. That turns a $500 repair into a $3,000 or more repair. Inspect carefully for exhaust leaks at the head. Also, because the exhaust runs through the head, the cooling system is doing more work. Any cooling system issue — low coolant, weak water pump, thermostat stuck closed — puts additional thermal stress on the head and the integrated exhaust passages.