An integrated Exhaust Manifold, or “headifold,” is an exhaust manifold cast into an engine’s cylinder head and cooled by antifreeze. This type of design, where the exhaust manifold is no longer a separate part, is becoming a lot more common in the auto industry (see 2017 Honda Civic Type R). Here’s why.
Let Jason Fenske of Engineering Explained walk you through the benefits of a water cooled exhaust manifold:
One of Fenske’s main points is that coolant collecting heat from the exhaust gets the engine up to temperature faster, which yields faster cabin warmup times and better initial engine oil flow (and thus better fuel economy, emissions and engine longevity).
Another benefit, he says, occurs at high engine loads where exhaust gases are so high, they risk compromising the life of the catalytic converter. Cooling these gases with coolant, Fenske mentions, prevents the engine control unit from having to dump more gas into the combustion chambers to lower exhaust temperatures. This means a staggering 20 percent better highway fuel economy on the 2017 VW Golf Alltrack he was driving. Not to mention, the lower CO2 emissions that go along with that.
Three benefits he doesn’t mention are weight, packaging space, and catalytic converter warmup. Weight is a big one, as typically, cars use a separate cast iron exhaust manifold bolted to a head with a gasket in between. This integrated setup gets rid of that manifold, all the bolts, and the gasket, the latter of which can also act as a potential failure mode.
Related to this weight savings is packaging space, as these tightly-integrated manifolds are typically smaller than a separate manifold bolted to a cylinder head. This benefit in packaging space can, according to a Ford patent on the technology, yield faster catalytic converter warmup times (and thus lower emissions after startup) and better turbo response, due to the shorter exhaust paths to the catalyst and turbocharger turbine, respectively.
Of course, as Fenske mentions, this integrated exhaust manifold adds more heat load to the cooling system, and makes tuning more difficult. But despite these drawbacks, the benefits in emissions, fuel economy, cabin warmup, weight, complexity and turbo response are hard to ignore, which is why “headifolds” have become so prevalent over the past decade or so.