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In four cylinder engines, why are some cars designed with the exhaust manifold towards the front of the car while other cars have it towards the back? Is there any advantage in one design over the other?

For example, the Toyota Corolla has the exhaust manifold toward the back while the Ford Focus has it in the front.

Toyota Corolla Engine Toyota Corolla Engine

Ford Focus Engine Ford Focus Engine

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up vote 6 down vote accepted

It all comes down to how the engineer designs it. The manufacturer sets how it's going to be put together. There's no real inherent design differences +/- ... It could come down to whether they want a turbocharger in front of the engine where it's easier to get to and where there might be more space. It could be that the design of a transmission limits space to one side or the other. There are little trade offs to doing it on one side or the other, but nothing which is earth shattering or where it really needs to be done that way.

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I think it's just a convention thing. Toyota's engineers decided to orient the head one way, Ford the other. – Zaid Jan 22 at 18:20
    
@Zaid - Exactly. Toyota could get new engineers in who think it should be oriented the other way. There's no real reason to it. The VW Jetta 1.8T was towards the rear (IIRC). There isn't a true way it should go ... but it has to go somewhere. – Pᴀᴜʟsᴛᴇʀ2 Jan 22 at 18:23

Exhaust at the front is the traditional arrangement, it allows the exhaust to be cooled by the air that's come through the radiator (ok, not cold air but still cooler than the exhaust) and allows an exhaust of a given length to be straighter when running beneath the car. Historically (like, in the 90s) intake manifolds have tended to be broader (in terms of the distance between the cylinder head and the opposite side of the manifold) than the exhaust which allows the whole drivetrain and front wheels to be placed closer to the nose of the car. The downside is that the engine has to be placed higher up to allow the exhaust to run beneath it which raises the car's centre of gravity.

Exhaust at the back allows a lower placing of the engine which lowers the car's centre of gravity and current intake manifold designs are (from what I've seen) less bulky than their predecessors so the drivetrain can still be kept close to the nose of the car. In designs that utilise a front subframe, the whole subframe can be placed lower as there's no need to leave space for the exhaust to run beneath it. The disadvantages are that there's a lot less of a breeze to cool the exhaust and it's much harder to get at when it needs replacing.

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Even though an automotive manufacturer may have dozens of models of vehicles, many of them share the main engine design. In particularly compact and low cost vehicles the engine is often chosen because they already make so many of them that it's the cheapest engine that meets the requirements.

This is particularly true when a new model is being introduced.

Without looking further into the specific details of these vehicles, it is quite possible that the engine chosen for these models as well as the engine configuration in conjunction with the transmission and drive components, necessitated a particular exhaust manifold design. Otherwise they would have had to redesign significant parts of the engine. This would raise the cost of the vehicle, reduce its reliability, and increase the service cost due to additional new parts that service places would have to store, order, manage, and install.

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