Why aren't inline engines mounted flat or at an angle?

I’m thinking of the Toyota Previa of course, which had an inline-4 mounted at a 75-degree angle underneath the seats. Why isn’t this more common, especially for more conventional front-engine layouts? The lower center of gravity should be a big bonus. Clearly maintenance would be more difficult but that’s the same with any boxer engine. And mounting at 45 degrees or something similar shouldn’t make it any more difficult.

Edit: another potential advantage could be better safety. If the seats are already mounted high like in a crossover, then the engine would pushed underneath them in a crash.


I suppose the main reason why they might’ve done it on a car like that is to use as much length as possible on interior space

Mostly because if a low center of gravity is your top priority, a Flat Engine is superior. Slant engines do have a purpose, however. Inline engines, particular inline 6 cyllinder engines are particularly hard to fit vertically, as they create a tall hood. Now, Slant Engines do have a use, but it is not to create a low center of gravity… It is simply to lower the hood line, because as inline engines get bigger, if they are mounted vertically, they cannot fit within short engine bays. The solution is to slant them, so they fit.

But boxer engines have a lot of disadvantages, especially their mechanical complexity and excessive width, which is why few automakers use them. A 90-degree, transversely mounted inline-4 could have a center of gravity just as low as a boxer while being much cheaper.

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I imagine complications with oil and coolant flow would arise. For starters you would need to design a completely different oil pan due to the crankcase now being 90 degrees lopsided, you would have parts of the crankshaft and pistons constantly in oil. Besides, car makers seem to be doing just fine with vertically placed inline 4s.

Yeah, I thought lubrication could be an issue, but again, it would be the same for boxer engines, right? Could be better since you can have the oil pan on one side of the crankshaft where the other pistons would be in a boxer.

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off topic, so like porsche for example uses a boxer for racing, would’nt hard cornering or any cornering flood the underside of the pistons on that bank of the engine and turn it into a bubbly froth?

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Somewhat related does anyone know why the opposed-piston engine isn’t used in cars since it also offers the advantages of a flat low centre of gravity design yet without the complexity of the boxer engine.

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I thought that if you angled the shaft like that you would introduce uneven rotational speeds, unless you evened it out by adding another join which cancelled the angle out?

Toyota did think of those issues, the sump is still at the bottom it’s just off to the side of a normal engine

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Emissions. There is a company that is trying to create a modern design, taking advantage of computer modelling to create an optimal piston shape, etc., to meet emissions and offer good fuel efficiency. https://en.wikipedia.org/wiki/Achates_Power


SAAB 99 and 900 had a I4 longitudinal mounted engine with a 45 angle.
Kind of half a V8.

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In fact it was derived from an engine which was literally half a V8 - Triumph slant-4 was designed from one bank of Stag V8 to simplify production. Early 99s even used Triumph engine :slight_smile:

That’s about the only way you can pull that off.

Race engines use windage trays to prevent that. Some people who are hardcore motorsport nerds even install them on autocross vehicles (particularly Dodge Neons back in the day, because despite being inline, they had that problem too)

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For the sake of completeness, there is another company also developing an opposed piston engine, but they also combine that with an opposed cylinder layout (‘OPOC’), to make a rather curious cylindrical engine with high specific output (power-to-weight is claimed at 1.8 W/g (1.1 hp/lb)). Compared to the Achates engine, power-to-weight and power-to-engine-volume is probably higher and reduced bearing loads from the symmetry mean reduced friction and so potential efficiency gains. Supposedly it has been backed by Bill Gates among others, and recently production was planned in China.