Are boxer engines overrated? – Pros and cons of boxer engines

Arif · Sep 24, 2020 10:00 AM

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Porsche owners and Subaru owners are two very different types of people. The former is stereotyped with having a mid-life crisis and the latter is stereotyped with endless vaping. They live in two different worlds. The only thing they have in common is the “Flat” or “Boxer” engines that their cars have.

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Boxer engines are just engines with horizontally opposing pistons*. Besides Porsche and Subaru, there have been several other cars that utilise the “boxer” configuration. The Citroen 2CV, Toyota S800, and the Chevrolet Corvair are some cars to name.

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Alfa Romeo also utilised the boxer engine in a few of its models like the Alfasud, Sprint, Arna, 145, & 146.

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Alfa Romeo Arna. Flat engine in a FWD car. Photo

So, the boxer engine is not special to Porsche and Subaru. In fact, the invention of the “Flat” or “Boxer” engine is credited to Karl Benz and his engineers in 1897. It was called the “Contra Engine” and only had two cylinders

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Benz Dos-à-Dos with contra engine (1899)

With only Porsche and Subaru still persistent with the boxer engine, it has become a niche engine for niche enthusiasts. If you’re wondering whether the boxer engine is good or bad, we have listed down its pros and cons for your own evaluation.

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Contra engine by Karl Benz

Pros - Low centre of gravity

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The boxer or “flat” engine is aptly named. The engine is flat and has all the weight at the bottom. 

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You can try out the Subaru Forester and the Toyota RAV4 to see the difference that a boxer engine makes. You could also try out the Subaru XV/Mazda CX-30.

They both have a similar suspension setup - Macpherson struts in front and double wishbones at the back. One thing that differentiates them is the engine configuration.

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There’s much less body roll in the Forester than the RAV4 thanks to the low centre of gravity that the boxer engine provides. Yes, the Subaru has symmetrical-all-wheel-drive, but body roll has more to do with the centre of gravity of the car.

Pros - Perfect Balance

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Boxer engines are perfectly balanced since both the primary forces and the secondary forces are balanced.

We'll just touch on this subject matter briefly...

A three-cylinder engine, by default, doesn’t have its primary forces balanced.

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Three cylinders, not four.

The pistons don’t cancel out each other’s weight. That’s why three-cylinder engines need further engineering (e.g. balancer shaft) to balance things out. Honda does some interesting work to balance three-cylinder engines. Naturally, in-line three-cylinders also don’t have the secondary forces balanced.

In-line 4-cylinder engines usually have the primary forces balanced.

How so?

Well, when two pistons are at the top (TDC), two pistons are at the bottom (BDC), naturally cancelling out each other’s weight.

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Two pistons up and two pistons down. Photo: autodesk

However, the secondary forces are not balanced. Even when two pistons are up (TDC) and two pistons are down (BDC), the upper half of the stroke is faster than the lower half of the stroke, causing imbalance of the secondary forces.

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Each cyllinder has another cylinder doing the exact action in the opposite direction, effectively cancelling each other out.

The winner here is the boxer engine with half of the pistons cancelling out the forces from the other half of the pistons. Two pistons are going left and two pistons are going right at the exact speed in the opposite direction.

Cons - Size

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Boxer engines are wider. Photo: Engineering Explained

Boxer engines are bigger than their inline counterparts. While space is saved up top, boxer engines are very wide. Have you ever wondered why Porsche never puts its flat engines in its front-engine cars? It's actually a bit challenging to fit boxer engines in the front. (Tell that to Alfa Romeo)

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Porsche 944. Photo

The Porsche 924, Porsche 944, and Porsche Panamera do not use flat engines.

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According to Tetsuya Tada, inline engines in the front are much easier to work on.

Since the boxer engine is wide, fitting the engine and allowing space for steering rack and everything else in the engine bay can be a challenge. This is even acknowledged by Tetsuya Tada, chief engineer of the Toyota 86 and the Toyota Supra.

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Eh?

Cons – Difficulty of Maintenance

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Porsche engines usually need to be lowered down for easier maintenance work. Photo: rennlist.com

A flat-four engine needs more parts than the standard in-line 4 engine. A flat-four needs double the engine heads, double the camshafts, double the head gaskets, and double the timing belts of an inline-4 engine.

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Compared to a V-engine, a boxer engine doesn’t add much more parts. The maintenance however, is a tiny bit more difficult (for a boxer engine) since there’s not much space to access the engine head. Replacement of the head gasket for a boxer engine usually requires the engine to be removed from the car.

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At least the oil filter is easy to access. Photo: Renegade motorsports

Summary

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Boxer engines offer the benefit of a low centre of gravity and perfect balance. The disadvantages of the boxer engine include the engine size and the difficulty of maintenance.

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Other manufacturers have meddled with the boxer engine, but Subaru and Porsche are the only manufacturers that still persist with this engine configuration until today.

 

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Arif

Writer

Previously an engineer in an automotive manufacturing company and a highway concessionaire. A part-time research student on biofuels and diesel engines. Obsessed with vehicle electrification and the future of transportation.

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