Actually, all other things being equal (materials, manufacturing tolerances, etc.), a boxer engine is, by design, more problematic than an in-line 4. Here are the main two reasons why.
1) A boxer has larger surface area-to-volume ratio. Unlike an in-line four, where the "hot" area (upper cylinder & head) is compact and nicely grouped, in a boxer you have two "hot" extremes separated by a "cool" block. This makes the design much more complicated in terms of keeping tight tolerances in light of differential thermal expansion. In a boxer it is nearly impossible to keep uniform temperatures across the cylinder / head area. Because the cylinder block is more compact in an in-line 4, it's easier to design a cooling system them will keep the entire block's temperature more uniform. The result is that in-line engines can operate within tighter tolerances.
2) An in-line block is much more rigid (for same material usage / weight). A boxer configuration make is inherently more "flexible", which in is bad to keep tight tolerances. Under high torque, the block / cylinders deform more than in an in-line 4 of equivalent displacement / performance. Engineers can address this by either "beefing up" the engine making it heavier (and more expensive), or have to relax tolerances to prevent seizing under high stress.
This issue with a boxer engine can be addressed to certain extent with careful engineering, but things like exterior temperature variations make it nearly impossible to match the level of thermal expansion control that can be achieved with a more compact, geometrically simple block of 4 in-line cylinders. The issues that have traditionally plagued Subaru boxer engines (head gasket failures, oil consumption) are direct consequence of the boxer configuration. They have been corrected to certain degree, but there is only as much as engineers can do without making the engine prohibitively heavy and expensive.
Additionally, a boxer engine of equivalent displacement / performance is inherently heavier and more expensive than an in-line four. This is because you have two heads and valve trains, so you need twice as many camshafts and VVT controllers, and in general, you have more exterior area to deal with in relation to engine displacement. So designers have to chose between making a good but expensive engine (like Porsche), and price themselves out of competition, or keep the engine cost similar to an in-line 4 and cut corners somewhere else (use cheaper materials or relax tolerances). You simply can not make a boxer engine "as good as" an in-line 4 with same materials and level of manufacturing precision for the same price - it either has to be more expensive, or you have to compromise on quality.
Boxer configuration was quite popular in the 1960's (just as rear mounted engine), but eventually engineers realized that it is a dead-end development unless you have a very specific reason to keep it. For example, boxer configuration is used in small aviation engines because it allows effective air cooling (large surface-to-displacement ratio). Porsche sticks to boxers partly because of tradition, but also because it allows them to maintain extremely low center of gravity. In many ways the boxer is like Mazda's rotary (Wankel) engine - it has some advantages, but the negatives outweigh the positives by a large margin.
The main reasons why Subaru uses a boxer are:
- It fits nicely with their hallmark "symmetric AWD" configuration. Departing from the symmetrical AWD configuration would leave them without one of their strongest marketing claims (although there are no good engineering reasons why a different configuration wouldn't be as good or better).
- Subaru is a relatively small company that has invested years of engineering effort into perfecting their family of boxer engines. They simply don't have the resources to invest in development of a newer alternative. Engine development is very expensive and even much larger companies frequently join forces to spread development costs (for example GEM alliance between Mitsubishi, Hyundai and Chrysler, join engine development between Nissan and Renault, cooperation between Toyota and BMW, etc.). Subaru couldn't afford to develop an entirely new engine family on their own - they would have to partner with somebody else, and this would most likely mean that they would have to abandon the "symmetric AWD" mantra - and this would mean end to Subaru's unique personality.
Last edited by katekebo; 05-23-2014 at 06:47 AM.
