The applied physics of boxer engines
Boxer engines: The darling crankcase configuration of Porsche and Subaru - among others. What makes them tick, and what makes them different from inline and vee-style engines?
Inline fours don’t rock very much, but they have poor secondary balance. (Which is why the high-revving, big displacement ones need balance shafts.) They like idling smoothly, but don’t like a rev, inherently.
Pretty much the reverse situation with boxer fours. They rock like a bastard in the Z-axis (that’s a ‘zee-axis’ in Trumpistan) because the pistons are offset. But on a more positive note the boxer configuration gives them excellent secondary balance characteristics - so they really do like a rev.
When I say they rock, imagine it like this. You’re standing on the bonnet of a Subaru Impreza. (That’s the hood, in Retardistan.) You’re holding a dirty big chrome-vanadium crowbar, and you drive it with Iron Man strength, vertically down, through the mass centroid of the engine.
Because this is a parallel universe where that doesn’t catastrophically destroy the engine (because: magic). What you would feel is the crowbar rotating slightly backwards and forwards, like it’s a spindle, and the engine is a top with Tourette’s syndrome.
This rotational rocking is because the pairs of cylinders are offset. There’s no getting around it.
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There’s another problem (challenge, feature, whatever…) The firing pulses are right-right, left-left (or vice versa). The point being that both cylinders on one bank fire, then the other bank fires, repeat.
And that means, if you want to have even scavenging of the cylinders, which is kind of essential to evenly refilling them with the next charge of air, you need long header pipes to merge the discharge from the ports from bank to bank.
You need to pair every exhaust pulse with another pulse 360 degrees away, or you don’t get uniform combustion in every cylinder. That means you need to join exhaust ports across across the width of the banks. OK? It’s a real estate challenge - because space in an engine bay is extremely limited.
So why don’t boxers and inline fours sound the same, given there’s the same number of combustion events per rev? Riddle me that.
It’s because in older designs they just merged the left bank exhaust into one manifold and the right bank into another, joined them up into a single tailpipe, and they lived with the uneven pulses, the uneven filling of the cylinders and the uneven combustion.
And that’s where the typical dak-dak burble/noise comes from.
Our friends at Subaru - which incidentally managed to teleport its somewhat niche business into the monolith it is today by building it on just two pillars: boxer engine and symmetrical all-wheel drive, put a magic ‘equal length, even pulsing’ exhaust system into competition in the WRC in the 1990s.
That system went into the Liberty (that’s a Restardistani Legacy) in 2003, then Forester in 2005, Impreza in 2007 - and the dak-dak burble receded from memory. WRXs lost it in 2015 - because the exhaust feeds a centrally mounted turbo.
But the STI - with its somewhat antique engine - retains the uneven length headers. But it’s probably next for the chop.
Of course one of the reasons WRXs that get tweaked heavily in the aftermarket game sound so distinctive is, obviously, they put the burble back with a suitably uneven exhaust. Like, it’s still there, waiting, to burst forth from the closet of conformity, in every factory flat-four Subaru.
Moving to flat sixes - they don’t have this latent dak-dak ability. Essentially a boxer six comprises two inline three-cylinder engines facing away from each other in bed together, lubed and hot, and yet still managing to engage in a form of perverted copulation by virtue of sharing the same crank.
The firing pulses are even per bank, so standard exhaust manifolds - one for each bank - are all you need for efficient scavenging. They rev like a bastard, too, because they have rock-bottom secondary imbalances, intrinsically, and they’re on a par with (or slightly better than) inline sixes on most other balance-type criteria.
This explains why Porsche is so historically fixated on them. If you want to build an engine gagging to rev its tits off, that’s also wide and low and doesn’t therefore mind riding right out the back, behind the rear axle, I think we’ve found a winner.
The big advantage of the boxer is its low height, reducing the centre of mass, and that reduces the roll effects when you’re cornering - without requiring you to slam the car onto the deck. (But you can of course do both if you’re building a race car, so that’s kinda nice.) And the main disadvantages are the cost and complexity of manufacturing - and the inherent width of the engine, from a packaging perspective.
Good luck changing the spark plugs - although you could say that about virtually every modern engine. They design them to be assembled, not worked on, that’s for sure.
