The Four-Cylinder Boxer Engine's Distinct Sound

or "Why does my new Subaru sound like an old Volkswagen?"

After purchasing a new 2.5L 4-cylinder Subaru, I was curious why the sound reminded me of old rear-engined Volkswagens, and seemed so distinct from inline 4-cylinder engines. Other than the flat-four cylinder configuration, my new Subaru's water-cooled OHC engine had far more in common with any typical inline-four than with the air-cooled push-rod engine in a Volkswagen Squareback.

Yet that's what its sound reminds me of. Most people don't even notice this distinction, and those who do, didn't know what caused it.

Here's one reference to this distinct sound I found in Wikipedia's description of the Volkswagen Vanagon:

This [Wasserboxer] engine is representative of the fact that boxer 4 cylinders produce a low pitch rumble, rather than a high pitch buzz/whine, when running. Some find this aspect of the engine to be pleasing, owing to the dislike of the "sewing machine" sound of I4 engines.

But why do they sound different? Both an inline-4 and boxer-4 have four even-firing cylinders, so why should they sound so distinct when running at the same RPM? This mystery bugged me for years.

The answer finally came to me while sitting at an outdoor cafe and hearing the cars pass by. I believe the distinct sound is due to the crankshaft and exhaust manifold designs required by the flat-four's cylinder orientation.

The Engines

HowStuffWorks® graphically illustrates the piston strokes of an inline 4 and a flat 4 boxer.

rear-mounted VW engine (top view) 

     front

 rear-drive axle
  transmission
   ---------
3 |[]--I--[]| 1
  |    I    |
4 | []-I-[] | 2
   ---------

Volkswagen firing order: 1-4-3-2

front-mounted Subaru engine (top view) 

     front
   ---------
2 |[]--I--[]| 1
  |    I    |
4 | []-I-[] | 3
   ---------
  transmission
front-drive axle

Subaru firing order: 1-3-2-4

While VW and Subaru have different nominal firing orders, they also number their cylinder locations differently. So in fact they are exact mirror images. Their respective crankshafts run in opposite directions likely due to their transmissions mounted fore versus aft.

So here's why they sound distinct...

The key thing to note is that while a boxer-4 does alternate firing fore and aft cylinders, it does not evenly alternate firing between its left and right cylinder banks. It cannot due to the 180 degree orientation of crankshaft pins selected for balance. So instead it must fire twice on one side and then twice on the other. And unlike an inline-4, a boxer-4 must have two separate exhaust manifolds (or expensive bulky header pipes).

One manifold exhausts [fire, fire, wait, wait] while the other side exhausts [wait, wait, fire, fire].

So in addition to the evenly spaced firing of each cylinder (just as from an inline-4) the boxer-4 has exhaust pulses exiting the left and right manifolds at half that frequency. This cadence is perceived as a half-pitch "rumble".

And here's how to make them sound like an ordinary inline-4...

Instead of collecting the left bank's exhaust pulses into one manifold and the right bank's into a separate manifold, connect each cylinder's exhaust port to an independent header pipe (of four equal lengths if you want a perfectly even cadence) before tieing them all togther to the tail pipe.

Thanks

to Noémi for making possible the vacation which allowed me the leisure to ponder and document trivia like this!

Controversy and Debate!

In an on-line forum, one individual characterized my conjecture here (I don't think he was referring to ignition timing ;-) as "retarded". In short, he maintains that the distinct boxer-4 sound is due entirely to the "uneven length runners" of the exhaust manifolds. You may read the entire "retarded" quote as well as reponses (none mine) to that post here.

What I believe the poster failed to take into account is that exhaust pressure pulses are not instantaneous spikes, but rather have significant duration. Each pressure pulse must be at least as long as its respective exhaust valve is open, with the restriction of the muffler and tail pipe stretching out its decay. Within any exhaust pipe or manifold, each exhaust pulse begins, builds, peaks, and decays. Considering one manifold's [fire,fire,wait,wait]: the tail of the first "fire" pulse has much more opportunity to overlap with the beginning of the second "fire" pulse than the tail of the second does with the beginning of the first (due to the intervening wait,wait). To the degree the two fire pulses tend to overlap somewhat, they form a [ FIRE , WAIT ] pressure pattern in each manifold which is half the frequency... sometimes referred to as a "rumble" sound which is superimposed upon the normal fire,fire,fire,fire sound in common with an ordinary inline-4 engine.

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