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.
front rear-drive axle transmission --------- 3 |--I--| 1 | I | 4 | -I- | 2 ---------
Volkswagen firing order: 1-4-3-2
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.
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".
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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