The reason behind Subaru’s signature BOXER RUMBLE!

Of course you’ve heard the one-of-a kind sound of Subaru, when cruising down the street. And you, fellow gearhead, know pretty well what that rumble signifies: a 2.5-liter "boxer" flat-4 engine.  The sound of a Subaru with an aftermarket exhaust has been likened to two Harley V-twins racing each other.  All Subaru cars that we love the marvelous  Subaru Impreza 22b , the 2016 Subaru Sti even the upcoming Subaru wagon have that sweet note !

The sound comes from the cylinder timing, and the unequal length exhaust manifolds. There's only 3 manufacturers that I can think of that use a boxer or flat engine. Porsche uses boxer 6 cylinder engines, and the original VW beetles used boxer engines.

So in case you’re wondering why this engine makes such a distinctive sound we’re giving you a chance to hear what our friend at Engineering Explained has to say regarding that.

First, the Subaru Boxer has a horizontally opposed flat design, but while that still matters, it is the design of the headers that makes all the difference. The 2.5-liter Subaru engine employs unequal length headers whose sound you’re actually hearing. Let us elaborate: the header piping from the 1st and 3rd cylinder has different length than the one that comes out from cylinder two and four. That's the basic design of the Subaru Impreza 2.5 i engine.

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".

The exhaust gas that’s coming from the engine then moves different lengths, and respectively the sound that’s produced is unevenly distributed in the exhaust and when exiting the car. Now you know why you get to hear Subaru’s unique ramble rather than a flat tone. If you are to get the new WRX you won’t be getting that rumble the latest 2.0-liter engine comes with equal length headers.

How to increase the "rumble" ? 

- The "rumble" is not due to the boxer engine design - it's due to the unequal length headers Subaru uses as we learned from above. This also decreases power - so more rumble = less power. You can make it louder - just lose the muffler. A friend runs a straight pipe on his lifted Subaru Forester wagon - stock exhaust. It's not that loud..... but he doesn't drive it every day either.

To find out which style of head you should go for click play on the video below.