Author Topic: The Four-Cylinder Boxer Engine's Distinct Sound  (Read 4033 times)

May 23, 2012, 08:10:51 AM
Read 4033 times

Manfred

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Taken from http://www.doudna.com/Subaru/

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.

Future work:

To verify my conjecture, record the exhaust notes of an inline-4 and boxer-4 at the same RPM, and compare their respective frequency spectrums.
Learn how to edit Wikipedia and add this information to Flat Engine or Flat-4.
Links to more Subaru stuff:

Joe Spitz's cars101 Subaru information research site
A ridiculous and stupid comparison of the 2.5 RS to the AMC AMX from self-proclaimed authorities of the unconscionable, unethical and immoral
Pretoria

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May 23, 2012, 08:35:23 AM
Reply #1

Stev0

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ex Prodrive Xt, X, XT, WRX STI, Outback 3.0R

May 23, 2012, 08:38:33 AM
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Manfred

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Schweet.
Do I get free Club brownie points..?
Pretoria

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May 23, 2012, 08:55:37 AM
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JPC

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Quote from: Manfred;4728
Schweet.
Do I get free Club brownie points..?

Well of course you do Manfred - we'll let you hang around another couple of days!!:p:D
Mine:
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Hers:
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May 23, 2012, 09:32:52 AM
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Manfred

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Excellent.
A couple more stickies and I'm sorted for the year LOL
Pretoria

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May 23, 2012, 10:01:09 AM
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JPC

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Quote from: Manfred;4744
Excellent.
A couple more stickies and I'm sorted for the year LOL

 
we will even give you a status as "Sticky Member 2012":D
Mine:
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May 23, 2012, 10:28:21 AM
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Manfred

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Pretoria

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June 20, 2012, 09:48:35 AM
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kevindbn

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Quote from: Steve;4725
Great post - Stickied :)
 
U cant beat that scooby sound.....;)

January 20, 2013, 04:53:04 PM
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Wikus

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The idling is also distinctly smooth and quiet.  Is it just me or have others tried to start an idling engine?
2010 White X Manual

January 20, 2013, 09:03:22 PM
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mauritsvw

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Quote from: Wikus;9039
The idling is also distinctly smooth and quiet. Is it just me or have others tried to start an idling engine?
Not yet with my Forester, but a few times with my Impreza!
2012 Forester XS MT - Steel Silver (Vossie 1)
2006 Forester XT AT - Platinum Silver (Vossie 2)
Ex 2005 Impreza 2.5 RS Sportwagon - Aqua Blue (Scoob y)

January 25, 2013, 09:38:46 AM
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Paul_photo

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I had engine troubles early last year and upon opening her up discovered an unknown projectile had been bouncing around her innards and mysteriously disappeared without trace after doing damage to one of the pistons. Lesley and the guys here at HRD in Trichardt also therefore opened the turbo to check if the blades had been damaged by the exiting debris but thankfully not.
When they gave her back to me I found (after the 2,000 km gentle running in period) that she had huge difficulty revving past 3500 RPM, with much tooth-loosening shuddering, shaking and bouncing about and general all-round painful unpleasantness.
So, of course I took her back and there was much fussing about timing belts and consulting of computers and, and, and, but still no high revving.
It was a desperate situation.
Lokasie skorro-skorros were running away from me and you can imagine how I cringed in embarrassment!
Eventually a 3rd party in PTA located a single cat converter mounted right before the turbo intake that had become severely clogged and was choking the turbo, hence her gasping for breath beyond 3000 RPM.
So, I asked them to take out all the cats.
Problem solved.

Now I know there are people who say removing them has no effect on performance and to them I say: "bollocks". She now pulls like an unladen steam-train that got poked with a red-hot pointy stick and is noticeably more exciting off the line than she was 115,000 km prior when I first bought her.

But before the admins take me to task for being off topic, my point is this:
Whereas before, the sound being emitted from the engine compartment while the world whooshed by in a blur was all electric belts and chains and no real drama or thunder, now the old girl has developed more of a voice. And it is mean.
And standing behind her while she idles you'd almost be mistaken for thinking she's related to a John Deere tractor, with a very distinct burble and cough.

Personally, besides adding a slight bit more of a warp factor to pull away, taking out the cats has added some personality and I love the voice she's developed now that her tonsils were removed!
« Last Edit: January 25, 2013, 09:40:47 AM by Paul_photo »

January 25, 2013, 09:56:22 AM
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Manfred

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Thanks Paul_photo.
I've been reading on subaruforester.org about the cat in the up pipe giving scheit.
First one I've heard of in South Africa.

Problem here is that it costs a small fortune to replace those pipes.
Pretoria

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2013 Ford Fiesta Ecoboost (Turbo Triple)

January 25, 2013, 10:10:51 AM
Reply #12

Paul_photo

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Quote from: Manfred;9243
Thanks Paul_photo.
I've been reading on subaruforester.org about the cat in the up pipe giving scheit.
First one I've heard of in South Africa.

Problem here is that it costs a small fortune to replace those pipes.


The repairs last year did cost me an arm and a leg, that's for sure...:eek:

January 25, 2013, 12:47:52 PM
Reply #13

Jacques

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Am I correct in saying that the SG9's came out without the cat in the uppipe?
2006 Forester XT MT - 20G'd

January 25, 2013, 12:56:33 PM
Reply #14

Paul_photo

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Unfortunately I don't personally know.
The guys at HDR were however quite adamant in saying this was the only model within the range of any year of Fozzy in which that particular Cat was installed, and that's why it took them so long to isolate that as the culprit. Therefore, based on what they told me I would assume the SG9's then didn't come out with that cat in the uppipe. Hope that helps...