Subaru Boxer Rumble – Exhaust Explained


[Engine start] [Unique rumble, better described at end of video] Hello everyone, and welcome. In this video, we’re gonna be talking about the Boxer rumble and how you get that iconic sound. And it really all boils down to the exhaust headers and how those work with the Boxer engine with its firing order. So first we’re gonna be checking out unequal length headers and then looking at equal length headers. And so here we have a Boxer engine that’s a four cylinder Lays flat, so the pistons fire like this. And we have cylinders 1, 2, 3, 4, and our firing order is 1, 3, 2, and then 4. And so what I’ve drawn here in red are the exhaust pulses. So basically, after your power stroke, once that exhaust valve opens up, you’re going to have a pulse of exhaust coming out a high-pressure front of that exhaust which I’ve drawn here basically as pulses, traveling through the exhaust. So you’ve got those, they’re going to collect into a single pipe, and then travel towards the rear of the vehicle. So with the firing order as shown, you’re going to have pulse one, then three, like this and then two and then four, like that. And so, as these meet up into the single pipe there’s going to be an uneven spacing depending on the RPM… you know, how much you’re flooring it, things like that, so there’s going to be an uneven spacing of these because these pulses have to travel a longer distance before getting to the pipe towards the exit, verses these pipes right here, as they travel. So you’re going to have an uneven spacing, and that uneven spacing is what gives it that rumble, because you’re hearing different exhaust pulses at different varying times, rather than just a single continuous stream of equally balanced-out exhaust pulses. And so… the benefits of doing something like this is that, well, first of all, it’s lighter, because you don’t have to have as long of runners to make an equal length headers, so you can have a lighter, shorter runner distance for cylinders two and four, and that can save you some weight. You’re also going to have better energy recovery for cylinders two and four, because you’re not going to lose as much of the energy from cylinder two and four as heat, before it goes into the turbocharger to help spool it up. So, as that heat is lost, the pressure is lost, as so it has less power to spin the turbocharger. So with cylinders two and four, you’ll have more than with cylinders one and three. And, of course, the sound. The sound is phenomenal, as you heard at the beginning of the video, and I’ll play another clip at the end. So, with equal length headers, basically what happens is you… you design the headers so that all of the four pipes, are the exact same distance. So when the exhaust pulses travel the exact same distance before they meet up into a single pipe heading towards the rear. So all of these are the same length. So once you get to that single pipe to go towards the rear, you can see that they’re evenly spaced, and that’s going to give you a very even tone coming out of the exhaust. And that’s what you have with the new WRX, verses the new STI, which is still sticking with this method but I think eventually, you know, this will kind of die out as the benefits of an equal length header… first of all you have better exhaust scavenging, um… and so this gives you more efficiency and better power, because you have equal exhaust pressure. So you have a lack of pressure, you know, behind the exhaust pulse, and that helps pull out the next exhaust pulse, and so that gives you exhaust scavenging, more power. It’s more efficient. You also have an even load for the turbocharger, so you’re not having these pulses, with some of them here and some of them here… uneven loading going into the turbocharger… you know, that could cause a little bit of a delay in spooling it up, or uneven power delivery, depending on how it works. So, having this even exhaust stream for the turbocharger helps it spool up and stay at a constant rate and provide you with a constant level of boost, or increasing boost as you press your foot down. So thank you guys for watching and if you have any questions or comments, feel free to leave them below. [The “rumble” is best described as a stream of quick double-pulses with a deep, hollow sound] [engine revving up to 4,000 RPM]

100 thoughts on “Subaru Boxer Rumble – Exhaust Explained

  1. can't stand that sound. the engine sounds broken even when it's perfectly tuned… I guess to each their own…. to me nothing beats a BMW inline 6.

  2. So if I were to want to fix the torque dip / problem with the FRS, for example, is there a difference in UEL vs EL headers? Either one would change the sound as well, no?

  3. I wish Subaru would make Punched out versions of there engines in a (Painless kit Crate engine) to go in the Chevy K and KC rigs form 1988 -1999. That would be AweSOME!!!

  4. Hey, maybe you can make a video about unique sound of Harley VTwin engines and how they compare to other high volume VTwins? What do you think?

  5. That was a reasonable explanation, but the bloke is Such a Wanker, I'm surprised he didn't have some headers just sitting around to show us the difference.

  6. Do you consider necessary to install headers for better response when power is needed to pass other vehicles?

  7. I care for fuel efficiency, and I'm happy with the one on my 2015 WRX, but if the increase in consumption is minimal, I would pay a tuner to loose some horsepower and get the sound of the STi.

    For how many years was the Evo better than the STi, but in the end, not everybody cared for the lap times, and boxer rumble experience turned the balance in favor of the STiย with the people actually purchasing the cars.

    If it were an option that came with a button, I wonder how many would not use the ๐’Š๐’„๐’๐’๐’Š๐’„ ๐’ƒ๐’๐’™๐’†๐’“ ๐’†๐’™๐’‰๐’‚๐’–๐’”๐’• ๐’“๐’–๐’Ž๐’ƒ๐’๐’†.

  8. "Rumble"? You mean "putt-putt".

    But UL headers doesn't explain it: most I-4 engines have UL headers and don't "putt-putt".

  9. Does the same concept(s) hold true for my wife's Toyota GT 86 using a version of the boxer engine, less the turbo?

  10. Yes, you've got it right on the button. I'm glad to see that I'm not the only person who knows how this works! Good one, dude.

  11. Uhh, one question: I was always under the impression that in boxer engines the two opposing cylinders fire simultaneously. The opposing pistons rods do not sit on the same crank pin (thats what you usually find in a V shaped engine) but each sits on its own pin with 180deg offset. These two characteristics are what makes a boxer run smooth even thou the distribution of moment per revolution is less advantageous than in an inline engine with a 90 deg crank pin offset.
    Was I so gravely mistaken for the past 40 years of my life? (sorry for my bad english, can't pronounce "th", german engineer here ๐Ÿ˜‰

  12. I feel like your drawing would have been better with the exhaust direction depicted going up the board, otherwise the cylinders are mislabeled…

  13. You could saved yourself some time and just said this " subies get the rumble from unequal length headers" plain and simple.

  14. If you get custom made unequal headers for a skyline EVO etc will it get more of a rumble ? Or can someone tell me how to get a EVO to get a deep rumble like the wrx ? Is this possible

  15. This is a good one. What I would like you to explain also is the science behind the change in exhaust note or tone between fresh and "broken-in" exhaust pipe/system and the different materials used for different systems.

  16. will unequal header lenght cause unequal normal engine wearing?.
    i mean will cyl 2 and 4 haver unequal wear than 1 and 3 because of the header?

  17. Could the same idea be used to recreate a crossplane V8 sound by shortening one primary tube and leaving the other three equal? Making two of the pulses meet at precisely the same time.

  18. How abaut diameter of pipe in equal and unequal ? 2.5" or 3" ? And collector use 3" or 3,5? My car subaru wrx sti 2009 2.5turboThanks

  19. How about the equal length 4-2-1 headers on the base Impreza? Isn't the new WRX 4-1? An explanation on the differences between 4-1 and 4-2-1 could be in order because I feel like the 4-2-1 has less rumble than the unequal length headers but, still has a slight rumble.

  20. Iยดve watched this video many times! This channel is great as one can come back to videos. Thanks for all the videos and the helpful info.

    Iยดm very motivated by the passion you put in all your videos. I'm now obsessed with the STI more than ever, that sound makes me want one very bad.

  21. It sounds nice, i wonder why does the frs, brz and 86 have such a horrible torque dip? Great videos man this is my 3rd time rewatching this vid, keep up the great work !! ๐Ÿ‘๐Ÿผ

  22. How long would it take for a car to stop working by simply starting it and letting it idle for 5 minutes everyday?

  23. Why do people, even yourself, even though you KNOW what causes the sound call it "Boxer Rumble". When it's nothing to do with the Boxer that makes that sound… it's just the header and unequal length exhaust pipes.. nothing to do with Boxer. How do we know? Well, go listen to that Boxer now, it's still the same one, and it has an EQUAL length header… and wow, no "Boxer Rumble". Yet when you put an unequal one on it? The Boxer rumble nmagically appears lol. It's an exhaust rumble from the header and unqual pipes. Nothing to do with boxer.
    Do you also know how i know it has nothing at all to do with the boxer?
    I traded my 98 GC8 with 81k Miles on it, for a 02 Lancer Evo 7 6 speed M/T. with 49k on it.
    I LOVE the car to this day, i find it to be SOOO much better than the old impreza, and WAY better than any new impreza. My only issue was, i really missed that beautiful Burbling Sound my impreza used to make. The Evo 7 makes a decent sound, but nothing out of the ordinary, the GC8 98 made this INCREDIBLE rumbling sound only an evo showed in those days. SO what did i do? I found a guy in Dublin who could take my Evo's header and pipes, cut, add and weld unequal lengh pipes onto that fitted underneath in the same place, all 4 pipes were custom made and welded for me, 2 shortened, 1 lengthened and 1 kinda REALLY lengthened, and it fitted just fine back in (little bit of rattling until i managed to get some plastic tubing around the area),. i started the car and i LITERALLY started crying. My Evo now sounds like my Old Impreza… I though it'd sound "SOMEWHAT" like an impreza, but i presumed you needed the exact same shap and the pipes needed to be cut at certain angles and lengths to get that correct burbling sound but to my surprise… NOPE. Providing the 4 pipes are different lengths from eachother, and they aren't making contact metal on metal (at least 4 inches of difference between the lengths of each pipe), you LITERALLY have an Impreza Boxer Rumble…. I ALWAYS thought the only reason i prefered the imprezas to the Evo's was because of that sound, i was like "If only Mitsu made a better sounding car". Not realizing at the time what made the impreza sound like that. So now i have the most beautiful car i can imagine, with the sound of the other most beautiful SOUNDING car i can imagine (Outside of an AMG 63 or the Sports Jaguar XKS's)

    Just a tip for you guys. Measure up your space you have left over under there, and get a custom set of pipes with the header, unequal lengths, pop them in, and you'll have yourself an impreza Burble. No joke, There's a shit*y little 1.4l Honda Civic on here with the "Boxer Rumble" lol, because he also fitted a UEL header system, go check it out!

  24. How come some Subaru's have the rumble but some Don't. Did they change things in certain years? I'm talking the N/A 2.5i? I've heard some with aftermarket exhaust that have the rumble and some that sound more like an inline. I'm looking at a 09 impreza with a 2.5 N/A. I want the rumble any ideas?

  25. Can you explain why 4 cylinder motorcycle engines sound different when they all have equal length headers? For example, the BMW S1000RR is an across the frame 4 as is the R1 but the R1 utilises a cross-plane crank. The result of this is the ignition timing in degrees is different from that of the BMs more traditional 180-degree (flat-plane) crank, so the BM has what would be called a screamer engine and the R1 has what is incorrectly known as a big-bang, I say incorrectly as my understanding it's correctly called an uneven-bang engine. The sound difference is noticeable, the R1 having a drone like exhaust note and the BM having, well, a screaming exhaust note.

    Interestingly enough, the Aprilia V4 engine shares the crank angle firing intervals as the R1 which is why they sound very similar despite one being an inline 4 and one being a V4. I saw a video on a flat-plane V8 and a cross-plane V8 and the sounds were different like the bikes I mentioned above.

    I'm not saying what you've said in this video is incorrect as I think the net result from uneven length manifold might be the same as a shift in crank angles but I am just interested in your take on it.

  26. I don't undertand why everyone called it "Boxer Rumble" basically implying that because the car has a boxer engine, THIS is whats causing that beautiful sounds to come out, when it LITERALLy has NOTHING to do with the fact that it's a boxer engine, A boxer engine is the same as other engines apart fro the pistons are are at a different angle, making NO difference in sound.
    What they're actually all thinking is the "Boxer Rumble", actually comes from the Exhaust system, The exhaust system used Unequal Length Exhaust Headers, THIS is what causes the cars to sound rumbly and awesome, again, NOT A THING to do with the "Boxer" rumble everyone thinks it is. How do we know? Well, go listen to other cars from other manufacturers who installed Unequal length headers… Honda Civic… wow Honda Civic has Boxer Rumble, also the biggest glaring fact, Subaru basically never changed that 2.0 or 2.5L Boxer engine at all, and after 2005 they stopped using Unequal length headers in their exhaust systems, and low and behold, there was no longer any Boxer Rumble, out of? a a pretty identical Boxer Engine… sigh

  27. All my life I saved for a new Subaru. By the time I had the purchase price saved up, they had decided to change over to equal length headers. I never bought a Subaru as a result. They just sound like any other 4-cylinder engine now. I bought a 6.2 liter V8 truck instead! ๐Ÿ˜€๐Ÿ‘

  28. So, Is this a way to combat the exhaust gas power robbing,?I guess my initial question would be: is that a subaru design to maximize the energy from the exhaust? I have a WRX with twin scroll turbo and i understand the benefit from that and it is phenomenal, but the sound is far from satisfying. The initial question would be: Does the un equal header length in the STI have a mechanical advantage to the system? Thanks

  29. Take the exhaust right off at the motor if you wanna hear how subies really scream. They are really loud.

  30. Not all boxer engines are turbo charged so that didn't explain exactly the reasoning. My Subaru is a ej25 naturally aspirated from factory.

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