The Rhythm of the Exhaust Pipe

Half a lifetime ago, I was listening to my Pontiac Bonneville’s engine running, listening for unevenness that I hoped to have eliminated with the tune-up. With satisfaction and pleasure, I listened to the engine under the hood, then for awhile enjoyed listening to the steady unhurried boom boom boom from the exhaust pipes at the rear.

I idly noted that the little boom-sounds and the accompanying puffs of exhaust air were coming at exactly the speed that I would cound off “one thou sand one” if I were counting seconds. OK, four per second, that comes to…240 per minute? Oh wait, I corrected myself, every other upward stroke of the piston is a compression stroke, it takes two revolutions to produce an exhaust puff to go into the pipe. So 480, which is nice slow idle for the big 389 up there. And I stood there pleased with this.

Until it occurred to me that the big 389 was not a one-cylinder engine. For the eight-banger to be releasing 240 puffs of air per minute, one presumably from every cylinder during every other revolution, it would have to be idling at an impossible 60 RPM! Oh, wait, dual exhausts…well, that raises it all the way up to a mighty 120 RPM. What the hell??!

Twenty two years later, this unsolved mystery floats back into my consciousness, along with the observation that all cars have an exhaust-pipe rhythm (sometimes you can even see the tailpipe dancing in time with it), usually quite a bit faster than the old Pontiac’s beat, but still way short of where I’d expect a viable engine idle speed to be placing it.

What explains this?

I think your math’s wrong, but I don’t know exactly where.

My Jeep (AMC 360 cid V8, duals) has a nice low rumbling tone at idle. Wanna hear it?

(Note: if link doesn’t work, tell me and I’ll put it up somewhere else)

Beats me what explains it… I’ll admit I was drawn here by the thread title. My WAG is that you don’t have a puff of exhaust coming out for each cylinder. Each vents through the header down the exhaust line. So at any time you have four of your eight cylinders venting in synch out one pipe. An off topic link you might find interesting if you listen to exhausts What makes a Harley-Davidson motorcycle have that special sound? from

Naaah, that can’t be true. Look at a crankshaft, or, better yet, at an engine with the heads removed. The four cylinders of the A bank are not at top dead center simultaneously, nor are the four in the B bank. Heck, if they were, you’d get a badly bucking engine slamming back and forth as first all four of the left-bank cylinders fired, then, as if in response, the other four slamming from an almost 90° angle. A V-8 crankshaft will hold the *cylinder 1 and 2 [right] rods on the same crank uh…thingie…you know, the offset part of the crankshaft. Then it bends back to center to pass through the main bearings that hold the crankshaft to the block (the part of the crankshaft that the crankshaft as a whole rotates around), bends off at a different offset angle to hold the cylinder 3 and 4 [right] rods, back to center to pass through another main bearing, and so on. None of the 4 rods and associated pistons on bank A are in the same position at the same time because the offsets on the crank that they are attached to are all offset at different angles. And the pairs that share an offset are from different banks, meaning that the angle from the offset up their respective cylinders differs by the angle of the V.

If you could hear the exhaust puffs from the left side, you’d hear the 1, a pause, then the 3, a shorter pause, then the 5, and right after it the 7. [The actual firing order is 1-8-4-3-6-5-7-2].

  • [I am going to use Pontiac cylinder-naming conventions here; you Ford folk will just have to translate]
    Anyway, my original question remains unanswered.

Gunslinger, I can’t listen to your Jeep because the URL you posted goes to some kind of members-only area.


Anyway, you know what I’m talking about. I’m sure there must be something I’m missing, and I’ll end up slapping myself in the head when I see the error in my figuring, but I sure don’t see it yet.

You have to look at the exhaust manifold(s) and the pipe layout, and get a good idea of how far each cylinder’s exhaust has to travel to get to the end of the tail pipe. Most manifolds seem to have a small difference in the distance each cylinders discharge “puff” would have to travel, so you get higher and lower volumes at different times and at constant speed give you the effect you are experiencing. In order to determine rpm from cylinder discharge you would have to take off the manifold(s), watch the valves lift, get real close, and be practically deafened and have your eyeballs scorched by burning hot exhaust gas. Or maybe not. Use the tachometer.

So it is a rhythmical artifact of the interaction of the gases from the 4 exhaust valves pumping their stuff into different portions of the exhaust manifold? That’s kind of cool, too…

Let’s see…

With a firing order of 18436572, the left bank gets a rhythm going of


while the right bank is doing


and the exhaust pipe bolts on…let’s see…uh, asymetrically, towards the middle of the left bank and at the very back of the right bank…we’d need to know the speed of the gases, which is volume expelled across the available diameter, given a compressibility of…

::grasps hair in hands::

aaaagh! it’s too complicated!



OK, I’m better now. I think you’re probably right, it’s probably akin to an interference pattern in wave behavior and has very little to do in any direct calculable way with the speed of my engine’s idle.

Scavenging. I can’t do math, but you have to include exhaust scavenging in your calculations.


I thought it would. Try this one

Several WAG’s, er, hypotheses:

  1. You’re feeling the 5-7 and 8-4 paired pulses as one long pulse. The exhaust flow from the other four cylinders get’s dampened in the mufflers to the point that you don’t feel them at the pipe.

  2. Some of the intakes are getting exhaust into them from (er, uh, you want to call it reversion?) I assume that this is a pre-smogger engine. The cam likely has enough duration to allow the exhaust to back up into the intake runners at idle. You might not hear a real lope to the idle, but some of the cylinders might not be hitting strongly.

  3. Only a few cylinders are really firing at idle. I know my Hog used to idle with one cylinder only firing half the time, this with a stock FLH cam. The flywheel kept the engine spinning at the same RPM.

  4. Someone snuck a Cadillac 4-6-8 engine into your engine bay. :wink:

Here I go correcting a WAG…

With #3, I meant to say that some of the cylinders might not be getting enough of air/fuel to fire every time, not because of dilution from the exhaust, but because the runner design or whatever didn’t allow enough flow.

Whatever. It’s just a WAG…

Since I had dual exhausts, the left exhaust pipe would be getting the 57 pulse and the right pipe would be getting the 84 pulse, which might indeed be the sourse of the audible booms and the puffs.

Meanwhile, up front, the old mill got a pretty good a/f mixture at idle thanks to the Edelbrock 2-plane intake, the tall bumps on the special camshaft, and the Carter 750 cfm AFB carb :slight_smile:

argh… the complexities of the V8… each cylinder rotates in a complex mechanism, except you’ve got to make sure the sum total of the rotational energies of each of the 8 in-line mechanisms cancels out so the engine doesn’t tear itself to shreds.

I used to have a similar dual exhaust Mustang GT convertible with a high-compression 289 engine (later versions would bore this out to 302). I bought it as a wreck, it had been rear-ended and the frame was bent. In retrospect, it was a miracle the gas tank didn’t rupture and the whole thing go up in flames, Mustangs are getting to be notorious for this design flaw. But anyway, it originally had high-performance exhaust manifolds and dual glass-pack mufflers, the guy who owned it had hot-rodded it up a bit. When I got the car, you could fire it up and the exhaust would really rumble, and the glass packs would boost horsepower quite a bit. I had the car restored, they stripped it down to the frame, straightened out the rear frame and body, and went back to a stock exhaust system. I was sure to get in trouble with the local cops with the glass packs, and the stock dual exhaust was pretty high performance anyway, even though I’d lose some minor horsepower. The car had plenty enough power to get me into lots of speeding tickets even in stock config. The glass packs made the car sound like a dragster. The stock dual exhaust was still pretty loud, it had increased flow over the single exhaust on other mustangs. Everyone said they could hear my car coming a mile away, just from the sound.
Well anyway, the difference in the mufflers and manifolds obviously has a huge effect on the sound of the engine. As I understand it, mufflers work to make a standing wave to cancel out the vibration. The muffler itself is sort of a tuned acoustic chamber, it slows the passage of the exhaust and keeps some sort of back-pressure on the valve assembly. I dunno, I never worked on this sort of engine stuff, its too complex. But you could hear the rhythm of the engine, in stereo, and feel the performance of each cylinder, when the engine was performing just right. I heard that Iacocca himself decreed that the Mustang GTs have more engine sound, so the drivers could feel connected to the car. And it was a great car.