AS mad science: understanding burble. Place for discussion and amateur science.

[bunglebus]

50 minutes ago, Dick Longbridge said:

This. I loved the sound of it on my old classic Impreza turbo. I ran a decat stainless exhaust on mine and found the soundtrack addictive. 

I do know some owners who went a step further and paid handsomely for even headers as an 'upgrade'. I assume it freed up a few horses, but the burble was gone. 

Fair enough - doesn't explain why the VW flat 4 does it too though.

I haven't heard an Alfa flat four but I'd be interested to know if they do the same

[New POD]

16 minutes ago, bunglebus said:

Fair enough - doesn't explain why the VW flat 4 does it too though.

I haven't heard an Alfa flat four but I'd be interested to know if they do the same

The Alfa sud my mate had in the 80s just screamed like a banchie.  On idle it sounded like any other 4 cylinder. 

If I remember correctly. 

[Barry Cade]

Just listened to a podcast on this very subject.. Overcrest, I think. Some English professor who explained why and how cars sound like they do.

Edit.. the podcast.

 

Edited May 9, 2021 by Barry Cade

[Asimo]

5 hours ago, bunglebus said:

Here's one I've yet to understand - why does a flat 4 cylinder engine (VW/Subaru) sound so distinctively different to an inline 4? Same number of cylinders and especially with the Scooby all exiting through a long single exhaust. 4 cylinders, common crank, why does the physical layout of the engine make it sound different? 

The fundamental reason is because the two front cylinders fire 360 degrees apart, as do the two rear cylinders. This  means that the two left / two right cylinders each fire 180degrees / 450 degrees apart. No other firing order is useable with a flat-4.

As installed in the car a flat-4’s exhaust headers are usually of unequal length ( because the front cylinders are further away from the silencer than the rears) and therefore the time taken for the pressure pulse to travel to the exhaust exit is not equal for each cylinder. Hence the burble. 

VW beetle is an extreme example of this withtheir stock exhaust system, as cheap and uneven as they could make it. However,  with a 4-1  equal-length “stinger” exhaust, there is no trace of burble.

Subarus with a single turbo have very different path lengths as the turbo is on one side of the car, hence their extreme burble. (Possibly deliberately designed to accentuate the throb)

Alfa did their best to rid the Sud of throb with a very good attempt at a true equal length exhaust.

 

[bunglebus]

Excellent explanation thank you - so in theory any 4 cyl would do it with the right exhaust design - but you wouldn't because it's not efficient.

So with the VW/Subaru you're hearing blub-bulb...blub-blub rather than blub-blub-blub-blub if that makes sense?

[Asimo]

Yes, that would be rather fun to try, 3 short branches and one long and curly branch.
Reminds me of when Dad welded up a one-to-six side exhaust for his Zephyr 6 and  was then disappointed by the lack of go and throbby sound. Took him ages to find that he had coincidently broken a sparking plug insulator and it was running on just 5 cylinders.

[Cord Fourteener]

58 minutes ago, bunglebus said:

Excellent explanation thank you - so in theory any 4 cyl would do it with the right exhaust design - but you wouldn't because it's not efficient.

So with the VW/Subaru you're hearing blub-bulb...blub-blub rather than blub-blub-blub-blub if that makes sense?

Yes.

You can't scavenge exhaust gasses with uneven headers or in the v8 with a crossplane crank, though as soon as you add forced induction you don't need the exhaust scavenging to draw the exhaust out of the cylinder as the inlet is under increased pressure so you can get big power gains by charging. 

[Zelandeth]

Here you go...V12 (no balance pipe fitted on this exhaust).

 

Flat twin.

I don't have any exterior footage of this one under load sadly.

Inline 4 normally aspirated diesel with essentially a straight pipe.

 

 

I think this is probably the most clearly obvious engine I've ever known with regards to when it comes on cam...There are some seriously funky harmonics/standing wave effects going on at the top end of the rev band too (about 3:45-55 in the second last video gives a good example).

[Cord Fourteener]

9 minutes ago, Zelandeth said:

Here you go...V12 (no balance pipe fitted on this exhaust).

 

Flat twin.

I don't have any exterior footage of this one under load sadly.

Inline 4 normally aspirated diesel with essentially a straight pipe.

 

 

I think this is probably the most clearly obvious engine I've ever known with regards to when it comes on cam...There are some seriously funky harmonics/standing wave effects going on at the top end of the rev band too (about 3:45-55 in the second last video gives a good example).

I'll have a go but I think i need audio only recordings at the specific rpms of 1000, 2000 and 4000  to be useful, and pointed towards to exhaust tip...

I'll see if I can get anything useful anyway.

Thanks!

[Zelandeth]

Yeah, just throwing it in there as example material really.  The van has a bit of a burble on idle, mainly as there's been zip done with regards to matching the length of the runners on the manifold as tended to be the way with old commercial engines.

The *inlet* manifold on the other hand has clearly had a lot more thought applied at the design stage, with the diameter decreasing as you get further along it, I assume giving a volumetrically matched path to each cylinder so they get an even charge.

 

Invacar is a bit odd as it has a left-to-right crossover pipe fitted that uses a small amount of the exhaust gas flow to preheat the inlet manifold.  That plus the slight offset of the left/right exhaust ports on the head means the runner lengths are essentially slightly mismatched.  The offside cylinder has the exhaust port on the front side of the head, nearside the rear.

This and the Jag I can at least get a 1000rpm sample of if it would be useful/interesting to you.  Van I've got no way of getting a tachometer reading from though...so it would have to be a case of find the fundamental best frequency in software and speed up/slow down to match it I think.

[Cord Fourteener]

11 minutes ago, Zelandeth said:

Yeah, just throwing it in there as example material really.  The van has a bit of a burble on idle, mainly as there's been zip done with regards to matching the length of the runners on the manifold as tended to be the way with old commercial engines.

The *inlet* manifold on the other hand has clearly had a lot more thought applied at the design stage, with the diameter decreasing as you get further along it, I assume giving a volumetrically matched path to each cylinder so they get an even charge.

 

Invacar is a bit odd as it has a right-to-left crossover pipe fitted that uses a small amount of the exhaust gas flow to preheat the inlet manifold.  That plus the slight offset of the left/right exhaust ports on the head means the runner lengths are essentially slightly mismatched.  The offside cylinder has the exhaust port on the front side of the head, nearside the rear.

This and the Jag I can at least get a 1000rpm sample of if it would be useful/interesting to you.  Van I've got no way of getting a tachometer reading from though...so it would have to be a case of find the fundamental best frequency in software and speed up/slow down to match it I think.

To be fair, whether or not I can get a suitable audio sample it's really fascinating in its own right.

I'll definitely be pouring over it and trying to understand what's going on!

Thanks, lots of details. Much appreciated 👍

[Zelandeth]

One of the best I can think of off the top of my head for distinctive and downright lovely burble at idle has to be the Saab 900 turbo.  Which is again down to runner length mismatch at the manifold.  The normally aspirated cars bad quite an elaborate 4-2-1 downpipe to try to keep things matched, and had a very smooth exhaust note as a result - not so the turbo though as there wasn't room for it.  Manifold looks not unlike the one on my van, just pointing forwards where the elbow leading into the turbo sat.

They've got a lovely but not intrusive "wub-wub-wub-wub..." At and especially just off idle.

 

[Cord Fourteener]

1 hour ago, Zelandeth said:

One of the best I can think of off the top of my head for distinctive and downright lovely burble at idle has to be the Saab 900 turbo.  Which is again down to runner length mismatch at the manifold.  The normally aspirated cars bad quite an elaborate 4-2-1 downpipe to try to keep things matched, and had a very smooth exhaust note as a result - not so the turbo though as there wasn't room for it.  Manifold looks not unlike the one on my van, just pointing forwards where the elbow leading into the turbo sat.

They've got a lovely but not intrusive "wub-wub-wub-wub..." At and especially just off idle.

 

My passat 1.9 pd diesel does a similar thing, for a similar reason.

I think these burbling noises are rev- specific, as you suggest. 

My former M54 straight 6 engine that @Sunny Jimnow owns is interesting; it is creamy smooth at idle and lower revs but gains a noticeable burble as you get to high revs, presumably due to the interaction of the timing / distance of the pulses with the 3to1 headers per bank.

I think there's something similar going on with your V12 but opposite way around?

[Zelandeth]

6 minutes ago, Tim_E said:

My passat 1.9 pd diesel does a similar thing, for a similar reason.

I think these burbling noises are rev- specific, as you suggest. 

My former M54 straight 6 engine that @Sunny Jimnow owns is interesting; it is creamy smooth at idle and lower revs but gains a noticeable burble as you get to high revs, presumably due to the interaction of the timing / distance of the pulses with the 3to1 headers per bank.

I think there's something similar going on with your V12 but opposite way around?

The Jag has an almost VR6-like warble at around 3000rpm on light throttle, other than that it's pretty much completely smooth aside from the slight rumble at idle. 

Exhaust on the Jag I believe comes off the manifold in two branches of 3 pots each and then merges a few inches down the downpipe.  Mirrored on both sides obviously.  Helpfully the only photo I have up there shows precisely half a manifold... I'll look tomorrow as I'm curious myself now.

I'm pretty sure the system on the Jag is sub-optimal, though it's mainly the way it is for packaging reasons I think!  There just isn't room for much variation.

Even at the back of the car you're having to fight for every cubic inch of space things have to occupy.

[LightBulbFun]

5 hours ago, Zelandeth said:

Flat twin.

I don't have any exterior footage of this one under load sadly.

 

interesting how no 2 Model 70's sound quite the same (admittedly mine does have a slight exhaust blow from one of the down pipes IIRC LOL)

pretty sure mine idles much slower then TPA as well

 

 

[Rocket88]

Commer 2 stroke diesel... the most surprising sound in lorrying....

[Cord Fourteener]

8 hours ago, LightBulbFun said:

interesting how no 2 Model 70's sound quite the same (admittedly mine does have a slight exhaust blow from one of the down pipes IIRC LOL)

pretty sure mine idles much slower then TPA as well

 

 

Yours sound like its missing every 4th firing stroke, so every second on a particular cylinder at idle, just because as soon as you touch the throttle is even again, if slightly throaty.

Either that or with the blow and the peculiar flat twin runners two pulses are causing interference at your low idle.

I'd like an audio of both ac7*s now! So interesting!

I'll see if I can get the audio from the video but that's never ideal for a number of reasons. 

[Zelandeth]

4 hours ago, colc said:

Commer 2 stroke diesel... the most surprising sound in lorrying....

Not sure...Detroit two strokes are quite something themselves, the V6 versions of which sound exceptionally angry.  Straight six is probably my choice though there...

 

[LightBulbFun]

4 hours ago, Tim_E said:

Yours sound like its missing every 4th firing stroke, so every second on a particular cylinder at idle, just because as soon as you touch the throttle is even again, if slightly throaty.

Either that or with the blow and the peculiar flat twin runners two pulses are causing interference at your low idle.

I'd like an audio of both ac7*s now! So interesting!

I'll see if I can get the audio from the video but that's never ideal for a number of reasons. 

interesting!

whatever causes it I will admit it does make a nice noise  

next time I am at the Field of dreams ill try and grab some better audio for you  

(PS mines an Invacar Model 70 Zels is an AC Model 70, only part of mine that is made by AC is the gearbox, which does at least have a ruddy great big AC logo on its cover LOL, granted theres not much in the way of physical differences tho they are both Model 70's still!)

17 hours ago, Zelandeth said:

This and the Jag I can at least get a 1000rpm sample of if it would be useful/interesting to you.  Van I've got no way of getting a tachometer reading from though...so it would have to be a case of find the fundamental best frequency in software and speed up/slow down to match it I think.

you have a tachometer for TPA?

iv been very interested to see just what the engine revs on a Model 70 do as one is being used etc (with the CVT and all that)

but iv not had anyone hook up a tach and record it yet sadly, so if you do have one for TPA please do post some footage of how it behaves out on a run id be very interested to see that  

(I am very curious to see just how close things are to whats stated in the book, for example it says that between a crawl and up to about 45Mph the CVT keeps the engine at 3500RPM (peak torque) and I am very curious to see how true that is in real life)

[juular]

3 hours ago, Zelandeth said:

Not sure...Detroit two strokes are quite something themselves, the V6 versions of which sound exceptionally angry.  Straight six is probably my choice though there...

 

Those sound absolutely glorious. I'd travel on that bus all day.

(And I hate buses).

[Zelandeth]

12 hours ago, LightBulbFun said:

...you have a tachometer for TPA...

Not specifically, but I've got an automotive multimeter which has a tach function.  Only goes down to 4 cylinder, but easy to just multiply by 2 to get an accurate reading.

Not really practical for use on the move though.

I've had a closer look today and can confirm the manifold structure on the Jag.  The front three and rear three cylinders on each bank are grouped together at the manifold, merging into one about 4" down the downpipe.

You get quite an interesting stereo audio effect standing behind the car because there isn't a balance pipe fitted - you effectively have two straight six exhausts in front of you but with a phase differential between them.

The rear three basically have a straight shot down the downpipe whereas the front ones have to make a 90 degree turn after a few inches and have a longer run as a result...my guess is that's where the very pleasant slight burble at idle comes from, rather than being totally smooth like a straight six in theory can be.

Speaking is straight sixes and having mentioned the Detroit earlier... can't believe I didn't post this earlier.

Listen to that thing howl...

Best bit is from about 2:30 on.

So sad we never really got them here.

V8 example too...

That was the first layout I came across a Detroit in, and it's nice...but I honestly prefer the straight six - it's that mix of smooth and angry sounding at the same time that makes it for me.  Very similar engine in a lot of ways to the Foden from a similar era, though they sound surprisingly different.

V6 however, they just sound downright mean.

Sorry for dragging this kinda slightly off topic, just differing engine sounds have always interested me.

[Cord Fourteener]

9 hours ago, LightBulbFun said:

(PS mines an Invacar Model 70 Zels is an AC Model 70...

That kind of talk might get you in trouble 😋

[Richard_FM]

I heard Kawasaki tried making a large V twin motorbike engine like a Harley Davidson.

They design team used their usual tight tolerances, & were disappointed to find it didn't have a Harley style throb to the engine note, as were potential customers who thought it wasn't as powerful because of the engine note.

[PhilA]

So, the whole exhaust design thing does come into play- have an 8 cylinder engine with a single 8-1 exhaust manifold. I could do 1000 and 2000 rpm recordings but you're stuck for 4000, it doesn't go that fast (well into redline). A recording at 200 RPM is easier...

Nothing scientific about this recording, other than a rattly bolt on the rear bumper. Microphone with a piece of fabric tape over it, taped then to the back of the car and taken for a drive.

I probably need to rerecord it because I've fixed a fairly major vacuum leak since then and that affects the sound, particularly low rpm.

Chieftain drive-1.m4a

[Cord Fourteener]

Amazing stuff everyone!

Please keep posting clips, I'm loving the general video clips as well as sound files.

And the discussion. Keep it coming!

I will get round to making more recordings myself, as much for the art as for the comparison. But for comparison I have a three pot and a four, I'll make visual sound files for those when I get a bit of time.

[Conan]

 

Here is my contribution. I hope it is of any help!

[Cord Fourteener]

36 minutes ago, Conan said:

 

Here is my contribution. I hope it is of any help!

That's awesome. Thanks.

[FakeConcern]

On 5/8/2021 at 11:26 PM, Tim_E said:

Phone is sophistimacated enough for now. 

Position it beneath the tip of the exhaust on something but not in the draught, get in and rev and hold at 1000, then 2000, then maybe 4000 then I can really get nerdy.

On Monday I used the T5 for the first time since you started this thread so I had a go at this. I positioned my phone below one of the exhaust tips and most of the sound is like a washing machine full of nuts and bolts!

On 5/14/2021 at 10:38 AM, Conan said:

 

Here is my contribution. I hope it is of any help!

Also I found it really hard to keep the revs at 1k, 2k & 4k, but did my best,I don't know how @Conan managed that as I couldn't do it haha...

No picture above as phone was under car in the dark.

This one is from the dashcam when pulling away up to 40mph...

I'll have another go with the phone positioned to one side and nt under the car when I use the it again.

 

[wuvvum]

On the subject of Detroit 2-strokes, this is still one of my favourite videos.

 

[HMC]

It’s more induction rather than exhaust but I will have to dig out a CAR article by LJKS where he discusses with a Honda engineer about induction  harmonics of variable length intake manifolds and the notes on a clarinet.