What is that exhaust sound ?

I have several tractors - some do this more than others. It is the sound the exhaust makes when you throttle down from high rpms. But I hear it when at low rpms and idle. It is not a miss like a bad plug or wire and it is random -  not like one cylinder does it every time around. Don't hear it at all under load or higher rpms. Just wondering, Larry Miller

At low RPM the fuel system isn't operating very efficiently, it could be either a lean or rich mixture. It could also be the valves passing some compression. I've heard it too, you get to thinking there is something wrong with the engine when all it really needs is a few more RPM.

My model b makes soft popping sounds. it could be the straight pipe on it or just some thing but I don't worry about it too much.

May be a burnt out valve by the sound of it

That is music you'll never hear anywhere else.

People are getting so used to hearing modern port-fuel-injected, high compression, electronic ignition with computer managed fuel mapping, ignition advance and knock control... 

One of the things that I've always been observant of, is the sound of every type and model of engine I hear.  Some of it has to do with a sick love of machines, and the physics and principles of operation, but the rest is probably the result of being a part-time musician with an endless love for unique sounds.

If you listen to a Model A Ford, you'll hear it's sound, which PARTLY is characteristic of the long, skinny exhaust, slow speed, but also it's flathead, low compression, mechanical points, firing order and chamber shape. Bore and stroke affect the timbre of sound, but rate of exhaust ejection and intake draught are also integral to the sound. The very first memory of my dad 'working' on a car, was when he was installing a freshly rebuilt engine into the chassis of a '29 Ford Tudor... I stood alongside while he and a neighbor lined up the mounting surfaces and started the bolts, and I watched while the electric starter groaned it over for the first time since rebuild.  It sounded nothing like the 455 in the big Buick wagon, and nothing like the neighbor guy's VW Bus.

I blueprinted and 'built' a 250 Chevy inline six several decades later, and it's nothing like the Model A, or the big Buick, or the wheezy-whistle of the flat four.

Here's some things to think about when you hear it...

The  Model A, and the Fordson, and the early Waukesha and Continental fours, you'll see they're all flatheads, low compression, short-duration camshafts, very conservative timing, small carb throat, and skinny exhaust.  The short-duration camshaft and skinny intake passage means high intake velocity at low RPM.  Good, 'cause these engines didn't run fast.  The A has a 'hissing' sound to the intake, because the Zenith carb's venturi is very restrictive, and running at a right angle... with a choke plate really close to the venturi... oh, and no air filter to 'quell' that noise.  Henry didn't believe in air filters, or oil filters, or hydraulic brakes... or...

Okay, so compare automotive engines of the mid-late '30's with say... the WC engine...

Start with overhead valves...  adjustable rocker arms... long, long stroke, moderate duration on the cam... centrifugal advance and air filter... full pressurized lubrication, in comparison, the WC engine was a musclebound heavy-breather... of course, it had to be!  When a Fordor rolled down the road heading to church, that 40hp mill was actually only being asked of oh... 10hp or so in order to make 40mph... splash lubricating all the way.  The WC motor... when it was put to a field, it was gonna be asked of everything... everyday... all the time.

And the farmer wanted it to use fuel at a 'sipping' rate.

Okay, so about the sound... take an engine that's made to really work... hard... constant speed, mildly varying load... make it work hard and run on as little fuel as you can... lousy fuel perhaps... so you've got long stroke, a moderate duration cam, overhead valves, low compression, in a hemispherical chamber (upside down, of course!), put it at working speed and make it do it's thing... tune it to the leanest-reasonable point (where power stays high, but fuel use is low).

Now, take an 'economizing' type carb, put it on there, and make it idle... slow... way slow... so it can calmly creep up to an unsuspecting implement, snag it, lift it up, and send it out to a field, where you'll throttle up, and drag that poor plow through dirt for eight-ten-twelve hours.  At idle, the engine will be pulling immense vacuum... and there'll be JUST ENOUGH FUEL for it to run.  On the Economizer, the fuel air ratio will be LEAN, particularly when you're drawing it into a BIG chamber.

Ultra-lean mixtures don't like to light.  HONDA experimented with solutions to this with their CVCC design (three-barrel carbeurators, extra valves, and all kinds of really cool ideas-in-progress)... you get cylinders that simply don't fire...  too lean...  it'd make the CVCC STALL if it wasn't set quite right.

But the Allis... big flywheel, long stroke... piston speeds changed LOTS more than the CVCC motor... didn't spin as fast, but long stroke, big flywheel, if it didn't fire, it'd just push that mix out the exhaust.

Into the hot exhaust... by the way...

And as soon as a little oxygen made it to that really hot mix...

"foomp"  <clink>

...

...

...


"foomp"  <clink>


:-)   Real engines clink after they foomp.

Ford was always bad about having a maze of vacuum lines, however it did allow them to meet emissions with out computers for 2 years after GM made the switch full line. Vacuum lines are going to bite them in the future on the 6.7 diesel in my opinion. Instead of using electronic actuators, they have a vacuum pump and lines to move the wastegate along with a few other things. 

DaveKamp wrote: That is music you'll never hear anywhere else. People are getting so used to hearing modern port-fuel-injected, high compression, electronic ignition with computer managed fuel mapping, ignition advance and knock control...  One of the things that I've always been observant of, is the sound of every type and model of engine I hear.  Some of it has to do with a sick love of machines, and the physics and principles of operation, but the rest is probably the result of being a part-time musician with an endless love for unique sounds. If you listen to a Model A Ford, you'll hear it's sound, which PARTLY is characteristic of the long, skinny exhaust, slow speed, but also it's flathead, low compression, mechanical points, firing order and chamber shape. Bore and stroke affect the timbre of sound, but rate of exhaust ejection and intake draught are also integral to the sound. The very first memory of my dad 'working' on a car, was when he was installing a freshly rebuilt engine into the chassis of a '29 Ford Tudor... I stood alongside while he and a neighbor lined up the mounting surfaces and started the bolts, and I watched while the electric starter groaned it over for the first time since rebuild.  It sounded nothing like the 455 in the big Buick wagon, and nothing like the neighbor guy's VW Bus. I blueprinted and 'built' a 250 Chevy inline six several decades later, and it's nothing like the Model A, or the big Buick, or the wheezy-whistle of the flat four. Here's some things to think about when you hear it... The  Model A, and the Fordson, and the early Waukesha and Continental fours, you'll see they're all flatheads, low compression, short-duration camshafts, very conservative timing, small carb throat, and skinny exhaust.  The short-duration camshaft and skinny intake passage means high intake velocity at low RPM.  Good, 'cause these engines didn't run fast.  The A has a 'hissing' sound to the intake, because the Zenith carb's venturi is very restrictive, and running at a right angle... with a choke plate really close to the venturi... oh, and no air filter to 'quell' that noise.  Henry didn't believe in air filters, or oil filters, or hydraulic brakes... or... Okay, so compare automotive engines of the mid-late '30's with say... the WC engine... Start with overhead valves...  adjustable rocker arms... long, long stroke, moderate duration on the cam... centrifugal advance and air filter... full pressurized lubrication, in comparison, the WC engine was a musclebound heavy-breather... of course, it had to be!  When a Fordor rolled down the road heading to church, that 40hp mill was actually only being asked of oh... 10hp or so in order to make 40mph... splash lubricating all the way.  The WC motor... when it was put to a field, it was gonna be asked of everything... everyday... all the time. And the farmer wanted it to use fuel at a 'sipping' rate. Okay, so about the sound... take an engine that's made to really work... hard... constant speed, mildly varying load... make it work hard and run on as little fuel as you can... lousy fuel perhaps... so you've got long stroke, a moderate duration cam, overhead valves, low compression, in a hemispherical chamber (upside down, of course!), put it at working speed and make it do it's thing... tune it to the leanest-reasonable point (where power stays high, but fuel use is low). Now, take an 'economizing' type carb, put it on there, and make it idle... slow... way slow... so it can calmly creep up to an unsuspecting implement, snag it, lift it up, and send it out to a field, where you'll throttle up, and drag that poor plow through dirt for eight-ten-twelve hours.  At idle, the engine will be pulling immense vacuum... and there'll be JUST ENOUGH FUEL for it to run.  On the Economizer, the fuel air ratio will be LEAN, particularly when you're drawing it into a BIG chamber. Ultra-lean mixtures don't like to light.  HONDA experimented with solutions to this with their CVCC design (three-barrel carbeurators, extra valves, and all kinds of really cool ideas-in-progress)... you get cylinders that simply don't fire...  too lean...  it'd make the CVCC STALL if it wasn't set quite right. But the Allis... big flywheel, long stroke... piston speeds changed LOTS more than the CVCC motor... didn't spin as fast, but long stroke, big flywheel, if it didn't fire, it'd just push that mix out the exhaust. Into the hot exhaust... by the way... And as soon as a little oxygen made it to that really hot mix... "foomp"  <clink> ... ... ... "foomp"  <clink> :-)   Real engines clink after they foomp.

Guys, don't you love the way other gearheads listen to engines the way you do? 4 cylinder OHV gas burner Allis motors have a music of their own for me. When their governers are starting to "sense " being loaded and begin opening up......music!!! And throttling back coming down a grade.,letting the compression hold  down the speed...still  more music!!