[MarCylinder]

Mufflers do not impact emissions.

In the case of gasoline cars, there are mechanisms for controlling intake air, recirculating exhaust gas, and filtering emissions directly out of the exhaust gases. All of these impact carbon emissions.

These defeat devices allow drivers to modify some or all of these attributes in favor of gaining engine performance.

[datavirtue]

Muffler delete is a known cause of misfires. It manifests as a popping sound surrounded by the other shitty sounds produced by such a modified exhaust. We have all heard it.

A misfire is raw fuel being expelled.

[MarCylinder]

That's a backfire. You're right in that a muffler can diminish the volume of a backfire, and that it is caused by combustion in the exhaust.

But the incomplete ignition of fuel can occur in both stock and modified vehicles. My car is completely unmodified, but can very easily have a low quiet popping noise during downshifts. With e removal of my muffler, it would be quite loud.

A modified engine can do this to a much greater extent, depending on the modifications. Some do so intentionally with what is known as a "burble tune", which is quite annoying in my opinion. It is inspired by rally cars that would intentionally inject fuel during the exhaust stroke and fire the spark plugs to keep exhaust driven turbines spinning even when under deceleration.

[mcronce]

A misfire is a failure to combust the air/fuel charge in the combustion chamber during the expansion stroke, and they aren't typically caused by muffler deletes.

They can lead to the popping sound you're describing, but they don't always, and they're far from the only cause, and muffler deletes aren't necessary for those either. Unburned fuel hitting oxygen in the tip of a hot exhaust and immediately burning is the direct source of the noise, and causes for that are numerous.

[sandworm101]

In my kneck of the woods we call that backfire. A misfire is when something goes wrong in the cylinder such as the spark failing. Misfire can cause backfire, but not all backfire is traced to misfire. A very rich mixture can backfire without misfire.

[Rumudiez]

A backfire is when the exhaust goes back out the air intake. An explosion in the exhaust system is called an afterfire, and is far more common. Most people just say backfire for both, unaware of the other term

[nullc]

Depends on the rest of the car, a turbocharged engine is going to have plenty of backpressure even with a straight pipe after it.

[omginternets]

That’s a backfire, not a misfire.

[datavirtue]

Ok, we have quite a few keyboard mechanics here it seems. The muffler is a source of back pressure. That back pressure is part of the highly optimized emissions systems tuning done by the manufacturer. When you remove that back pressure there is high likelyhood that the reading from the 02 sensor are going to change and cause the computer to alter the air-fuel ratio based on that change. Hence, you will likely lose power, mpg, and introduce the possibility of misfires and other unwanted behavior.

I saw this analyzed on a Chevy Silverado that would randomly misfire. Everything else checked out: fuel pressure, spark plugs, firing timing, and fuel injector pressure. The remaining item: muffler delete.

[omginternets]

The part I was objecting to is "A misfire is raw fuel being expelled." That's just plain wrong.