What causes backfire?

[Limbo]

The poping noise is not actually backfire.
Its just a change in pressure, usually due to exhuast change
Backfire is a loud cracking noise like a fire cracker. You see this in movies with old cars

[JohnL]

The poping noise is not actually backfire.
Its just a change in pressure, usually due to exhuast change
Backfire is a loud cracking noise like a fire cracker. You see this in movies with old cars

My understanding is that the really loud explosive backfire is an external explosion of unburnt fuel mixed with very hot gas as it exits the pipe and comes into contact with oxygen. The unburnt fuel is hot and ready to go but is in an oxygen deprived environment so can't burn, add oxygen and bang. If it's dark you'll probably see a blue flame accompany the backfire.

'Popping' is smaller explosions (or more muffled) that occur internally within the pipe due to unused fuel mixing with unused oxygen inside the pipe, both coming from the combustion chambers. This has been known to split mufflers etc.

[Limbo]

I know what your talking about JOhnL, but i was sure they mean air poping noise from the exhaust, usually when you get a larger exhaust. I know when i changed my exhaust the noise went away. Was told by the exhaust shop wasn't an engine problem. I think that's the one they are talking about.

The other is a loud popping noise which i agree is backfire.

[minix33]

its all the same - caused by a unburnt fuel being ignited in the exhaust [slightly more detailed in my first post of this thread]

[EGB18CT]

ive got the popping rumbling noise usually get it when the cars cold, and on deceleration/gear changes, i run no resenators and the exhaust is very loud. Nothing to worry about its just the exahust is amplifying the poping noise so to speak.

[hengz]

(mrwillz)myn does it too sometimes
eg. if i cruise at a high ish rev around the block , or down shifting a bit early and sits on higher revs

sounds somewhat cool

[JohnL]

I know what your talking about JOhnL, but i was sure they mean air poping noise from the exhaust, usually when you get a larger exhaust. I know when i changed my exhaust the noise went away. Was told by the exhaust shop wasn't an engine problem. I think that's the one they are talking about.

The other is a loud popping noise which i agree is backfire.

I can't see any non explosive changes in the rate of exhaust gas flow (even abrupt) causing any 'popping' noises. The "loud popping noise" is typcally more like a bloody loud BANG!

Theory; When you get a larger diameter exhaust it has a larger volume. When you back off the throttle the gas flow in the exhaust gas slows down, but if you have a larger volume exhaust the gas will slow down even further. you now may have a larger 'pool' of unburnt fuel in the pipe, so when it ignites it does so with a bigger and more audible 'pop' (?).

Or, it might just be that with the smaller diameter exhaust the gas flow at zero throttle is just that much faster than with a larger exhaust, and the unburnt fuel shot down the pipe just before you backed off (and if you were at WOT the mixture would have been rich) is already exiting the pipe before it gets caught by a following slug of oxygen rich gas (as you would get on the overrun due to the injector cut-out mechanism). With a larger diameter pipe perhaps the fuel rich gas is too slow getting down the pipe, and gets 'caught up' by a following slug of oxygen rich gas, possibly in the resonator or muffler (?).

My old Nota clubman used to pop (a lot) and bang (sometimes) on the overrun. There is a distinct difference in the sounds, one is polite by comparison to the the rudeness of the other. Proper backfires also shoot a blue flame out the exhaust.

[JohnL]

its all the same - caused by a unburnt fuel being ignited in the exhaust [slightly more detailed in my first post of this thread]

With respect, it's not all the same. They are somewhat similar, but one is an external explosion and the other an internal explosion, and the causes are not identical.

[JohnL]

ive got the popping rumbling noise usually get it when the cars cold, and on deceleration/gear changes, i run no resenators and the exhaust is very loud. Nothing to worry about its just the exahust is amplifying the poping noise so to speak.

Happens more when the car is cold = happens when the mixture is richer, therefore more unburnt fuel in the exhaust gas. I would guess that with a cold pipe the unburnt fuel and gasses have cooled down too much by the time they reach the end of the pipe for a proper backfire to occur when the fuel hits atmospheric oxygen (?).

I'd say it's not that the exhaust is amplifying the 'pop', more like it's not muffling it as much. A fine distinction I know...

[JohnL]

Another cause of backfiring and 'popping' is an exhaust leak nearer the engine. On the overun, due to the exhaust gas moving relatively fast and having inertia, the pressure in the pipe usually becomes negative when the throttle is closed (more negative the closer to the engine), and air is then sucked into the pipe through the leak, bringing oxygen with it.

This oxygen (possibly added to oxygen escaping the combustion chamber with the injectors shut off) will then almost immediaterly cause a popping (small explosion) in the pipe, or the amount of air sucked in may not be quite enough to cause an internal pipe explosion, but when the oxygen and unburnt fuel reach the end of the pipe more oxygen is added and a backfire results when maybe it wouldn't have without the exhaust leak. That's the theory in any case.