What is "backfiring"?

[Alfadog]

Hi

Backfiring is something that I know about but I'm not 100% sure of WHY it happens and HOW it can be bad for your engine.

My car backfires a bit when I am at 3000 or over going down a hill... it sounds kinda cool but I'm not sure how bad it is, if it is at all?

Could someone please explain for me and probably some others here too what the cause(s) are for backfiring engines?

[That Ozzy Guy]

It happens due to a rich mixture of fuel and it's bad for your plugs (ie it fouls them) and your cat (in newer cars) over time. That unburnt fuel ignites in your exhaust and walah, a pop.

You can also experience a popping sound due to gas flow through an aftermarket exhaust. As you change gears fast gas flow is mixed with slow gas flow and a pop occurs due to the reverberations.

[Alfadog]

Ahh OK... so... why does it only happen when I go down a hill?

I only have to go down hills when I pick up a friend who lives up a mountain, so it's no biggy

[That Ozzy Guy]

Hmm, you got me there Lachlan. I'll let one of the wiser fellas answer that one.

Is this the Z or the K you're talking about?

[Alfadog]

The Z... I think it might have something ot do with not enough back pressure in the exhaust system???

It's just annoying me that I dont know why it happens.

[Alan Pugh]

OK this is what I remember without getting out the books.

Backfire, when fuel is ignited in the induction system back to the carbie (not usually an issue on F.I.).

Afterfire, when excess fuel is ignited in the exhaust system which can be caused by a combination of different faults in the engine,

Valve timing incorrect,

Ignition timing incorrect,

Running too rich,

Throttle opener solenoid not working.

I'd have to go back to the text books to give you the complete list.

Alfadog in your case it's only happening going down hills at 3000 rpm or over which suggests it's the throttle opener solenoid. This is only fitted to manuals, not auto's.

I'm taking it for granted you are decelerating when it happens or you are just getting back on the accelerator.

The dangers are that you will in extreme cases blow your muffler apart. Other than that it's not going to do much damage.

The fouling of the plugs will occur but only if the cause is from running too rich which will foul the plugs anyway, even without the afterfiring.

Hope this helps.

Alan.

[Mike]

Depending on the type of backfire... I only know of two ways it can affect your engine system. If the backfire comes out of the carbs, it can cause excess strain on the carb mechanisms and loosen things up. Not to mention that it will cause fouling. Backfire through the exhaust can be hell on your muffler. I had extensive backfire after rebuilding an engine some time ago... the backfiring screwed up my muffler so bad I could see 'bulging' of the metal. I had some pretty bad backfires going through it before I discovered that my timing sprocket (under the distributor) was put in 180 degrees off! BAM BAM BAM!! Loud as hell..... muffler is dead.

Anyway, depending on the backfire you can cause different types of problems.

[Alfadog]

Ah, awesome. It's not doing any *real* damage then, it's only a pop and splutter backfire, so no real chance of buldging out the muffler or anything.

I was more interested in the cause and I think I know why now! Thank you Goerge, Alan and Mike!

p.s. yes it is when I was not touching the accelerator down a hill and the sounds come from the exhaust. Sounds kinda cool anyway, in a rally car kinda way

[sjcurtis]

Hi Guys,

this looks like mixture theory 101

BACK FIRE - thru the carby = "lean mixture seeking Fuel".

AFTER FIRE - thru the exhaust = "rich mixture seeking Air".

These two theories apply to normally aspirated carb & injected

and both types of boosted engines using a 4 stroke cycle.

My 2c

cheers

Steve:classic:

[Alfadog]

oooooooh that sounds good.

Afterfire. Cool.

[EScanlon]

If I recall what an old trucker told me some time back.

When you're going down the hill, you're forcing the engine to continue to turn over, via the transmission. This maintains the engine speedd as you close the carburator to decelerate, however the vacuum generated by the intake cycle through each of the cylinders, still pulls air THROUGH the carburators.

That SMALL amount of air pullee by each piston is still enough that the air / fuel mixture that gets sucked in, small that it may be (remember you closed the carb), is still very rich. As it gets sucked through, and eventually it fills the exhaust manifold. At that point it now has enough air / fuel to ignite the first time that one of the cylinders IS able to fire.

Remember, ignition essentially stops if you reduce the fuel / air mix going in to the cylinders, and now there isn't enough combustion time for the spark to ignite what little fuel there is. So the exhaust is now primed and when it backs up enough, one of the cylinders can actually fire. The now hot gas exhausting, ignites the mixture in the exhaust manifold, you get B A N G !

Now the system is cleared, and ready for new priming. If you continue going downhill, with the carburator at a lower setting than what is needed to accelerate the car or just maintain speed, you'll repeat the above cycle.

In Mexico, trucks use this all the time to coast downhill and INCREASE the detonations. It's kind of a "macho" thing, the more your truck backfires down the hill showed how much power you had to accelerate. (I rode the buses a lot when I lived in Mexico.)

Those guys would "spike" the downhill by oscillating their gas pedal. Get the rhythm right to the speed of the bus and the hill and you could literally make your truick play a tune. Did I mention I had to ride a LOT of buses when I lived in Mexico. Heck, the SCHOOL bus ride was 45 to 70 minutes EACH way.

Now, it may all be hogwash, but the "peseros" and the Ruta's would routinely make all sorts of "barking" or finger pulling sounds at each other by using their buses.

Thanks for allowing me to reminisce.

Enrique

[beandip]

the back fire in the exhaust is due to a rich mixture of fuel coming in contact with the hot exhaust systime , it can blow the muffler clear off the car , or destroy the muffler as mike said his buldged his. I wouldent think it would be prevelent with SUs just because of the way they work, that is if they are tuned properly. It normaly would occure when coming down in RPM like down a hill or when down shifting going into a corner, slight rapping does no harm .