backfire

[mgaffar]

i was just wonderin how does this happen?... always wondered this but never found an answer on google.
thanks

 

[skip0110]

https://www.gtplanet.net/forum/showthread.php?t=38616&highlight=backfire

 

[Jedi2016]

As I understand it, there are two types of backfire.

1) The timing is off, or something in the engine isn't working properly. A valve opens prematurely, while the cylinder is pressurized, causing a loud "bang". This is the "bad" type, and indicates something is very wrong with the engine. I could be mistaken about the exact cause of this type of backfire, since I've never owned a car that did it.

2) Highly-tuned cars and cars running on a very rich fuel mixture will sometimes not ignite all of the fuel in the cylinders. This mostly happens when you back off the throttle.. in highly tuned cars, this causes the engine to slow down very rapidly, and results in unburned fuel being sent out through the exhaust. Since the exhaust is usually a little warm (i.e. don't touch it or your hand will melt), this causes the fuel to ignite, causing a sort of "burp" sound a small puff of flame from the exhaust. Unlike the first type of backfire, this doesn't necessarily indicate a problem. Most of the time, it's just a result of the engine tuning and the way it processes fuel through the cylinders. I was watching an episode of "Top Gear", and they were talking about the Ferrari F40. He's driving down the track, and he says "Listen to this!" and backs off the throttle. From inside the car you can't really hear it, but it made that little "burping" sound, and he says something to effect of "When you back off, it just DUMPS all this unburned fuel into the exhaust... for FUN!!" He was saying it in a good way.. hehe.

 

[McLaren'sAngel]

Hiya! Meow! (='.'=)

Sorry to ask, but what are the usual procedure that professionals do in order to fix backfire problems?

 

[Boz Mon]

Hiya! Meow! (='.'=)

Sorry to ask, but what are the usual procedure that professionals do in order to fix backfire problems?

I could be wrong but I dont think that backfires are really a "problem" just a side effect of a highly tuned race car

 

[Emohawk]

That's the 2nd type. The first type would be fixed by adjusting the timing (ie: when the valves open and close vs. where the pistion is at that time), I think. In more serious cases it could be some problem with the valvetrain that I don't know anything about.

 

[Jedi2016]

Yeah, the first type (the one that makes the loud explosive bang) is a bad thing, and should be repaired immediately. Ever notice that when you watch a movie, and someone pulls up in a thirty-year old rustbucket of a stationwagon? It backfires when they shut it off.. you know, when all the kids duck and all the cops pull their guns? That kind of backfire. And believe me, it ain't "highly tuned".

 

[Taurine]

My little Ford Laser backfires, not with a flame but upon backing off the gas there is a very noticable blurble through the exhaust.

On lower tuned cars you mostly get a pop with no flame, with a little more power you often get a bang with a flame.

 

[Jedi2016]

Is your Laser supposed to do that? My old Camry did that, but that's because it was running on four cylinders. Which might not be that bad, except it had a V6 engine So it was bad. So two cylinders' worth of gas was getting thrown out the back end constantly. It had happened slowly, so when I finally got it fixed, I was actually surprised by the rather significant increase in power.

 

[Taurine]

No it's just badly out of tune

 

[xcsti]

I havent read that link so it may say exactly what I'm saying (heck I might have posted there)

Racecars are usually set to fuel scavange, that is the intake valve is opened a while the exhaust valve is just barely open. This helps pull the fuel air mixture in quicker and with less resistance. This can allow some of the fuel to be thrown into the exhaust.

 

[McLaren'sAngel]

My little Ford Laser backfires, not with a flame but upon backing off the gas there is a very noticable blurble through the exhaust.

On lower tuned cars you mostly get a pop with no flame, with a little more power you often get a bang with a flame.

Hiya! Meow! (='.'=)

O no...I think I may have that kind of a backfire like your Laser does. Very rarely, when I let go of the gas pedal, I would hear my exhaust quickly quiet down and I would hear something that sounds like a growling noise. BUT the way it sounds, is like the tailpipe is sucking something in.

 

[Takumi Fujiwara]

Oddly enough, my new motor backfires like crazy. It's normal on the street, but when I run the car, it shoots flames when I let off the gas at high RPM. Pretty bizarre, as it's a stock motor.

 

[Drifting Thunda]

When I had nothing but long tube headers on my 5.0L V8 it backfired many times. Now with full exhaust or even out the cutout (after the exhaust reducer) it doesn't backfire. So I think if there isn't enough backpressure backfire can occur, if that's not true then it was backfiring because the oxygen sensors weren't on and therefore the motor was running rich.

 

[Super Jamie]

taurine, your laser is just running rich. my car does the burbling too at low throttle opening and high vacuum, i have tuned it that way to be rich down low to cover the overlap of my cam

thunda is right on the money, the backfire you're talking about is when air-fuel mixture collects in the exhaust and sits there until a burst of fire from the cylinder head ignites it. yes, fire does come out the head, as the remanants of combustion escape into the exhaust manifold

acutally, if you have an old carby car, you can do this yourself. turn the ignition off while you're driving, so air-fuel builds up in the exhaust. sometimes you need to pump the accelerator, sometimes you need to hold the pedal flat, sometimes you need to back off totally, it depends on the car. after about 5-10 seconds turn the ignition back on and the air-fuel in the exhaust will ignite, causing a huge "keybang"

tops for frightening people walking on the side of the road, or getting rid of someone tailgating you. once i had a 4wd up my ass at night, i keybanged him and and got this HUGE roostertail of flame out the back, it looked like a jet on afterburners. the guy stayed WELL back after that

keybangs are also a good way to blow your muffler apart (my last car had a big oval "football muffler" i'd done it that much!) and possibly crack pistons as the huge explosion sends force back into the engine as well as out the exhaust

 

[Super Jamie]

backfire can also be caused by (most likely) incorrect ignition timing, lighting the air-fuel while the inlet valve is still open, and the manifold is still full of air-fuel as well. it is for this reason you should never look directly into the throat of a carburettor, use a small convex mirror (makeup mirror) and you can see better than with your eyes anyway. multipoint fuel injection systems have pretty much done away with this as a possibility. if your car does this, get it seen to. i blew my last holley apart by running it WAY too rich and enjoying the tough "pop pop pop" i got on decelleration

as mentioned above cam timing can also be off, if the engine has been built incorrectly in the first place. it's pretty rare but it does happen, and it's difficult to figure out when it does, you're sitting there wondering why a motor runs so well when it's way off tune, but like a bag of crap when you set everything to what it's sposed to be. realistically, an engine left to the state of neglect where the cam skips a tooth is likely to not be running anymore anyway

hope this helps