Tech Info: Afterfire vs. Backfire (Clearing up the common Misconception)

*Interesting read via TGNY originally from 6Crew.com

Through My time of modding cars and etc. and yours as well im sure you always hear people talking about backfires, their car making popping noises, and etc. Sometimes these convos come up in reference to adding BOV's in draw-thru MAF setup equipped vehicles and some NA vehicles with open exhausts.

Anyway, these conversations seem to have caused massive confusion about what phenomenon is actually happening. this thread will explain the differences between the two and hopefully help a few people learn something while at it.

BACKFIRE:
A backfire is an explosion that's created by an engine that occurs in the air intake system rather than inside the combustion chamber where it is normally supposed to happen. Fuel and some unburned hydrocarbons are ignited somewhere in the intake prior to making it into the combustion chamber or somehow gets blown back past the intake valves. This condition causes a looud abrubt popping noise and will cause loss of power and forward motion and can and will damage the intake tract or possible other parts IE- turbo, sensors, and etc. A backfire is a separate phenomenon from an Afterfire.

WHAT CAUSES A BACKFIRE?
Backfires can occur when running too far of advanced timing, running cams with a massive overlap and having the car not tuned properly for the application as well as just having mechanical valve failure, IE- somehow the valve timing is thrown off or the valves arent seating properly and fuel is push back up past the intake valve during ignition. While there can be more things that cause a backfire, these are just a few.

AFTERFIRE:
An Afterfire, sometimes called Afterburn, is an audible popping sound, individual or multiple that often occurs when the fuel/air mixture is too rich or has a momentary instance of a rich condition. This pop at times can and do resemble small caliber firearms going off. Afterfire occur in the exhaust system post turbo or post engine typically.

WHAT CAUSES AN AFTERFIRE:
Richer fuel mixtures are also slow burning. This results in the exhaust gases to have some amount unburned fuel passed on to it as it exits the engine. As it exits the engine, that unburned fuel and any air leftover from combustion mixes and ignites somewhere in the exhaust. Afterfire's are more common where long exhaust ducting retains greater amounts of unburned fuel due to rich fuel mixtures. Afterfiring can also be caused by cylinders which are not firing because of faulty spark plugs, defective fuel injector nozzles, or incorrect valve clearance

Of the two phenomenon's a true backfire is not something any car modder wants to happen. especially a nitrous backfire. An Afterfire is normal and ever vehicle does it to some extent. more pronounced when dealing with higher flowing and free flowing exhaust. but they are common and typically do not pose any immediate threat to the engine and surrounding parts.

PICTURES:

Afterfire




Backfire





This was just a brief description of the differences between a Backfire and Afterfire. Hope this is helpful.

 

I learn things every day. I knew they were two different things, but I had no idea they had different names.

 

Hah! I think just the pictures would have been a pretty good explanation.

 

Okay, now one of the RX7 guys needs to make an explanation on "Fore Fire". Is that just a RX7 thing?

 

hmm interesting. I can see how there can be confusion about the two

 

no, fore fire is when your car after burns out the intake lol

Back fire can be used on either end of the engine really. Its just a term regarding incorrect location of a detonation of fuel vapors, before or after the engine. The term "Backfire" is just a descriptive term for the explosion itself, not its location.

 

So back fires out the Carb on a RX7 is a Fore fire???????

 

Yes, although on eriks its hard to say cause there was no compression areas left in it, so if was really just shooting fire out of every end at once.

 

what is it called when u have an extra spark plug back there?????