back fire ???????????
 

what causes a car to back fire ? i just put on headers and 2.5" exhaust on my 65 289 any ideas

With my '95, I backfire when i downshift and scare other drivers away. I watch them hit their brakes. One guy even asked me if I got shot at after a backfire. I really love it, but I belive it has to do with a number of things. First, I've heard that with SN-95s, the computer pulls timing on a downshift. I'm not exactly sure how that plays out. Secondly, I've heard whenever you open up your exhaust system, your letting the outside air enter more freely. Also, the air velocity is increased in the exhaust. This outside air mixes with the hot exhuast, and the result is similar to thunder. Another story Ive heard is that if your timing's off, the pressure is actually building up in the heads adn has to release at some point causing a shockwave. Which story do you like :confused:

Most of the time backfires are usually caused by lean running conditions, sometimes by ignition/timing problems.

Your exhaust is not directly related, but if it causes the engine to run lean, could be an indirect cause.

The one exception is when you have a hole in your exhaust. For example, if you are missing a collector bolt, or if the collector is bent, you will cause a backfire when suddenly backing off the throttle. This is caused by the vacuum that is created in the pipes when you back off, pulling air in from outside. The oxygen rich outside air explodes when it hits the high internal heat in the pipes. I had this problem with my Chevelle in the mid 80's, and it drove me crazy. There was a collector bolt that had backed off, and I couldn't see it because of it's location. Once I finally found it, and fixed it, the backfire went away.

Take care,
-Chris

thanks for the info guys .