Does any one know if you can get a back fire through the cats? I was driving yesterday, just taking off and shifting into 2nd when there was a loud bang from under the front about where your feet are. It was so loud I thought either something must have fallen off or I hit something. The engine never missed a beat and nothing happened; I pulled over and checked under the car and under the hood and nothing was amiss. There was nothing in the street that I could have run over, and the car ran perfect all day. It freaked me out a little because I had just installed the 87 performance tune the day before. Any thoughts?

 

Were you driving through the ghetto?

 

My car makes popping sounds decelerating. My harley does that also, as well as backfiring. It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm. The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. Lean mixture can be the culprit for exhorbiant backfiring, but popping is considered normal. It is worse though if you car is running too lean. If the backfires and popping is very loud and intermitent, you should suspect that that fuel is not igniting at the right time, thus it is a timing issue. I am sure you are fine, but if it keeps happening, get your tune changed on the XCAL2.

 

My car makes the same popping sound but only when the engine is cold. After the engine is warmed up I dont hear it.

 

just to throw this in, when the throtle plate is closed your engine would run rich, because the throtle plate controls the amount of air that enters the motor, but also because we a fuel injected our computers read directly how much air enters to supply the appropriate amountof fuel we wouldnt run rich or lean with the t/b closed

the poping happens because of the scavenging effect (a performance plus) what happens is that on the exhaust side of our system a vaccum is created within the combustion chamber to help draw the air fuel charge through the intake at a faster pace then at atmospheric pressure. but since our trottle plate is closed theres a small hole that still allows air to pass through (for idle) now this scavenging effect is trying to draw air through that small hole, like sucking through a straw, so for the RPM your combustion chamber isnt getting enough air fuelcharge to fill the combustion chamber, so when exaust stroke hits, the amount of combustion leaving your system isnt enough to be smooth. which is why all performance engines pop and backfire on decel because in a short way of saying , they are not "smooth"

 

Thanks for the feedback but the sound I heard (and felt) was a loud bang not the typical popping of deceleration at the mufflers. This was up higher in the exhaust system if it was in the exhaust system at all. I guess there's nothing I can do but drive it and see if anything goes wrong; there are absolutly no sings of anything being wrong with any of the systems and the enging runs strong and smooth. I was just concerned because it was such a loud "explosion".

 

I have not heard anything like that before or after I did my 87 tune. Even though ours are from different people I don't think the tune would cause that. You could pull your O2 sensors and check though. If one is more scorched than the rest at least you know with reasonable certainty what happened. If it happens again you could call Doug and tell him what happened.

 

I'll take a look, BTW I really like the look of your car!

 

u notice a difference with the tune in ur 5-speed??

Paul

 

Thanks. I'm so envious of your vert I'm considering my next mod to be a targa top. I love riding at night with no top.