I recently swapped a 3400 sfi engine into my fiero and have not yet put on an exhaust. In other words the exhaust exits strait out of the manifolds. The engine runs pretty good but when letting off the throttle it will pop..pop..pop..pop..pop, I believe the correct term is "afterfire"? anyway is this caused by anything other than not having a cat or pipes back there? or could it be something else. thanks

Depending on engine configuration, Many things contribute to Afterfire including: overlap, fuel mixture, high engine speeds and sudden decelleration with no exhaust system etc. Sometimes cold air enters the hot exhaust ports when running open headers / when letting off the throttle . You will usually have some unburned gases that will ignite.

If it was running fine before and your ECM etc is controlling timing properly, your exhaust should take car of your problem.

It's weird that you mention this as today, I fired up my SBC with only Corvette Rams horn manifolds. Pop, Pop, Pop, .....
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I'm not driving too fast, . . . I'm flying too low.

[This message has been edited by 3084me (edited 09-04-2007).]

Well i have never actually ran this engine and setup with an exhaust (only have had it running for about 1 week) so it may be one of the other things you mentioned, also to clarify it never once has done it under acceleration only when letting off the throttle at rps above about 2k or so.

thanks for the reply, plus for your help

Do you have the VSS hooked up? On decel fuel is usually cut to prevent that. Of course if your computer doesn't know how fast you're going, it might not do that....

yes the vss is hooked up, but if still does this after i put on the cat and pipes that will deff get looked at, thanks a lot.

I have a few lengths of stainless steel flexible exhaust pipes here and sometimes I'll "fab up" a short length of pipe to a "test flowmaster" just to hear how everything sounds. (on a SBC , BBC or Big Block Olds, I have a few sets of test headers and some cherry bomb header mufflers that I use). You could always fabricate a "quickie exhaust" if you want to be sure everything is ok before you install it.
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I'm not driving too fast, . . . I'm flying too low.

[This message has been edited by 3084me (edited 09-04-2007).]