Goin70

I posted this on the C3 Tech Forum, but thought there might be an expert on this Forum that doesn't comment over there --- I have an LS 376/480 crate engine in my 1970 Corvette restomod. The engine runs great, but I'm experiencing a "popping" and "crackling" sound when I decelerate while the car is in gear. Sometimes, I even get one or two "backfire" pops. What is causing this? Can it be tuned out of the engine? The car has a stainless steel Magna-Flow exhaust system that has been installed by a pro, so I know there's no flow obstruction or other problems. Could it be inadequate air intake through the K&N filter setup? Help ...

crowz

Ive seen something similar with spark plug choices. If its too long it will cause ignitions with out actually being fired. I had all kinds of popping like that with torque master plugs I ran once. Great under full throttle but a royal pain at cruising and slowing down.

C409

....... Inadequate air on decel ? ..........

Tom400CFI

Air fuel and ignition in the exhaust system. The Ignition comes from hot exhaust parts or flame from exhaust ports from normal combustion. Air comes from incomplete combustion in the cylinder, or exhaust leaks. and fuel comes from incomplete combustion in the cylinder.



Yes, If you tune so that DFCO enables sooner on decel, there will be no fuel while decelerating, to cause the combustion.


Uh Oh. A "professional"? That always raises a red flag for me! I'd carefully inspect the entire exhaust system for leaks, from the manifolds (including any AIR plumbing) to the mufflers.

Goin70

The "pro" who did my exhaust is a real craftsman, so I'm not too worried about the quality of his work, but I'll still have it checked. This guy has 40+ years experience as is the "go to" shop used by all the classic car people in my area. He is the only one who touched my car and he did the whole system from front to back, with beautiful welds (no clamps or connectors anywhere). Everyone who's seen it says, "Man, your exhaust is a work of art"!

DanielRicany

This is common with LS1s in the C5 Corvettes that I have noticed. I think it comes down to the free flowing exhausts that people install on them. Seems pretty common if you google it.

8a8mfh

Did you ask him what might cause it? 40 years is a long time, one would think he'd have been asked that before.

Well anyway, if it's not a mechanical problem it can be tuned out. DFCO, throttle follower, decay rate, etc. Closed throttle timing. Although I do believe it's normal.

Goin70

Yes, I asked him and he didn't feel it was exhaust related. Since my LS engine has the hotter cam than a standard LS3, do you think that could be contributing to this problem? Most other responses on this Forum and others have focused on tuning issues, excess fuel during deceleration, spark plug issues, etc. I just hope by "normal", you don't mean it can't be fixed.

I appreciate your input and help!

Tom400CFI

It's normal in that most if not all C5's and C6's will do it. My C6 did w/stock exhaust...more noticeably w/mufflers.


A couple people have already told you how to fix (eliminate) it....