Exhaust popping at deceleration

[SBMIANO]

Whilt the sound is very cool is the rumble/popping noise from the exhaust normal when decelerating?

Thanks

 

[goldcup]

I believe it's normal but not totally sure I hope so cause my car does the same thing occasionally!!

 

[mike & juli]

Whilt the sound is very cool is the rumble/popping noise from the exhaust normal when decelerating?
Thanks

Yeah, quite normal--can have the 'popping' tuned down if you have a VECII or VECIII computer...probably as we are, running a tad rich...could lean it down a bit more if we wanted, but we're satisfied with the sound-air/fuel mixture we have now. ENJOY~~!
~juli~

 

[DAMN YANKEE]

Complicated question as the answer can range from "no, that is not a good sign" to "some minor popping can be expected." Simply put, you can get popping in deep vacuum state, and usually from a lean mixture dumping unburned fuel into the exhaust system which then ignites and pops . Think of it this way...during deceleration, in deep vacuum, the mixture of air to fuel has very little gas to it...it is therefore very lean and that mixture does not ignite in the cylinder. That unburnt mixture moves from the cylinder and into the exhaust system. All that unburnt gas can collect and, upon the interminent good ignition in the cylinder, those ignited gases move to the rear exhaust and ignite those unburnt collections...hence the popping. Got a RICH bias and you get alot more popping!

Having said that, more things can be at play. Air leaks can cause it...and, just to make things really complicated, a poorly functioning ignition system can create the conditions for excessive popping as well.

Most high performance cars have some popping. Most folks keep adding bolt on performance imporvements to their Viper...smooth tubes, headers. cat work, hig flo exhausts...and never attend to the fact that the "tune" of their car in open loop is "locked in" by the PCM to stock settings. A good way to limit the effect is to properly tune the car, how do we do that when we have a PCM? Using engine control devices. Will all the popping go away, no, just most of it....and there will be no BANGS whatsoever! In a well sorted car (one that isnt a total monster) the reoccurance of popping is a sign that all is NOT well. By the way, cars with some kind of cat usually experience much less popping.

 

[mike & juli]

Hey Dan--We missed you at the TatorToga!! We wanted to meet the 'man behind the tatortoga design' t-shirts! Great Job...hopefully there will be another event where we can meet....I've heard more of a richer mixture causes the fuel burnoff--hence the 'popping'?
But, we're not running real rich, leaned down about as far as we can go, and still have the 'popping' on decel. ~juli~

 

[DAMN YANKEE]

Sorry I couldnt make the even, looked like a blast! Man, I got a couple of the dash plates...were they ever a surprise, great!

Ok, in your case, my guess is that you still have too high a fuel value in deep vacuum. Remember... do not confuse proper air/fuel in deep vacuum with your air/fuel settings in any other state! Using a VEC? that relationship I talk about in the tuning section between fuel and ignition timing is inescapable as well.

Too rich IS still the reason for your popping.

 

[mike & juli]

Dan, yeah that was an AWEsome event, a day I will NEVER EVER forget---as we met the NYC/MA/NJ/CT guys coming up from the thruway and all HELL broke loose on the Northway up to Saratoga...unreal...20 or so Vipers slithering all over the highway!!! What a RUSH!!! Yes, we love the dash plates, so you didn't know about them? They are a great memento. We are very sorry you could not attend.
Okay...so now....yeah--we are running a bit more rich than we wanted, but Sean Roe read our dyno results, and said that we shouldn't lean down anymore. Now, I've learned something from you...about the deep vacuum and the ignition timing...I take it that those can all be tuned with the VECII more precisely? Thanx for your input as ALWAYS! ~juli

 

[DAMN YANKEE]

We have to be careful here...Sean is always the go-to-guy. I think he is refering to your air/fuel in boost. Right? not vacuum? Right?

Also alot of people reduce backpressure in their exhaust, some amounts of raw fuel (and fresh air) will be "scavenged" into the exhaust system. The exhaust system's temperature will rise. Exhaust gas temperatures will increase and there will be a tendency of back-fire (or popping) during deceleration of the engine. A rich condition can make tis much worse.

 

[mike & juli]

Yes, Sean is referring to a/f mix in boost. And, yep, he IS the go-to guy for dyno interpretations...he's done wonderful jobs with not seeing our car personally, but accepting emailing him dyno results (from same dyno each time). And, he says he would not lean it down anymore than it is now...the 'popping' on decel is minor, not excessive.
Acceptable for us...but may not be for others. Thank you again, Dan, as always I learn from you!!! ~juli~

 

[GR8_ASP]

I agree on the lean side. Popping increases noticeably after the a/f steps up during a prolonged decel. And, as I have an a/f gauge installed, I doubt the too rich syndrome. Lean misfire resulting in exhaust system burn.

 

[DEADEYE]

Mine pops only when engine braking.

 

[Racer Robbie]

Complicated question as the answer can range from "no, that is not a good sign" to "some minor popping can be expected." Simply put, you can get popping in deep vacuum state, and usually from a lean mixture dumping unburned fuel into the exhaust system which then ignites and pops . Think of it this way...during deceleration, in deep vacuum, the mixture of air to fuel has very little gas to it...it is therefore very lean and that mixture does not ignite in the cylinder. That unburnt mixture moves from the cylinder and into the exhaust system. All that unburnt gas can collect and, upon the interminent good ignition in the cylinder, those ignited gases move to the rear exhaust and ignite those unburnt collections...hence the popping. Got a RICH bias and you get alot more popping!

Having said that, more things can be at play. Air leaks can cause it...and, just to make things really complicated, a poorly functioning ignition system can create the conditions for excessive popping as well.

Most high performance cars have some popping. Most folks keep adding bolt on performance imporvements to their Viper...smooth tubes, headers. cat work, hig flo exhausts...and never attend to the fact that the "tune" of their car in open loop is "locked in" by the PCM to stock settings. A good way to limit the effect is to properly tune the car, how do we do that when we have a PCM? Using engine control devices. Will all the popping go away, no, just most of it....and there will be no BANGS whatsoever! In a well sorted car (one that isnt a total monster) the reoccurance of popping is a sign that all is NOT well. By the way, cars with some kind of cat usually experience much less popping.

The above is a wonderful explanation of why vipers pop. A perfectly tuned modded car should never pop as Dan says above. You can cut down the popping by sticking a thin shim between the throttle arm and the throttle stop button/screw. I believe that a shim .004-.005 works best from my foggy memory. Installing a secondary throttle spring will also make sure the throttle stays positively closed against the shim. I have done this on a lot of customers vipers and it has eliminated most of the popping.

Robbie