More exhaust pop and flame questions

Discussion in 'Performance Modifications' started by MoparMap, Mar 6, 2014.

  1. MoparMap

    MoparMap VCA National President

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    I have a few things I'd like to try to get sorted out, some not exactly Viper related, but similar. Like many, I love the nice crackle and pop of my exhaust when coasting down and have looked into the subject a fair amount. I've read every explanation from lean to rich to no cats and beyond. From what I can tell, it seems both rich and lean conditions can cause it. This makes sense to me as either way could potentially leave fuel unburned in the cylinder (either too much to burn or not enough to start combustion in the first place). Some claim the popping is bad for the exhaust and will ruin the cats and mufflers in cars, though this was a factory feature in the earlier gens and doesn't seem to have caused any major havoc. Others claim some of the methods of achieving the pop are actually beneficial and help to cool the engine down (not quite sure how on this one, need more info). Also along these lines are both the small bursts of flame (typically between shifts), and even flat out afterburner style consistent blue flames as seen in cars like the new P1 and Aventador. Obviously this can't be too detrimental to anything or I highly doubt cars so ludicrously expensive would let something like this happen. The main question I have on this is how they acheive a constant flame like that? Typically these are mid/rear engine cars, so the exhaust is pretty short and I can see this have a contributing factor, but is it possible on a longer system? The no cat argument also seems to be invalid here as I'm fairly sure the stock P1s and Aventadors are required to have them, so there must be something more to it than just dumping fuel in the exhaust. Both my mother and I are fans of the race car appeal of entertaining exhaust, but I'm curious what all it takes to pull it off.

    On a similar note, but not quite as Viper related, I have a 5.7 Hemi I transplanted into a 67 Dart with an aftermarket ECM. I'd love to try to put a similar crackle and pop into the exhaust if I can as I have come to like the racier nature of the sound. Would any tuners out there be willing to divuldge some of their exhaust popping secrets? I've read the somewhat obvious ones of dumping fuel in the exhaust and lighting the spark off late, but I could see this causing bad things eventually. Seeing as Dodge found ways to do it to stock cars without hurting them, I'm curious what the least dangerous method is of tuning this in.
     
  2. Dan Cragin

    Dan Cragin Enthusiast

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    We can tune in and out the decel pop and make it shoot flames between shifts if you like that.
     
  3. MoparMap

    MoparMap VCA National President

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    I'm aware that most tuners have the capbility and knowledge to do this on Vipers, but I'm wondering if it can be applied to other cars as well or if the exhaust setup of the Viper is one of the main reasons that it happens to begin with? Mine is pretty much right where I like it, but I'd like to try to get my 5.7 Hemi to have a similar burble and pop on decel. Half the people say shut off fuel cut on decel to keep fuel moving through the chambers, and others say that the fuel cut off is what causes it to begin with. Realistically both could be true (too much or too little can both cause it if I understand correctly), and there's some balance in the middle that quiets it down. I have full access to the tuning on my 5.7 with the aftermarket computer I have and would like to play with the settings myself, but want to try to do it the safest way possible by learning how manufactuers pull it off to begin with. I know I could just dump fuel into the exhaust and retard the spark to light it off really late into the stroke in over-run style conditions, but I feel like this is not a safe way to do it as I'd essentially be trying to use the headers as the new combustion chambers and wasting tons of fuel. My Viper still has mufflers and all 4 cats and it hasn't blown anything up and acts just fine, so it seems like the way it accomplishes the sound is the preferred one.

    Rant time, so feel free to skip: On my Viper, I'm pretty sure it accomplishes the pop through fuel cut. I say this because I have settings on my 5.7 that seem to parallel the conditions I see pop under in the Viper. In the Viper, if I wind it up a little bit, say over 3000 rpm, and roll off while still in gear, it will usually pop on me. This popping continues either for a certain amount of time, or if I just barely tap the gas and let back off it will keep going down to around 1200 rpm. At this point, I notice the engine feels just slightly different, like the amount of engine braking has changed just a bit and the exhaust pitch changes. This is the main reason I think it's fuel cut. Once it comes back down close to idle conditions, I think it fires the injectors back up and the pop goes away. It also seems like there's some set interval of cut, so it will either start injecting again after X amount of time, or if rpm falls below X. The settings I have in my aftermarket computer on the 5.7 are as follows:

    RPM threshold to enable over-run fuel cut (engine has to exceed this RPM to "arm" the fuel cut logic)
    KPA below which fuel cut is active (manifold pressure required for the computer to determine engine is really braking and not just cruising)
    TPS below which fuel cut is active (parallels the manifold pressure, but used as a double check to assure the engine is really coming back to idle)
    RPM below which fuel cut is deactivated (start regular fueling again below this RPM, similar to what I think my Viper does)
    Delay before engaging fuel cut (delay fuel cut from happening immediately when unneeded, like between shifts perhaps)

    I figured I'll just try playing with this to see what happens as I'm less worried about huring things when removing fuel than adding it, but I'm not sure if the Viper does anything unique with timing as well. Timing is the main thing that scares me. The 5.7 ring lands are well known to be quite thin and the most common comment I read is "if you hear knock it's too late", so I've tried to be pretty conservative to this point and not experiment. For what it's worth, my carb'd motorcycle also pops quite a bit on decel and I'm almost certain it's a lean pop. The bike is well known for this trait and screwing out the idle mixture helps to quiet this down, though I can't screw it out enough to remove the sound without messing the mixture up so bad it will burn your eyes once it does get to idle rpm (not that I want do).

    TL;DR
    How do you make the pop happen? More fuel/less fuel, more timing/less timing? Doing this on a car that is not a Viper if this makes any difference. The Dart has no cats and just a set of "turbo tube" mufflers (I think they're more like a chambered glasspack than a fiberglass stuffed one). Also have electric cutouts right behind headers I can open at will.
     
  4. CHAP4034

    CHAP4034 Viper Owner

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    Hi Dan

    YES I like that ! :)

    I have a 2003 with the MOPAR VIPER SRT10 POWER KIT
    -Engine Controller - Race P4510172AB
    -Camshaft - Race P4510171
    -Exhaust System - Race P4510176

    I want decel pop and flames and more power :2tu:

    What can you do for me ?

    ps I still have the stock PCM if needed
     
  5. MtnBiker

    MtnBiker Viper Owner

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    I have the Mopar ECU on my Gen IV. It took away a lot of the decel pops but still lets out a very loud crack when shifting above 5000 rpms or sudden decel from above 5000 rpms. Can the Gen IV be tuned to shoot flames? That is what I want.
     
  6. VIPER GTSR 91

    VIPER GTSR 91 Viper Owner

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    Hey John, are you leaning toward Nascar races at night now? LOL
     
  7. MtnBiker

    MtnBiker Viper Owner

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    Hi Mark, no I just like the thought of flames coming out the exhaust :). I saw a Gen 2 at CoTA spitting flames as it went into turn 1. I shot this video.

    http://youtu.be/isY6pVq3YNg
     
  8. NO HEMI

    NO HEMI Viper Owner

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    Mine's a Gen 3, has no cats, and a roe tune, shoots pretty big flames!!
     
  9. MoparMap

    MoparMap VCA National President

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    Also, anyone have any insight into the blowtorch style constant blue flame out of the exhaust under full tilt? Keep seeing this on the P1 and Aventador and wondering how it can do that with cats. Seems like boosted cars do this more often, but pretty sure the Aventador is NA, so evidently it doesn't matter completely.
     
  10. VIPER GTSR 91

    VIPER GTSR 91 Viper Owner

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    I have seen that a lot at various races in the past. Excess fuel being ignited and my old GTSR replica did that too a tad. You capture some great videos I must say. I will have my GoPro attached at VOI but have to figure out the problem with battery life in the thing. I have the extended battery too but believe the problem is my setting at 1080p.
     
  11. Indy

    Indy Enthusiast

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  12. Camfab

    Camfab Viper Owner

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    That guy probably burns a quart of oil every two hundred miles. Who drives like that?
     
  13. MoparMap

    MoparMap VCA National President

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    May have found a bit of a hint regarding this. Saw a weekend car show working on an old Ferrari 355 engine and they mentioned how the factory exhaust has a cat bypass valve that opens at higher rpm. Guess it makes sense if you're only really testing emissions under cruising style conditions. Running wide open would allow a rich mixture to get around the cat issue and potentially cause some impressive fireworks.

    The question still stands though, what do Viper tuners do to achieve exhaust pop? What specific settings do you change? I've read several people say to just dump extra fuel during overrun, but that this will eat up cats in a hurry and potentially burn stuff up. If the factory tune has exhaust pop, I know this can't be what they do since emissions would never let them get away with this. Cats are supposed to last the life of the car and then some, so a factory tune that destroys them is a liability to Chrysler. I'm still betting it's a lean pop setup, but in my experiments with my tune so far I haven't had much luck. I did get one or two loud cracks on a coast down, but that's not really the style I'm going for. I'm looking for more of a consistent quieter pop and crackle.
     
  14. Dan Cragin

    Dan Cragin Enthusiast

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    To give the car more decel pop or flames, you need to increase the decel fueling. Its the unburnt fuel that causes this. Unless there is a big explosion this will not hurt the cats, a richer mixture makes the cats live. One side effect of adding decel fuel is you will get less decel engine braking. You will also get soot and possible paint damage from more decel fuel.
     
  15. MoparMap

    MoparMap VCA National President

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    Okay, sounds there's a somewhat non-dangerous number then. Is there a specific AFR kind of number or range that you would typically shoot for? I have a feeling I might be fighting a bit of an uphill battle as I have a wasted spark ignition setup that might be cleaning up any fuel before it gets in the pipes.
     
  16. vprtech

    vprtech Enthusiast

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    Retarding the ignition timing on decel will cause more popping and burble on decel. It would take quite a bit of unburnt fuel and heat in the exhaust for there to be flames. If you have the capacity to do your own engine calibration work, I suggest you get the car on a roller dyno, and just experiment.

    1996 Roadsters with the JTEC/GEN1 engine had very noticeable decel pop/burble.
     
  17. MoparMap

    MoparMap VCA National President

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    I think I've somewhat let this one go for now. Just finished up a cam swap in the Dart and am working on retuning it all over again. I think the wasted spark setup on the Hemi is probably going to burn up any extra fuel I try to put in, so unless I disable it I don't think I'd get too far. I did get a few pops out of it with a fuel cut on decel, but it was really more of a loud crack that's not really what I was looking for. I like the consistent sort of rumble and burble I get on my Viper over the loud pop and bang I got with the Dart so far. May try to pick it up again later after I get the new cam settled in.
     

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