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Thread: Exhaust backfire on deceleration

  1. #11

    Default Exhaust backfire on deceleration

    JWZ28 wrote:
    Right now the PV% is still at 19. When I romp it from a stop the car just launches like a rocket ship and powers through every gear. To begin with, the popping only occurred on deceleration, so acceleration was no issue. When I first drove the car today, I had two consecutive pops on deceleration, one on each bank (my exhaust does not have a crossover pipe, each bank is independent). I noticed a slight difference after adjusting the PV down to 22, and an even bigger difference when I dropped it to 19. After the adjustment down to 19, I went for a cruise with a buddy for about an hour. He drove behind me the whole time and we both only noticed 1 or 2 small pops during the whole cruise, and they were consistent with what you would expect with a dumped exhaust system with straight through mufflers. I plan to drive it for a while as is, and possible drop the PV a little more if the change remains consistent.

    I was getting the popping sound with the mufflers and without the crossover pipe too. The PV value is 19% at reset so I would try 17% and I am guessing that that issue will completely go away. Reason I asked about acceleration was that I noticed that PS setting was 22% on your setup. At 25% I was getting backfires through the manifold and I had to go to 27% PS. I guess your motor likes 22%. Well it sounds like you are close now, keep us updated.

  2. #12

    Default Exhaust backfire on deceleration

    I didn't mention, but I actually had a slight stumble/stutter on acceleration this morning with the PS at 22. Kicked it up to 25 and it's all go now. It wanted to blow the rear tires off every time I pulled away from a stop. I may try 26 or 27 to see if it improves any further. I'll also drop the PV to 17 to see how she acts.

  3. #13

    Default Exhaust backfire on deceleration

    Let me know how it goes. As far as the PS I was wondering why you weren't having acceleration issues.

  4. #14

    Default Exhaust backfire on deceleration

    Yeah it wasn't too bad, I was just having to milk the clutch a little from a stop and on hard acceleration it ran out of fuel and stuttered a tad. 25% on the PS seemed to clean it right up. She really pulls through the power band now.

  5. #15

    Default Exhaust backfire on deceleration

    Gerry, I unfortunately can't give a good explanation for it because I can never get a straight detailed answer from MSD on how the PV feature works (maybe it's classified). I actually thought it was an exhaust leak to begin with, but I tried to chase one down for weeks and even replaced all my header gaskets with no success. Lowering the PV setting immediately made the condition go away. Maybe since a lower PV setting requires less vacuum to add fuel, it actually does so when the throttle closes and the engine sees a large rush of vacuum. Maybe it's the exact opposite, and the lower PV setting requires more vacuum to add fuel, so lowering the PV setting on mine cause less fuel to be left over to pass into the exhaust and ignite. But in that case I can't imagine why the AFR would read lean.

    Who knows, I sure would like a detailed explanation of how the PV feature works based on the adjustments we make.

  6. #16


    Can anyone give me a light? I have the same symptoms. Today my values of pump squirt and enrich valve are at 10% and I still have backfiring through the exhaust.
    SBF 302
    Cam Setting: MILD
    Fuel Pump Type: NO PWM WITH REG
    IAC - 20-30 at operating temp
    Power Valve - 10%
    Pump Squirt - 10%
    A/F ratios: idle 14.4, cruise 14, WOT 13.0
    Idle target: 900 RPM
    Ignition Timing:
    Idle RPM - 925
    Total RPM - 2600
    Idle ADVANCE - 18°
    Total ADVANCE - 36°
    Vacuum advance - ported 10°

  7. #17
    Join Date
    Sep 2018


    ADD Pump Squirt and watch your AFR reading as you cycle through the throttle opening. More PS should help (depending on your build). My 302 SBF has a Super Victor single-plane intake and an aggressive cam (narrow LSA). I had to increase my Pump Squirt over 25% and lower my Power Valve setting below 10% to get the lean stumble/backfire to subside. I run E85 so my AFR targets are more aggressive and my engine only likes the Street/Stock calibration (which I believe has the richest fueling strategy of the three cam settings).

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