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Discussion Starter #1
Does anybody know why my car seems to back fire when i let off the gas pedal sometimes? Is this normal for a blown bullitt. THANKS
 

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It is not normal. You need to get someone that knows what they are doing to tune your car and make sure the bypass is working correctly.
 

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If it happens when you let off after a medium to hard pull, even a short one you may have an issue with the blow off valve not relieving pressure properly. If it happens no matter how hard the pull, even if just a light acceleration maybe a timing/tune related issue. Has this been happening since day one of the supercharger? Also-define backfire-is this a loud pop that really gets your attention or just some mild popping through the exhaust-and also what are you running for exhaust? My SLP catback is prone to some popping on decel even without a supercharger.
You'll get more responses on this tomorrow when everybody returns to real life. There are far wiser people than I who may have better answers to your question....Good luck....John R :cool:


Alan & I posted at the same time,beat me by milliseconds-he is one of the wiser people I referred to in my post.
 

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By backfire I think engine, if it is engine then no no no it is not normal ever for engine backfire.
 

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MaineBullitt said:
Alan & I posted at the same time,beat me by milliseconds-he is one of the wiser people I referred to in my post.
Very kind of you to say that John, however, I am sure there are several peeps on this board that would disagree with you!
 

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What kind of exhasut do you have. I noticed that mine does do it a little bit too. I have magnaflow with a bassani x. for me it's not enough to start worying though. Maybe its the Calgary air. lol
 

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P-51 Bullitt said:
Very kind of you to say that John, however, I am sure there are several peeps on this board that would disagree with you!
:lol: Don't feel bad Alan-there are a few people on this board who I dont even rate high enough with to warrant a response to a question. I apparently was mistaken when I thought originally that this board was exempt from the snobbery & flaming that goes on elsewhere. It's still way better on this board than some of the others....John R
 

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Discussion Starter #8
I have the bassani catted x pipe with the stock catback .Maybe back fire is the wrong word,it is more of a popping sound after i get off the gas.Yes it has been doing this right from the start.It seems to do it in the higher RPM range more though.How do you check to see if the bypass is working right.Do i need to get to the dyno and check this out.I checked all my clamps and found one had not been tightened at the bypass valve so i tightened it up.Could that be the reason why my car goes pop pop pop when i decelerate sometimes. THANKS GREG
 

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bullitt00623 said:
I have the bassani catted x pipe with the stock catback .Maybe back fire is the wrong word,it is more of a popping sound after i get off the gas.Yes it has been doing this right from the start.It seems to do it in the higher RPM range more though.How do you check to see if the bypass is working right.Do i need to get to the dyno and check this out.I checked all my clamps and found one had not been tightened at the bypass valve so i tightened it up.Could that be the reason why my car goes pop pop pop when i decelerate sometimes. THANKS GREG
Greg-that's tough one to diagnose without actually hearing the car. Have you called MPH yet to ask them? Did your car pop at all before the Mongoose kit? As far as the blowoff valve-when you let off the gas, especially after a more than gentle accelaration all that boost that the charger is making has to go somewhere-your throttle plates are closed, so the boost gets relieved through the blowoff valve. On my car I would lamely try to describe it as a low pitch growl until the motor is back down to about 1500 rpm or so. Hopefully some better educated forced induction guys will chime in here soon.....John R :cool:
 

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Discussion Starter #10
The car didnt seem to do this before the blower install.I will take the car out tonight for a good run and see if it still does it or not.If so i will contact Tim at MPH tomorrow and see what he has to say.THANKS JOHN! GREG
 

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I'm no expert, but here is my .02

The by-pass valve vents pressurized air to the intake tract before the blower. It does this to prevent pressurized air from bouncing off the closed throttle body blade and back to the impeller inside the supercharger. Pressurized air vererberating into the blower can cause damage to the impeller. It is not a blow-off valve, blow-off's are for turbos and vent excess exhaust gasses to the atmosphere to prevent the turbo from producing more than the desired boost. I really don't think an actual backfire (if that is what you are hearing) could be casued by a problem with the bypass valve.

Backfires are usually timing related problems, which in your case would mean a tune problem. It is also a problem that would likely be more evident at higher rpm where accurate ignition events are more critical.

Does your car have a custom dyno tune? If not, you may need to look into getting that done. If it does, I would suggest getting the car to your tuner, they should be able to monitor the car on a dyno and see if you have timing or ignition related problems that are causing the backfire.
 

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Discussion Starter #12
My car has the custom MPH tune that came with the mongoose kit i bought.I think this could be tioming related,i guess i will have to talk to Tim at MPH. GREG
 

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stang4.6 said:
I'm no expert, but here is my .02

The by-pass valve vents pressurized air to the intake tract before the blower. It does this to prevent pressurized air from bouncing off the closed throttle body blade and back to the impeller inside the supercharger. Pressurized air vererberating into the blower can cause damage to the impeller. It is not a blow-off valve, blow-off's are for turbos and vent excess exhaust gasses to the atmosphere to prevent the turbo from producing more than the desired boost. I really don't think an actual backfire (if that is what you are hearing) could be casued by a problem with the bypass valve.

Backfires are usually timing related problems, which in your case would mean a tune problem. It is also a problem that would likely be more evident at higher rpm where accurate ignition events are more critical.

Does your car have a custom dyno tune? If not, you may need to look into getting that done. If it does, I would suggest getting the car to your tuner, they should be able to monitor the car on a dyno and see if you have timing or ignition related problems that are causing the backfire.
You are 100% correct-my terminology has been wrong in this thread-the correct term is indeed Bypass Valve. That's what happens when a lifelong musclecar geek like me tries to get contemporary :lol: Thank you for correcting me-I'm never too old to want to learn.....John R :smile:
 

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If it's a popping sound under decelleration than it is normal. Mine has been doing this since day one of the blower install. I get dynoed before every shootout and not once has the tuners been concerned about this.
 

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DOUBLE J said:
If it's a popping sound under decelleration than it is normal. Mine has been doing this since day one of the blower install. I get dynoed before every shootout and not once has the tuners been concerned about this.
Mine does too and always has since I added the Magnapacks.
 

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I think popping on decel with magnapacks (or slp loudmouth, or bassani stingers) and some other aftermarket mufflers designed to be loud is normal. With stock mufflers, I don't think I have heard that before. Mine never poped on decel with the stock mufflers and a bassani x, and it never poped with the magnaflow cat back (magnaflow mufflers, not magna packs) and the bassani x.

All that aside every car is different, and you really won't know until you talk to your tuner.
 

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Same here my pops on Decel. Nothing abnormal about it.
 

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Mine also with the MPH Mongoose V2SQ does this now. It didn't before the x-pipe but afterwards it does occasionally on high rpm decel. It's not a backfire, one that sounds like a gunshot, but is a moderate volume popping at high rpm. I occasionally get a burble on hard shifts but it's clearly from the rear of the car and not up front next to the engine. This is basically unburnt gas in the exhaust iirc. Similar to how excessively rich carb'd cars do this at idle.
 

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Discussion Starter #20
Thanks for claering this up as i was a little concerned.If other blown bullitts are doing the same thing then its nothing to be worried about.THANKS for all the replys. GREG
 
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