Exhaust pops

TurboRex12 · 12-29-2004, 11:13 PM

I have the Blitz Nur Spec Cat Back on my 02 WRX, it sounds great except when i let off the gas and let the rpms slowly go down. When i do that it makes a poping noise. Sorta reminds of the sound that old muscle cars make. Can anybody give me some advice about this, is it normal or does something need to be fixed? All my car has done to it is the catback, a cold air intake, and a BOV.

IBAUCLAPlaya · 12-30-2004, 12:14 AM

Normal, just a little bit of backfire probably. BOV causes it.

jdmjones · 12-30-2004, 12:27 AM

My past cars have always done it. No problem.

WRXtreme · 12-30-2004, 08:54 AM

Its normal, mine did it with just a catback and stock BOV, it did it w/ HKS SSQV and catback and it does it now w/ HKS SSQV and open downpipe.

six3jag · 12-30-2004, 10:33 AM

How does the BOV cause it? I have this same sound and its doesnt bother me too much but if a differnet BOV could fix it i would get it fixxed.
-Jacob

dumdum · 12-30-2004, 05:27 PM

Quote:

Originally Posted by six3jag

How does the BOV cause it? I have this same sound and its doesnt bother me too much but if a differnet BOV could fix it i would get it fixxed.
-Jacob

Because a BOV causes an overly rich condition when it opens durring the shift.If you had someone drive your car and you drove in back of it you would see a puff of black smoke come out the exhaust when the car was shifted.This is raw fuel.At some point the heated exhaust gas will ignite some of this fuel inside the exhaust system causing the back fire.

Rain_Racer · 12-31-2004, 09:08 AM

regardless of the BOV situation in the car, the Stock ECU adds extra fuel when you let off the gas after hard acceleration. This is an anti-knock safeguard and has nothing to do with the BOV.

Cosmo · 12-31-2004, 10:49 AM

First of all, the BOV has no effect on it. It has no effect on AFR at all. Period. I don't understand why people keep insisting that it does. I have logged several different cars with several different BOV's with my wideband, and see absolutely no fluctuation in AFR than with the stock bypass valve. And, my car makes the "exhaust popping" sound in question, and I don't have a BOV.

Second, your exhaust always did that, but your stock muffler muffled it. With the less restrictive muffler, you hear it more.

tech1145 · 01-02-2005, 07:01 PM

Cosmo, what happens to lean out the mix in the brief time you shift the car then? Or is a shift to short of a time to register the rich condition?

dumdum · 01-02-2005, 07:17 PM

Quote:

Originally Posted by tech1145

Cosmo, what happens to lean out the mix in the brief time you shift the car then? Or is a shift to short of a time to register the rich condition?

This was to my understanding too.I mean the atmospheric BOV is venting air that has already been metered.The recirculating valve re-inters that air back into the stream.The ECU isn't going to stop fueling the motor when that air is being let out into atmosphere.On speed density vehicles this isn't a problem because the metering is done in the manifold so the venting of the BOV is done before the air/pressure is metered.

Cosmo · 01-03-2005, 08:25 AM

Well, yes, that air has already gone by the MAF. But, the BOV opens when you let off the throttle, which cuts the amount of fuel you are putting into the engine anyways. And, for the recirculating arguement to be valid, you would have to slow time down completely. the BOV/recirculating valve is open for a fraction of a second. There isn't enough fuel injected in that small amount of time to effect anything.

Plus, the throttle body is closed when the BOV opens, so that air isn't going into the engine no matter where it goes. So, the engine is getting the same amount of air with either a BOV or a bypass valve.

dumdum · 01-03-2005, 10:45 AM

When you vent to atmospere your are also releasing the pressure built up in the chargepipe system.That causes the system to have to be re-pressurized.Where with the divertor valve ,the air re-enters the system keeping pressure in the system.I would think the ECU would be expecting this mass of air to still be there once the throttle is re-opened?With the BOV the pressure/mass of air is not pressent when the throttle re opens and the turbo will have lost alot spool time because the divertor valve actually cause a "pinwheel" effect on the compressor blades as it passes through durring the shift.But your points are all very valid though.I've researched on many other forums and 80% seemed to think the BOV's are wasted money and do less than more.

Cosmo · 01-03-2005, 12:15 PM

Well, I'm not saying a BOV is or isn't worth the money. If you like the sound, then it's a good mod. If not, then spend your money elsewhere. But, I'm just saying that the whole "BOV makes you run rich" arguement is invalid. At least, according to the AFR logs I have taken.

dumdum · 01-03-2005, 07:27 PM

I just can't see how it doesn't run rich.The ECU doesn't just forget about that metered air.I guess we agree to disagree!

I absolutley admit to liking the sound they make.Its pretty intimidating to here those things blow off next to you!

Cosmo · 01-04-2005, 08:18 AM

No, it doesn't forget about it. And, if the BOV were opening at partial throttle or full throttle, I could see it making it run rich. But it opens when you let off the throttle, which is telling the ECU to cut back the fuel anyways.