Subaru WRX Forums banner

1 - 20 of 21 Posts

·
Registered
Joined
·
421 Posts
Discussion Starter #1
"Pop" in Cat-Less Exhaust Questions.

As i understand, when you have an exhaust with no cat and a BOV, you'll exprience a "pop", similiar to a small explosion of unburnt fuel, in the exhaust after ur BOV releases the air pressure.

My understanding is that when your BOV releases pressure, your engine has already pumped in enough fuel to burn with the previously large amount of O2 to burn. However, with the release of pressure, suddenly the fuel air mixture is rich, hence some of it goes into the exhaust unburnt.

For cars with Cat, the thing stops in the cat. However w/o a cat, the excess fuel goes into the exhaust, and finally gets enough oxygen to combust.

Ok please correct me if my logic is wrong.

The question here is, how do we fix that? Is there any negative impact on the engine/any other car part's integrity?

Looking forward to the solution!
 

·
Registered
Joined
·
752 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

As i understand, when you have an exhaust with no cat and a BOV, you'll exprience a "pop", similiar to a small explosion of unburnt fuel, in the exhaust after ur BOV releases the air pressure.

My understanding is that when your BOV releases pressure, your engine has already pumped in enough fuel to burn with the previously large amount of O2 to burn. However, with the release of pressure, suddenly the fuel air mixture is rich, hence some of it goes into the exhaust unburnt.

For cars with Cat, the thing stops in the cat. However w/o a cat, the excess fuel goes into the exhaust, and finally gets enough oxygen to combust.

Ok please correct me if my logic is wrong.

The question here is, how do we fix that? Is there any negative impact on the engine/any other car part's integrity?

Looking forward to the solution!
is your car doing that?
 

·
Registered
Joined
·
1,843 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

is the sound's coming from the engine bay or from the exhaust ?
 

·
Registered
Joined
·
421 Posts
Discussion Starter #4
Re: "Pop" in Cat-Less Exhaust Questions.

Yes its being heard on my car now.

Its from the exhaust. I can hear it's from the back of the car...
 

·
Registered
Joined
·
752 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

recirculate the BOV, its messing up your AFR.
 

·
Registered
Joined
·
752 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

recirculate the BOV, its messing up your AFR.
hmmmm..i'm sure there's a way to fix that?
i guess you can get it tuned then, if you aren't going to recirc the bov.

the car is running rich because it isnt getting the air from the blow off valve and wants to use the same amount of fuel.
then there is a small explosion when the extra fuel finally does ignite. trouble shoot this by recirculating the bov. if it doesnt fix your problem, then i have no idea what's wrong.

the same thing was happening to me. I have a WideBand 02 sensor on my car and when i would let off the gas and just coast, the A/F ratio would become very rich. I recirculated the blow off valve and the A/F ratio was fine.
 

·
Registered
Joined
·
243 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

this is my first turbo car and im trying to learn as much as i can about it... anyways im curious as to what you mean as recirculating the bov and how you go about it?
 

·
Registered
Joined
·
2,522 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

Perrin makes a good Recirculating BOV. What it does is it puts the pressurized air back into the intake system as to keep the air fuel ratios proper. I would highly recommend going with one that does that. The stock BOV is not very good and if you plan on boosting higher then stock then you will have to do it.
 

·
Registered
Joined
·
496 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

uhhh, my race cat back makes lots of little pops as well when i rev match downshift. But my car has OEM DP and BPV... Is my cat back alone causing my car to run rich?
 

·
Registered
Joined
·
3,474 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

The popping sound is quite normal for our vehicles even when tuned (albeit less frequently and not to such a high degree without). You have all explained the situation as a rich situation and a backfire sound because of it. It happens in between shifts or when you release the throttle rapidly enough that there is not enough air for combustion internal to the engine. That bit of fuel gets into the exhaust and reaches flash point either due to the heat and/or air it receives. The cat holds the flame back and just comes out as a dark puff of smoke. Without the cat the flame or a flash actually comes out.

Non recirc BOV makes this happen more frequently and to a much larger degree as alot more unburnt fuel reaches the exhaust due to richer AFRs in these situations.
 

·
Registered
Joined
·
496 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

The popping sound is quite normal for our vehicles even when tuned (albeit less frequently and not to such a high degree without). You have all explained the situation as a rich situation and a backfire sound because of it. It happens in between shifts or when you release the throttle rapidly enough that there is not enough air for combustion internal to the engine. That bit of fuel gets into the exhaust and reaches flash point either due to the heat and/or air it receives. The cat holds the flame back and just comes out as a dark puff of smoke. Without the cat the flame or a flash actually comes out.

Non recirc BOV makes this happen more frequently and to a much larger degree as alot more unburnt fuel reaches the exhaust due to richer AFRs in these situations.
What about a situation like mine where the car is stock but after putting on a non resonated race CBE, now i'm getting lots of popping in between shifts. This can't mean I am running rich... can it?
 

·
Registered
Joined
·
493 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

if you have a DP you need to get it tuned. As for the popping it will occur even with a tune but, with the tune the car's AFR's wont go nuts. Stock tune plus DP = crazy AFR's.

Enjoy the popping sound it's just unburnt fuel. If you want no popping sound get a catted DP and it will stop the sound.
 

·
Registered
Joined
·
496 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

if you have a DP you need to get it tuned. As for the popping it will occur even with a tune but, with the tune the car's AFR's wont go nuts. Stock tune plus DP = crazy AFR's.

Enjoy the popping sound it's just unburnt fuel. If you want no popping sound get a catted DP and it will stop the sound.
yea, but like I said I am running STOCK DP with JUST a cat back. Is this normal for that situation?
 

·
Registered
Joined
·
493 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

if you have a DP you need to get it tuned. As for the popping it will occur even with a tune but, with the tune the car's AFR's wont go nuts. Stock tune plus DP = crazy AFR's.

Enjoy the popping sound it's just unburnt fuel. If you want no popping sound get a catted DP and it will stop the sound.
yea, but like I said I am running STOCK DP with JUST a cat back. Is this normal for that situation?
Hmmm, that doesnt sound right. Mine never poped with the spt catback, which catback do you have. I mean it is possible but with stock DP very rare because of cat in it...
 

·
Registered
Joined
·
3,474 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

if you have a DP you need to get it tuned. As for the popping it will occur even with a tune but, with the tune the car's AFR's wont go nuts. Stock tune plus DP = crazy AFR's.

Enjoy the popping sound it's just unburnt fuel. If you want no popping sound get a catted DP and it will stop the sound.
yea, but like I said I am running STOCK DP with JUST a cat back. Is this normal for that situation?
Yes it still is. your exhaust has no resonator so you are hearing it echo through the exhaust. Even stock, there is still unburnt fuel present and igniting.
 

·
Registered
Joined
·
493 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

if you have a DP you need to get it tuned. As for the popping it will occur even with a tune but, with the tune the car's AFR's wont go nuts. Stock tune plus DP = crazy AFR's.

Enjoy the popping sound it's just unburnt fuel. If you want no popping sound get a catted DP and it will stop the sound.
yea, but like I said I am running STOCK DP with JUST a cat back. Is this normal for that situation?
Yes it still is. your exhaust has no resonator so you are hearing it echo through the exhaust. Even stock, there is still unburnt fuel present and igniting.
+1 for the guy driving the red lightning :)
 

·
Registered
Joined
·
239 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

if you have a DP you need to get it tuned. As for the popping it will occur even with a tune but, with the tune the car's AFR's wont go nuts. Stock tune plus DP = crazy AFR's.

Enjoy the popping sound it's just unburnt fuel. If you want no popping sound get a catted DP and it will stop the sound.
yea, but like I said I am running STOCK DP with JUST a cat back. Is this normal for that situation?
Hmmm, that doesnt sound right. Mine never poped with the spt catback, which catback do you have. I mean it is possible but with stock DP very rare because of cat in it...
Mine did it with a CB and still does with a TB (SS 3 inch catted and resonated) and tune. I've had no problems with the car.
 

·
Registered
Joined
·
1,643 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

popping with JUST a catback is just you hearing the unburnt fuel igniting in the downpipe BEFORE the cats. you don't see the flame because it's hitting the cats.

you should probably get a tune either way though. OpenPort 2.0 is out now and supports CANBUS (08 and 09 WRX and STI) -- well worth it, way better than COBB.

I have the popping too... it's nowhere near as bad as catless setups or AntiLag
 

·
Registered
Joined
·
496 Posts
Re: "Pop" in Cat-Less Exhaust Questions.

if you have a DP you need to get it tuned. As for the popping it will occur even with a tune but, with the tune the car's AFR's wont go nuts. Stock tune plus DP = crazy AFR's.

Enjoy the popping sound it's just unburnt fuel. If you want no popping sound get a catted DP and it will stop the sound.
yea, but like I said I am running STOCK DP with JUST a cat back. Is this normal for that situation?
Yes it still is. your exhaust has no resonator so you are hearing it echo through the exhaust. Even stock, there is still unburnt fuel present and igniting.
Thank you, this is the information i was looking for!

EDIT: +Karma for a perfect answer
 
1 - 20 of 21 Posts
Top