Popping in L98 Exhaust
Thread Starter
Race Director
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From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
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Popping in L98 Exhaust
Just installed oversize (2.5") front "Y" without cats. The main cat was also replaced with a hi-flow cat. The catback section has 2.75" pipe and a very mild muffer.
Until the car warms up (e.g., 5 min), I get a LOT of popping in the exhaust now. Between shifts and on deacceleration, it sounds like a fireworks show! After it warms up, everything smooths out and sounds great.
I realize backpressure has been lowered, but it would seem if this is the cause that it would pop all the time. I'm sure it's richer during warm up, so is the MORE unburnt fumes that can fire off in the exhaust until it's warm. A coworker thinks nothing is changed and I only hear this because the system is considerably louder.
I'm wondering what you think.
Thanks
gp
Until the car warms up (e.g., 5 min), I get a LOT of popping in the exhaust now. Between shifts and on deacceleration, it sounds like a fireworks show! After it warms up, everything smooths out and sounds great.
I realize backpressure has been lowered, but it would seem if this is the cause that it would pop all the time. I'm sure it's richer during warm up, so is the MORE unburnt fumes that can fire off in the exhaust until it's warm. A coworker thinks nothing is changed and I only hear this because the system is considerably louder.
I'm wondering what you think.
Thanks
gp
Team Owner
Joined: Oct 2004
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From: altered state
St. Jude Donor '05
You may be right to an extent about unburned fumes, a larger pipe will always magnify the sound. I get it a bit too upon decel, have both hiflo pre's and main. I know if I removed them it would be much more noticeable. Have you had your A/F ratio checked/scanned?
Burning Brakes
Joined: Aug 2005
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From: North Central Indiana
Mine does this also. The exhaust is much louder than it once was. It smooths out about the time I would expect it to go into closed loop. I just assumed it ran better because of that. It was strange to actually be able to hear the difference.
Last edited by Scott-89; Mar 30, 2006 at 10:24 PM.
Heel & Toe
Joined: Jan 2006
Posts: 23
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From: Miami Florida
same here poping happens allot till the car is warm. Then if i shut her down and start her back up in she pops again. Then it goes away, L98's are weird i guess. Since there is no engine performance lost due to the poping.
Maybe others know exactly why this occurs and are willing to share information?
Maybe others know exactly why this occurs and are willing to share information?
Thread Starter
Race Director
Joined: Nov 2003
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From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Originally Posted by cuisinartvette
You may be right to an extent about unburned fumes, a larger pipe will always magnify the sound. I get it a bit too upon decel, have both hiflo pre's and main. I know if I removed them it would be much more noticeable. Have you had your A/F ratio checked/scanned?
As with the posters above, it smooths out fairly quickly. I'm thinking about adding hiflo precats tommorow (cause I need to go back to the shop anyway). But it's hard to decide.
Listening to the car as it drives down the road (with someone else driving), it just sounds "healthy". In fact, I'd leave it like it is -- if I only LISTENED to it drive by. OTOH, I have to drive it and inside the car it's nearly twice the volume! It sounds like I have headers and glass packs inside.
My friends (who were teens in the 70's) say it's perfect. They say it's what a badazz V8 SHOULD sound like. But I cringe when I hear it popping (wondering if that's damaging). The interior volume doesn't help that either. Kinda makes me feel self concious.
gp
Intermediate
Joined: Mar 2006
Posts: 38
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From: Louisville Kentucky
popping in cold start
ive seen this condition before, popping during warmup and the most common culprit was thermactor air being pumped into exhaust with aftermarket exhaust.try to disconnect youre pump and see what happens
Team Owner
Joined: Oct 2004
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From: altered state
St. Jude Donor '05
I wouldnt worry about it, the burbling is pretty common...The interior resonance on these C4's seems like it comes with the territory. Cosrsa or LT1 muffler can take care of that though.
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Thread Starter
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From: Overland Park Kansas
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Originally Posted by dominr8
ive seen this condition before, popping during warmup and the most common culprit was thermactor air being pumped into exhaust with aftermarket exhaust.try to disconnect youre pump and see what happens
Originally Posted by cuisinartvette
I wouldnt worry about it, the burbling is pretty common...The interior resonance on these C4's seems like it comes with the territory. Cosrsa or LT1 muffler can take care of that though.
I have to think a combination of mixture (during closed loop), decrease in backpressure, and maybe even a weak valve spring/two.
gp
Thread Starter
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From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
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Originally Posted by Jcrow
It does that when you remove the cats. The air pump is injecting oxygen into the hot exhaust. When the hot unburned fuel mixes with the oxygen it pops.
Are you also saying that removing the air pump (or disconnecting it from the main cat) would cure the problem?
I suppose this makes sense. I've heard the air pump only runs during warm-up and that's the only time it pops.
gp
Intermediate
Joined: Feb 2006
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I dont have the front cats and it dosent pop, maybe the high flow cat is too high flow to burn off the unburned fuel? I have seen this on other GM 350's when the air injection goes into the exhaust manifolds. Personally I think it sounds kind of cool. At night I have seen actual flames out of the tailpipes
Team Owner
Joined: Sep 2002
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From: OBAMA IS HITLER
i get plenty of pop when shifting from 1rst to 2nd, and upon deceleration. i enjoy it. other than the neck snapping acceleration and explosive exhaust sounds, its another reminder that i got rid of stock performance and enjoy the car that much more because of it.
Thread Starter
Race Director
Joined: Nov 2003
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From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Before posting this thread, I did a search on exhaust popping. Over the past year there have been numerous threads asking the cause of popping during warm-up -- especially if the front cats on an L98 have been removed. No one seemed to know the answer -- but many needed to know.
Though some cases can be attributed to tuning problems, it appears that the removal of front cats creates the issue. And, the culprit is air being injected into the exhaust manifolds. Though I'm guessing this provides enough oxygen to re-ignite unburnt fumes, I fail to see how pre-cats -- further downstream -- suppress the popping!
At any rate, one solution is to divert air from entering the manifolds. There is a black, plastic box next to the A/C on L98s. This box is a switching valve that routes air between the manifolds and cat. During warm-up it goes to the manifolds. A few minutes later, air is diverted to the cat (via the air tube). Presently, I've bypassed the black box and connected the air pump hose directly to the cat tube. This prevents the popping during warm-up -- because air cannot get to the manifolds.
I will continue to investigate this solution and whether additional mods are required in an adjacent thread entitled "Need help with AIR pump on L98".
Gregg
Though some cases can be attributed to tuning problems, it appears that the removal of front cats creates the issue. And, the culprit is air being injected into the exhaust manifolds. Though I'm guessing this provides enough oxygen to re-ignite unburnt fumes, I fail to see how pre-cats -- further downstream -- suppress the popping!
At any rate, one solution is to divert air from entering the manifolds. There is a black, plastic box next to the A/C on L98s. This box is a switching valve that routes air between the manifolds and cat. During warm-up it goes to the manifolds. A few minutes later, air is diverted to the cat (via the air tube). Presently, I've bypassed the black box and connected the air pump hose directly to the cat tube. This prevents the popping during warm-up -- because air cannot get to the manifolds.
I will continue to investigate this solution and whether additional mods are required in an adjacent thread entitled "Need help with AIR pump on L98".
Gregg
