Exhaust Sound/Note

[pzero]

Ok guys, I know we have a bigger problem on our hands than exhaust sounds, but I thought I bring this up and get some other Z owners opinion.

I replaced my stock exhaust manifold with Kooks headers. From header connection down to stock H pipe in the middle to stock Z mufflers.

Basically straight pipes with stock H pipe and Z mufflers.

Now the problem is at high speed or acceleration if I let off the gas it pops. Like pop, pop, pop back pressure I'm guessing. Is there any way I can get rid of popping noise or is that part of the fun.

Other than that it sounds awesome in idle, acceleration and steady speed down the freeway.

Any suggestions...I'm all ears.

I got my old exhaust manifold, cats, and resonator with me in the garage.

[Jeff71]

Nothing concrete but you could try altering the DEFCO tables via the tune. Other than that, trial and error by changing the exhaust system set up. I.e. adding a resonator back in the mix and/or the cats. Do you have the exhaust fuse pulled full time? If so, put it back in and go for a drive. If it resolves the issue, look into an exhaust controller so you can open/close the flaps when you want.

[Flannel-On-A-Camel]

There could be a couple of reasons for this.

First of all, you're on to something. Large, high-flow exhaust systems often burble or pop on decel because of the lack of back pressure. High engine speed creates high manifold vacuum, which is increased even more when you let off of the throttle and the butterfly valve in the throttle body snaps shut. This creates a low pressure condition in the exhaust: a small amount of air going in, a small amount of exhaust coming out. The weak exhaust pulses aren't able to overcome the difference in barometric pressure between the outside, ambient air and the intake manifold, causing a reversal of exhaust flow, pulling cool air into the exhaust system. When the cool, outside air and the hot exhaust mix, the colder air rapidly expands and creates a burble. It's the exact same event that occurs when ambient air around lightning quickly expands, and produces what we know as thunder, just smaller. Having large, high-flow pipes facilitates this exchange due to the reduced velocity of the exhaust pulses, which translates directly to a reduction of back pressure.

In addition, some manufactures tune cars to burble on decel. When the throttle is cut, the ecu injects a small amount of fuel and greatly retards the engine timing. This allows the spark plug to fire while the exhaust valve is open, creating the burble sound. I honestly have no idea if this applies to GM or the LS7, but I'm sure someone on this forum will know something about it!

I hope this helps, please feel free to clarify or expound on anything. I'm still learning about LS7's (I'm new to the Z/28 world), so I won't be too much of a help with solutions. I've read somewhere that in 2011, GM actually made an effort to accentuate the burbles and pops in LS7's because they found out that most customers actually enjoyed it. I, admittedly, am one of those people!

[MX621]

My money is the H pipe causing the pops. I have ARH headers with cats, stock H pipe and it pops on decel some to, but the child in me kinda likes it.

My stock C5 zo6 poped on decel as well, and when I went to an X pipe it never did it again after.

[1970judge]

Originally Posted by Jeff71

Nothing concrete but you could try altering the DEFCO tables via the tune. Other than that, trial and error by changing the exhaust system set up. I.e. adding a resonator back in the mix and/or the cats. Do you have the exhaust fuse pulled full time? If so, put it back in and go for a drive. If it resolves the issue, look into an exhaust controller so you can open/close the flaps when you want.

Highlighted. This reply, combined with the highlighted is the answer.

[Bigdumogre]

Running high flow cats on mine instead of off road pipes made a difference. Still pops at times just not as much

[pzero]

Originally Posted by MX621

My money is the H pipe causing the pops. I have ARH headers with cats, stock H pipe and it pops on decel some to, but the child in me kinda likes it.

My stock C5 zo6 poped on decel as well, and when I went to an X pipe it never did it again after.

Thanks for the input. I was kinda sorta was thinking about that. X pipe they say it's got the smooth flow Euro sound, while H pipe got the choppy American sound. I sure hate to mess with the obnoxious choppy American sound....

Originally Posted by 1970judge

Highlighted. This reply, combined with the highlighted is the answer.

Thanks buddy. Its been tuned. I'll contact the tuner and maybe he can retune it.

Here's clip of my Z sound. At the last decel you can here one pop. If I accelerate harder, its more like 3-4 pops.

https://youtu.be/9-nDmbceveo

[1970judge]

Originally Posted by pzero

Thanks for the input. I was kinda sorta was thinking about that. X pipe they say it's got the smooth flow Euro sound, while H pipe got the choppy American sound. I sure hate to mess with the obnoxious choppy American sound....



Thanks buddy. Its been tuned. I'll contact the tuner and maybe he can retune it.

Here's clip of my Z sound. At the last decel you can here one pop. If I accelerate harder, its more like 3-4 pops.

https://youtu.be/9-nDmbceveo

Let me know how that goes. I'll be genuinely curious to know if the tuner knows how to modify defco.

[WHAMMO]

Just wanna add that it definitely can be tuned out, on LS3 too. In my case I asked to put it back cause I love the sound

[joelster]

IMO if they totally tune it out, it's probably way too lean. I looked at the fueling and timing tables of my car on Friday while it was being tuned.

[Yessa]

Headers with factory cats left in place should produce very little of the infamous gasish odor? Am I correct or....?

[NVNO1]

on a side note get some ZL1 add-on splash guards before that beautiful silver paint gets trashed

[DasBaldDog]

Originally Posted by Yessa

Headers with factory cats left in place should produce very little of the infamous gasish odor? Am I correct or....?

I would think so..... but (there had to be a but coming), neither Kooks nor ARH offer a header that works with the factory cats.

To work with the factory cat, you'd need some sort of shorty header, and would probably be fairly expensive since you technically already have a factory shorty header.


From my uneducated understanding, the gain comes from better gas velocity from the longer uninterrupted flow of the LT's combined with both getting rid of the cats themselves (or going to freer flowing cats) as well as the 2" restrictor section between the first and second cat on each side.


So with that, possibly wrong, understanding, if you want to make power/noise for the cheap, the better way (instead of replacing the factory shorty header with another shorty header) would be to replace the factory catted section with a 3" pipe with a hi-flow cat and no restrictor section. But no one makes that as far as I am aware, so it would need to be custom.

If you find someone that makes that section, please let us know, I'd love to have one, possibly swap out my ARH headers.

[Brutale arancia]

Originally Posted by DasBaldDog

I would think so..... but (there had to be a but coming), neither Kooks nor ARH offer a header that works with the factory cats.

To work with the factory cat, you'd need some sort of shorty header, and would probably be fairly expensive since you technically already have a factory shorty header.


From my uneducated understanding, the gain comes from better gas velocity from the longer uninterrupted flow of the LT's combined with both getting rid of the cats themselves (or going to freer flowing cats) as well as the 2" restrictor section between the first and second cat on each side.


So with that, possibly wrong, understanding, if you want to make power/noise for the cheap, the better way (instead of replacing the factory shorty header with another shorty header) would be to replace the factory catted section with a 3" pipe with a hi-flow cat and no restrictor section. But no one makes that as far as I am aware, so it would need to be custom.

If you find someone that makes that section, please let us know, I'd love to have one, possibly swap out my ARH headers.

And if I could produce the pipe with high-flo cat with a bung for O2 sensor, would it eliminate the need for a tune?