He could be fine with the stock exhaust because his resistance is at the DP. I would think anything after that wouldn't matter. At that point it would be a matter of sound choices.

2005 I have to get some slicks. Our mods are different but I would bet with slicks it would be close. I posted a couple of tenths under you in the 1/8 trap on eagle gt's with an 8.77. The 60' is still in the 2.1 range instead of a decent 1.8. Incidently, that 8.77 was a loss to a 2011 Camaro SS, he ran a 7.60. Not a bad loss in my book considering he had over 100 more hp... Not to mention rwd, and 4 more pistons

 

Stock exhaust is fine.
Every run now is 8.0s in the 1/8, with a best of 7.99
Don't forget that E47.5 is worth at least 30+ HP
I really don't have many mods.
K&N intake
Motor mount
2nd cat delete
I'm also 100lb lighter , with different wheels/tires/rotors, good for only a tenth at the track.

 

Figured I'd give a heads-up here.

If anyone is interested in making these kind of modifications to a stock manual SS downpipe, I just put mine up for sale here: https://www.chevyhhr.net/forums/show...598#post681598

Spread the word if you know someone who is interested, I'm trying to get rid of it by Christmas!

 

I mentioned this earlier but got no response, I buddy of mine in school had a custom 4" downpipe on his STI with a cutout, I have never heard turbo whistle that compares to that ever! didnt even sound like car more like a plane and dump truck combined.

I know k04 wont do that but would a 4" downpipe lose power or be beneficial? (by seeing that everyone in the world uses 3" I sort of know the answer im just wondering why people do it)

 

Maybe it has something to do with who has the bigger pipe ............just saying.
Realistically, with a turbo at WOT..you cant get the pipe big enough...but for normal driving? Another story....

 

Huh… Hmm… if you have been following this thread, I've done a lot of testing of different DPs...
The stock 1st cat is NOT a problem, it is huge, & flows really well.
The resistance is at the 2nd cat, & 'coring' it, removes the resistance...
A stock DP, with a 'cored' 2nd cat, works better than a 'catless' aftermarket DP...


Here's an update on my exhaust system:
https://www.chevyhhr.net/forums/hhr-...-hhr-ss-50328/

Here's an update on my brake rotor selection:
https://www.chevyhhr.net/forums/hhr-...lection-50326/

 

As far as the downpipe a 3 inch supports enough flow up to 700 or so horsepower that's when Ryan@zzp had to go to a larger 3.5 due to the restricion.

 

Correct...
The stock DP is a true 3" DP...
Some guys think that there is a restriction at the O2 bung area, of the
stock DP, it still is effectively, 3", in this area also.
The 2nd cat is the restriction, & most guys find it's easier, to just replace the
whole DP, then reworking the 2nd cat, by, "coring" it.
It's so easy to "core" it, without removing the DP, that I wish that guys would
try this 1st, then do so testing, before replacing it.
The loss of low end torque, with the same top end performance, should convince
most, especially the automatic guys, that there's no need to replace it.
But most will replace it, because, "racecar"....

 

Your saying that if I just remove all the CAT guts from the second cat, I will have a better performing DP than an aftermarket downpipe with a straight smooth mandrel bend, also not mentioning the weight savings (I know 15lbs isn't worth much)

 

Just read this thread from page 1 to 6 and have 2 questions for 2005HHR auto.
When coring the 2nd Cat using 2 3/4" tubing, you say to push/pull it. Is the core of the cat that soft? Or did you make some sort of a core drill out of the tubing?
Also, of the pictures I've found of the muffler, it looks more of a chambered design, then a straight though as you say.
Do you have a picture of a cut open one to show the straight through flow?
Thanks