HHR08SS

that's it? do the "guts" just straw put after ?

2005HHRauto

Yes, just the 'core' that remains in that coring pipe, is removed from your 2nd cat.
There will still be some cat 'guts' left inside your cat,
in your DP, LEAVE it there, it is needed, & will still work…

What's straw put after ?

HHR08SS

meant "stay put after being cored" great info, thought I read somewhere to weld in a pipe after coring, this will be much easer!

2005HHRauto

With my 1st attempt, years ago, I did 'tack' weld a piece of pipe in there.

I couldn't believe it, that the remaining cat 'guts' stayed in place, after I
removed the pipe , a year later, when 'evaluating that DP.

So no need add support, as those cat 'guts' won't go no where…

2005HHRauto

A couple of pics, of the 2.75" 'core'.
Also in the pics, is a cat from the 1st position, & it was removed,
from one of the DPs that I was testing, & replaced with a straight piece of tubing.

That piece of tubing with the 6 rows of 'holes' in it, was 'tack' welded in one of my
DPs, when I 1st removed the 2.75" 'core'. I removed it one year later, & discovered
that it wasn't needed, as the remaining cat 'guts' left in the 2nd cat location, remained in place.
That 6 rows of holes tubing is from a stock automatic muffler that I had cut up. By the way, a 5 speed
muffler only has 2 rows of holes vs the stock automatic muffler, which has 6 rows of holes.

How weird is that, 6 rows of holes was needed in the automatics, for better sound control,
to meet federal standards… You would think, to save money, just make all with 6 rows of holes,
& call it a day. Maybe the engineers got their way, & said hay, the 5 speed buggys sound better
with 2 rows of holes…LOL


.
.
A collection of DPs that I did my testing with, both on/off the track.
With all this testing, just simply 'coring' out the 2nd cat, a simple free-mod,
is the best DP for our buggys…
I spend a lot of time, money, & welding, cutting up these DPs, to find what worked best…LOL

Why does simply 'coring' out the 2nd cat, is all that's needed…
1. The 1st stock cat is HUGE, & isn't a problem, in our stock DP & our little K04 can easily, & quickly, boost 30lbs thru it.
2. The ECM has 100's of tables, & some of these tables are probaby 'tuned' around this 1st cat/intake track volume.
3. The 2nd cat is a problem, it is small , & it does restrict flow.
4. Removing the 2nd cat, removes the restriction in our buggys exhaust system.
5. 'Coring' the 2nd cat, retains the volume of that 2nd cat position, & has a scavenging 'effect' on the 1st cat,
causing an increase in off idle & low end torque.
6. You can replace the 2nd cat with a 5",high volume, 100 cell, $450, Random Technology cat, & it will also cause,
an increase, in off idle & low end torque.
7. Caution, just plain gutting the 2nd cat, removing 100% of the cat material, causes turbulence, & will NOT increase,
low end torque, you need to carve/core a 2.75" hole thru the 2nd cat to have a scavenging effect, on the 1st cat.
RESULTS OF TESTING A 'CORED' 2ND CAT DP
12.39 best , at the track .
30 MPG with a 50/50 mix, [E47.5] , of Ethanol & Gasoline, on a 45 minute mileage test,
of really gentle driving, while staying out of 'boost'. You can't believe how hard it was, to stay out of boost…LOL
LINK TO A THREAD THAT SHOWS MORE TESTING / RESULTS
https://www.chevyhhr.net/forums/hhr-ss-45/dp-testing-ruthless-pursuit-power-43170/

jczboy

having the hardest time finding a 2 3/4 OD pipe. Everyone has 2 1/2 or 3. Did you get yours through an exhaust shop?

2005HHRauto

I had a LS1 Z28 before the HHR SS , & had some 2.75" 304 stainless left over
from doing an exhaust system on it.
Try a custom exhaust shop in your town.
They usually have a 'barrel' of left over pieces...

Firebuff

Thanks for the tip !!!!! Will be trying it soon on a spare DP.....So it takes a perforated tube or a solid one will do ????

jczboy

So after hunting everywhere in my area, with no luck, for the piece of exhaust. I starteded to look aroung the garage. Found a 2 3/4 hole saw and extention. worked like a charm.

Nightwalkerhemi

Notice performance?better sound?
Happy with the mod?I might do it this weekend.