Sorry this has been over done.

 

High flow cats only rob you of about 3HP, but keeps the car from stinking of un-burnt fuel every stop light instead.

As for placement of the high flow cats, at least few inches after the header collectors, so the high flow cats do not disturb the collectors scavenging to gain hp instead.

And to point out, you loss more HP by using 1-7/8" headers on a stock'ish LS-2 or LS-3 than the cats rob you, then using the correct 1 3/4" pipe LT headers isntead.


Understand header collector scavenging, that you need to get enough air flowing through the collectors soon enough to start the collector scavenging correctly from the other pipes to start to get gains on the lower RPM's, and the fact that even with the 1 3/4" header pipes, you are in no fear of them becoming restrictive at top end on a stock'ish N/A LS-2 or Ls-3 instead.

Hence headers do not work since they are just larger pipes, but the amount of air that the collector can draw out of the other pipes before the motor shoots gasses down them instead. The header pipping too large, and it does not get the needed volume speed through the collector for it to start scavenge the other pipes soon in the lower RPM instead.




As for the high flow cats now moved away from the head heat source, then become less effective, and most of the time there is not a big enough differential reading from the Front 02 sensors, and the rear O2 sensors isntead (not that the ecm wants to see and will throw a cat not working correctly code). The rear O2 sensor only monitor the cats, nothing more, and since the engine does not need them to run in closed loop (less than WOT, when the car goes into open loop and even the front o2 sensors are not used), the with long tube header and high flow cats, the rear O2 sensors are deleted, and turned off with the re-turn needed for the headers.

This should cover your question, and if you have other, then feel free to post.
Note, once you get to the muffler, any performance muffler just becomes about exhaust note only.
As for 2 1/2" over 3" X pipe and back, it goes back to the motor, and if there is enough exhaust flow to make a difference. LS-7 and blowers, yes, but with a stockish LS-3 and LS-2, maybe a HP or two to be gained by the bigger piping from the headers back.

As for the LS-7 and larger engine, its single problem to start with it not the exhaust side, but the intake side instead. Hence there is not enough room under a OEM hood to get a big enough intake manifold for that engine to breath right from the start. If you take a look at dyno's on the OEM ls-7, it will rev out to 7K, but is starving for air around the 5.6K mark isntead. So here, you use a blower not to boost the hell out the compression, but instead feed it enough air so it not starving instead (why the 505HP motor turns to a 770hp motor with just a LS-9 blower and not a ton of boost either).

 

Thanks for your reply. Very informative. So just to be clear,you are recommending 1 3/4 tubing over the the 1 7/8? Most of the manufacturers recommend the later. Are they wrong? Plus if I am thinking of doing cam later would go 7/8?

Thoughts

Appreciate your time!

 

LS-2 and LS-3, 1 3/4" unless you are going to a very wild cam and heads, and plan on pushing the RPM's way up their as well.

It only with the LS-7 that you have it breathing correctly with a different intake manifold (and new hood) so it can breath correctly up to 7K, or super charging a LS-2 or LS-3 that you want to go with 1-7/8" header pipes.


On the super charged LS-2 and LS-3 your still going to loss a little lower rpm HP to gain some top end flow room (added boost means more exhaust out the heads) with the 1 7/8" pipe headers, but your not worried about the small loss in HP with the larger tube headers at the lower rpm range, since the super charger pressure is going to pile HP on in this area instead.

To add, always love this video, since the header pipe OD was way too large to begin with (1-7/8"). Hence when they started to dent the headers, they reduced the OD diameters down to the correct size (1-3/4"), and the motor made more power instead.

 

As always great information from Dano523.