Cat Positioning....

[GBX]

I notice that certain HFC's have the cats in the same place as OEM (a la Fabspeed) and some have them further back in the x-pipe (akra). What, if anything, is affected by cat placement? Are you less likely to throw a code w/ the cats closer to the motor? how is performance affected, sound, etc?

[aus]

The only way to not throw a code is to have them in the stock primary location. Even then, you seem to need 300cpi cats to not throw codes.

[SleepyE90]

The OEM midpipe has two sets of cats, primary and secondary.
The Fabspeed pipe has high flow cats in the primary location just like the stock midpipe.
However, the Akra deletes the primary cats and has high flow cats where the secondary cats would normally be in the OEM midpipe.

[FrenchBoy]

Interesting question. I would love to get some with some knowledge chime in.

Also, on a related topic I believe, I feel like I get more gas smell with my Akra exhaust than I did with the stock system.

[aus]

Stock has FOUR 400 cpi cats, while the Akra has TWO 200cpi cats.

[FrenchBoy]

Originally Posted by aus

Stock has FOUR 400 cpi cats, while the Akra has TWO 200cpi cats.

We all know that (actually the Akra has two 100 cpsi sports cats). That's not what the OP is asking.

The questions is: Why do some vendors place their sports cats between the 2 sets of O2 sensors (i.e. to presumably somewhat affect the reading of the second set of sensors), while others like Akrapovic places them further down the exhaust line after the second set of O2 sensors.

[mixja]

The most important part of the exhaust system in relation to performance is in the first 25% of the exhaust (i.e. extractors and down pipe).

Eliminating the primary cats either totally or moving them to mid-pipe area of the exhaust basically reduces back pressure (because exhaust gases can flow far more freely through an open pipe) WITHOUT detrimentally affecting exhaust gas velocity (it in fact improves the velocity due to lower back pressure and the fact the open pipe has less volume than a catalytic converter) assuming all other things remain equal. So the flow is less impeded and the gases can exit faster.

Because exhaust gas velocity is very closely related to exhaust gas temperature, removing the primary cats and leaving secondary cats (as opposed to leaving the primary cats and removing secondary cats) has far greater effect, since the exhaust gas velocity is much less due to cooling by the time it has reached secondary cats (hence any restrictions imposed by the secondary cats have a much lesser effect on performance that those imposed by primary cats).

So assuming the pipe diameter is the same or close to OEM (a bigger pipe diameter = lower exhaust gas velocity so generally avoid this unless you are purely focused on high horsepower), the primary cat delete will not only increase maximum power due to greater overall flow capacity, it will also increase low to mid range torque due to the improved exhaust gas velocity.

One caveat is that a reduction in back pressure can cause over scavenging, where intake charge is siphoned straight out to the exhaust system during intake/exhaust valve overlap. This is my theory about why the Akrapovic system causes a dip in torque @ 2500RPM, whereas the OEM system with just a primary cat delete does not (because the Akrapovic system will have a lower back pressure than the OEM system even with primary cat delete). This could be rectified by altering the valve timing at the affected engine operating ranges.

[Bob MG]

^^^ I found supercharging eliminated my dip at 2500 with my Akrapovic.

I replacing my X pipe with a catless GT4 Akrapovic X pipe as soon as it arrives. Just one set of resonators for me thanks