Anyone feel like giving a quick tutorial on the various pieces, their purpose, and ability to work together or take out of the system?

Primary bypass
Secondary bypass
Cat bypass
Primary Muffler
Secondary Muffler
G Pipe
Headers
Others that I didn't mention

Maybe some of these are the same thing? Please forgive my ignorance.

Finally, anyone care to give a recommendation? I know opinions will be all over the place. Right now I'm bone stock. I want something that will pass emissions (or easily changed back for a test), but give me a little better sound and maybe a little better performance. Thanks!

 

Heh, I am almost in the same boat as you, trying to figure out all of the names and whatnot.

The primary/secondary bypass goes hand in hand with the primary/secondary muffler. I think you basically replace the muffler with a bypass (a straight pipe in essence).

Cat bypass is the same idea, a catalytic converter delete. This is what would most likely cause you to fail emissions, very horribly too actually.

G pipe is the secondary bypass for the secondary muffler.

Headers are the first part in the exhaust system, and is attached to the engine directly.

Hopefully I got those correctly. I came from owning only E36 M3s, to owning my first 964, so sometimes things get mixed up or there are different names for the same things.

 

Quick overview of the 964 exhaust system:

1. A pair of headers/heat exchangers sit underneath the two banks of cylinders they join together and exit on the left hand side of the car (black).
2. A catalytic convertor sits on the left hand side (red).
3. The primary muffler/silencer is along the back of the car below the bumper (blue).
4. The secondary muffler/silencer is on the right hand side and has the tail pipe exiting from it.

The Turbo system is virtually the same, but the cat section is replaced by the pipework which feeds the turbo, and the primary muffler is replaced by a cat.

 

Thanks!

 

Great visual!

On a secondary note, could we co a bit more in depth and ad some of the benefits behind replacing each part such as weight savings and power advantages VS noise level changes?

 

Replace the primary is the biggest first step - saves weight and increase HP with a nice sound improvement. Next step is to by pass the CAT - major HP increase and better sound / or replace with a sports CAT. You can also do a secondary muffler by-pass - by the sound and weight savings is not as good. All this is short of the third (major) step whcih would be going to custom headers or 993 headers and modifying all after. Depends on waht you want and how much $$$ you want to spend. There are some header systems out there that allow you to keep heat and others that don't -your choice.

 

Didn't I read (here) a while back that somone dyno'd the secondary bypass and found no gain in hp/tq?

 

I think that virtually the only difference from removing the secondary is increased noise.

In order of preference you would remove: cat -> primary -> secondary. Of course this is only where it is legal to do so.

 

This was not my experience. I passed my WA state emission test quite easily with a test pipe in place.

YMMV

 

In Oregon they require visual confirmation of a Cat... when they think about it... but I have failed it before, and had to go from B+B Headers to the stock system in order to pass.

 

I think someone said they passed sans cat in Texas as well.

 

same in georgia - -I had to go back to the stock system with only the primary by-pass the emmissions test - Tried it without the CAT once - but failed / sports CAT works though .

 

Yes, myself and another Texas lister posted that we both passed emissions with a cat bypass. In terms of visual inspection, quite honestly, I'd be shocked if the grease monkey doing the inspection could visually tell the difference between a cat bypass (with O2 in place) and an OEM cat. They usually are only vaguely aware that it is a "Porsh" 911 and always ask me if that thing is a turbo (kinda like the Dodge Hemi commercials). Why does everyone always assume that if its a 911, it must be a turbo....

 

I've been thinking about doing some exhaust experimenting using a dyno to determine where the most bang for the buck is. Right now, I'm running Cat Bypass, OEM primary muffler, and secondary bypass. The sound is great, but I'd rather have the horsepower and torque if there is more to be made. I've thought about trying two different alternate configurations to what I have now:
1) cat bypass, primary bypass (cup pipe), and OEM secondary
2) cat bypass, primary custom built 'sport free flow muffler', and secondary bypass.
Of the two options above, #2 should offer the most weight savings because I'd lose probably at least half the weight of the primary muffler and still lose the secondary weight too. I've thought about taking an OEM primary up to my local exhaust shop and having them cut both end pipes off and mate them to a straight through free flow muffler (like a glass pack). The whole custom muffler assembly would fit in the same spot as the OEM muffler and mate up on both ends using the orignal connecting pipes. My concern is that it may be too loud for street.

 

Have a go Greg and let us know. What we need is here is more imperical data (like your dyno graphs). And when describing sound levels to the 993 forum of my exhaust set-up, I used a sound pressure meter from Radio Shack (= cheap) to give absolute and relative values. You might consider the same.

What was the cost of the dyno btw? Is it by time or pull? Do they give time and space to make the changes you want?