Cat bypass
 

Does removing the cat really make a noticeable change in performance?

Nope

You will gain a very small HP increase in the upper RPM ranges at the expense of some low end response. At least that has been my experience.

I'd recommend going with a Euro-style pre-muffler. M&K and Dansk make nice ones.

do you consider back firing on deceleration an improvement?

damn thing!

I installed a Dansk bypass and a Steve Wong chip at the same time; the improvement was quite noticeable. Placebo effect maybe ...?

My car came to me with an M&K pre-muffler and Wong chip already installed. I've not driven it with the stock cat and chip. It does pull hard at high RPM, but I suspect most of that is the chip, not the lack of cat.

Mine has never back fired on me, ever. Not once.

It depends on the car. You should put which car you have in your signature so the responses you get can be more fine tuned to what you're looking for.

it will if your cat was plugged up!

I put the M&K premuffler on mine and noticed a gain but it wasn't huge. I got more bang for the buck from the Steve Wong chip. I have never had mine backfire either.

Audible hp

The benefit is small but it does remove quite a bit of heat form that side of the engine. When I changed out Oil return tubes the O rings on that side of the engine seemed a little more heat-fried than the right side.

also, there is a weight reduction!

Removing cat effect:
 

I actually dyno'd my car with and without a cat (I have an '82 SC; it also has a modified stock muffler, as in all the baffling is out save one). There was a 9 HP difference without the cat. I have a test pipe with an O2 bung and a functioning 02 sensor. The high reading is also with a modified airbox cover (cut both ends off, made a .3 HP difference. Hey, .3 HP for free in a 911 is a very good thing).
http://forums.pelicanparts.com/uploa...1323311336.jpg
The first two runs were with cat, the last was without. No other changes.

Cheers
Scott

Notice the drop in HP and torque below 3K rpm with the cat removed. This mirrors my "on the street" butt dyno and shows that the reduced back pressure with the cat removed negatively affects the low end response.

I'd love to see a similar dyno chart but with a pre-muffler. I'm guessing that the results would be similar.

Thanks for posting the chart, Scott!

true that, but:
 

Yep, you lose a bit off the line. But, in the working RPM (in my car's case, 2800-5200 RPM), you gain both torque and HP. For my purposes (AX/some DE/general street use on nice days) it works well.

I'd bet the "sport" stock muffler has more to do with HP/torque gains then the lack of a cat. Cat weighs a ton, as well.

The next performance upgrade will be SSI's and a two in two out. I'll dyno it again after that.

Cheers and hope to see you at a local event.

scott

I like to think of the backfiring as adding "character".;)

M&K one-in/two-out and premuffler, not cheap.

Third to second downshift and back out of the throttle in tunnels - priceless.

And, before anyone asks, I have no intention of growing up.

Nice to see some documentation of added power, not that any rationalization was necessary.

Agree with "Placebo" performance for daily driving. Probably more at the track. My bypass (not pre-muffler) made the car louder. Mostly I'm glad I removed the cat as the insides were mostly melted/gone/junk. Bit cleaner look now. Plan to get the SW chip and most say thats about all you can do to get more power without big $$.
I'd say removing the cat is a good first step, prepare for more noise.

[QUOTE=rfloz;6419396]I like to think of the backfiring as adding "character".;)

I haven't had any backfiring issues since I've removed the cat. Ran it at 120 mph today and 80+ consistently. Did some quick deceleration with that too.
Have the 3.0 mixture set at 3.5 CO.
I think I'm going to weld a Small retractor in the pipe for a bit of BP to see if I can sharpen up the off the line.
I do notice an improvement without the cat when really on it above 3k though.

backfiring?
 

Hmm, not sure what I'm hearing is backfiring. I like to think of it as a nice crackling sound. It doesn't bang, it crackles melodiously.

When I leave for events early of the AM, I am careful with the throttle. My wife says she can hear me accelerate through the gears on the main road outside our subdivision. Many complements on the sound from other track/AX types. Who cares what the civilians think.

Oh yeah, I've got the decel valve out of the fight as well, don't know if that makes any difference to backfiring or not.

Cheers and happy cat removing.

Scott

I think the proper auto word for that is "burbling" isn't it. What all hard run cars should do btw...