could adding a cat bypass induce stalling?

[pjm1]

We've been doing some experimenting with our recently acquired 1986 Carrera and my 2 sons are definitely pushing for a more powerful sound. One experiment we did was install a cat bypass to see how that changed things. It seems that one side effect of this was that the engine sometimes stalls now when it's cold and when the car is not in gear. I've poked around on the forum and have heard some discussion about a lack of back pressure potentially causing stalling. I think perhaps that could apply here given the free flowing nature of a cat bypass. Is that right? It seems, at least anecdotally, that the stalling goes away when we put the cat back on. I'll do some more testing to make sure but wanted to run this by you guys to see if it made sense.

Also, I believe the best way to play with the sound of the car would be to get a sport muffler of some sort. So that's on our list of things to investigate next. Any thoughts on that are certainly welcome. I'll read up on this topic more as I'm sure there's lots of discussion already out there on this.

Thanks, guys, and have a good weekend!

[tirwin]

I don’t have much experience with the 3.2 and Motronic. Do you have an O2 sensor installed?

I just swapped out the factory heat exchangers with headers on my 3.0 SC and I had to adjust the mixture. It was too lean at idle. That sounds like what’s happening in your case. I would think the Motronic ECU would adjust for that.

I think when people get Steve Wong chips this is the kind of info that is necessary input for choosing the right tune. So do you possibly need a chip too?

[BER]

I replaced the cat and OEM muffler on my 84 911 with a Dansk pre-muffler and Dansk sport muffler. Good sound, a bit more power, engine runs cooler and runs great. Make sure your pre-muffler has the O2 sensor bung and the O2 sensor is connected. The sensor is an integral part of the Motronic system.

You might also consider a Steve Wong performance chip. Plenty of posts on this forum about the benefits of a SW chip. I have one and it really awakens the 3.2 engine.

Have fun!

[proporsche]

i would like to see the strait pipe you put on..however i do not like the oxy sensor connected..if you have it connected -disconnect..and see.It is possible that you need to adjust the fuel mix-might be a little too rich.
The oxy sensor`s main thing is adjusting your c/o level for emission purposes...no need to have it...in CT you have a an emission test or not? i think not..

Ivan
big YES for Steve W.chip that is for sure;-))))

[RarlyL8]

The un-tuned OE Carrera heat exchangers sound terrible when straight piped, both in tone and volume. The only way to get a good tone is using tuned headers. These and a minimal muffler can be had used on the cheap if just playing around, just make sure they are 1-5/8" version. Ditch the O2 sensor and you're ready to terrorize the neighborhood.

[RedCoupe]

I have a M&K bypass on my 86, and have no problem with stalling, whether the O2 sensor is connected or not. I would check for vacuum leaks. The gaskets from the intake runners to the heads are known to "suck in" when they get old, so you might check around the base of all the intake runners for leaks as well as the rubber boots on the manifold and all the vacuum hoses. Also, the vacuum hose that runs up to the brake booster is likely to develop cracks if it has never been changed. You can clamp it off or make a plug with another hose and see if that makes it run better.

[pjm1]

Originally Posted by tirwin

I don’t have much experience with the 3.2 and Motronic. Do you have an O2 sensor installed?

I just swapped out the factory heat exchangers with headers on my 3.0 SC and I had to adjust the mixture. It was too lean at idle. That sounds like what’s happening in your case. I would think the Motronic ECU would adjust for that.

I think when people get Steve Wong chips this is the kind of info that is necessary input for choosing the right tune. So do you possibly need a chip too?

Hi Tim. Yes, we did reinstall the oxygen sensor in the cat bypass. Thanks for your thoughts!

Originally Posted by BER

I replaced the cat and OEM muffler on my 84 911 with a Dansk pre-muffler and Dansk sport muffler. Good sound, a bit more power, engine runs cooler and runs great. Make sure your pre-muffler has the O2 sensor bung and the O2 sensor is connected. The sensor is an integral part of the Motronic system.

You might also consider a Steve Wong performance chip. Plenty of posts on this forum about the benefits of a SW chip. I have one and it really awakens the 3.2 engine.

Have fun!

Thanks for the advice. I'll need to look into the Steve Wong chip and check out the Dansk sport muffler!

Originally Posted by proporsche

i would like to see the strait pipe you put on..however i do not like the oxy sensor connected..if you have it connected -disconnect..and see.It is possible that you need to adjust the fuel mix-might be a little too rich.
The oxy sensor`s main thing is adjusting your c/o level for emission purposes...no need to have it...in CT you have a an emission test or not? i think not..

Ivan
big YES for Steve W.chip that is for sure;-))))

Hi Ivan,
We put on the fabspeed cat bypass here:
https://www.fabspeed.com/porsche-911-carrera-european-pre-muffler-cat-bypass-pipe/
Interesting - we'll have to try taking the oxygen sensor out - we haven't done that yet. And you're right, for a car of this age, we don't have emissions tests in Connecticut.
Thanks for your advice!

Originally Posted by RarlyL8

The un-tuned OE Carrera heat exchangers sound terrible when straight piped, both in tone and volume. The only way to get a good tone is using tuned headers. These and a minimal muffler can be had used on the cheap if just playing around, just make sure they are 1-5/8" version. Ditch the O2 sensor and you're ready to terrorize the neighborhood.

Thanks!

Originally Posted by RedCoupe

I have a M&K bypass on my 86, and have no problem with stalling, whether the O2 sensor is connected or not. I would check for vacuum leaks. The gaskets from the intake runners to the heads are known to "suck in" when they get old, so you might check around the base of all the intake runners for leaks as well as the rubber boots on the manifold and all the vacuum hoses. Also, the vacuum hose that runs up to the brake booster is likely to develop cracks if it has never been changed. You can clamp it off or make a plug with another hose and see if that makes it run better.

Thanks, RedCoupe. I think that makes sense. I'm on the very low end of the knowledge spectrum here so I'll need to learn a little more about how to check for leaks so i'll poke around on the forum on that. One thing I did that could be an issue was I didn't yet install the new gaskets as I only wanted to compress them once we were sure we wanted the cat bypass and we were installing it permanently. Right now we are switching it in and out, trying various combos of things so I'm just using old exhaust gaskets that I imagine aren't sealing so well. So that could be an issue for me. Although it seems, even with the old gaskets, that the car doesn't stall with the cat but does with the cat bypass. Probably a big rookie mistake to use the old gaskets as we test but I just wanted to save the new gaskets for final install when I then hope to not touch them for a few years. Thanks a lot for your advice!

[E Sully]

I installed an '86 engine in my 1973. I used my '73 SSI's and a low back pressure Bursch Turbo muffler. It ran very smoothly, with and without O2 connected.
https://youtu.be/93wgjvunlTo
I think RedCoupe may be on the right track. Make sure all intake runners are snug, and check for vacuum leaks. One of the issues the engine I had was a leak in the area indicated in the picture. The rubber was cracked and leaking.


Also make sure the Idle Control Valve is clean and set properly.