to go catless or catted opinions needed
 

sorry duanne 996tt should have been clearer, I am thinking about gutting all four cats out of a stock exhaust.
My options are:
gut all four cats
-might smell gassy not sure if this is only at startup?
-my flash will take care of the o2`s

remove secondary cats leaving primary cats intact
-will not smell
- pass emissions, but this is not an issue for me
-not be as free flowing? Not sure how much is left on the table

removal all four, reinstall 2 100 cell cats in the primary cat location
-wont smell
-probably wont pass emissions, dont really care
-flow just as well as catless? Not sure?
-cost of purchasing 2 new 100 cats sucks

What I really want to know with these options is running fully catless on a stock exhaust going to stink me out of the car at startup, traffic lights etc. with the windows down?
Or not?
Why would I do this instead of buying an aftermarket exhaust? I have had two, and I find them too loud for my taste inside the car, and obviously my car came with a stock exhaust so this is a cheap alternative allowing me to save for the higher end exhausts but still helping out my higher flow needs.
Let me know what you guys think

Straight pipes w/o out cats will give you an increase in torque and power but you will sacrifice some low end torque. I've not run an OBD 2 Porsche without cats. It must drive the ECU a bit crazy unless you have it remapped assuming no cats. Depending on your taste of sound, turbo's don't sound too terribly well with straight pipes (IMHO). I did have an OBD 1 (single sensor in front of the cats) '95 993 3.8 n/a that I ran without the cats and 100 octane chip hence no ECU issues. Without the cats and 100 octane it smells like a real car. Several board sponsors make modular exhaust systems that let you swap out between cats, mufflers, or straight pipes.

HI,I heard a complete gutted cat system with the muffler pipe hole saw drill trick on a X-50 flashed.. and to tell you the truth,my opinion,it sounded like a boat,,,,,Hollow sound,weird.......But the PCA member said it did perform alot better??Hope that helps :)

Thanks for the info John, I don't plan on doing the hole drill sound mod, just removing the cats. I have been told by an exhaust manufacturer and a well respected tuner who both have done this to there cars at one time (gutted cats only) they both say it retains all the stock sound(quiet) but flows way better. I have been told it's about 80% of what any aftermarket system gives you.
My concern is just fuel smell and what tools made the job easier. Spade bit, start small get bigger etc.
And if adding 100 cells in the primary location help with smell or just hold back flow?
Somebody here must run this, please help.

You know with my old muscle cars,I use to gut the cat,,,,then put a sraight pipe in for two reasons,,,,better sound[less hollow] and performance in which keeps the exhaust gases at a higher velocity and more uniform flow then going into the casing body and getting squeezeed down again.....I know Stef fro Europipe put a thread out recently about how to modify the stock cat system.....Hope this helps......

I does John, he has been helping me with this already. He just quite couldn't remember how he used to do it as it has been awhile for him. He says it will smell somewhat but again for him it has been a long time. Looking for someone doing this now and his tips to doing this efficiently and advice on the smell.

I don't have a 911tt but I got some test results that may help, my car comes with 2 huge brick cats stock. I tested in limp mode on my car just wastgate pressure. I was catless for a while and hated the smell.

Test with stock 2 cats on my car got 7psi in limp mode.

Test with only stock testpipe cat in car. 10-11psi in limp mode.

Test catless 14-15psi in limp mode.

Test with 1 vibrant 3inch 200 cell metalcore high flow cat welded into the 3 inch testpipe and 14psi almost like being catless! Drmatically reduced oder no more rotten eggs and only cost about 1/2 psi of boost. The one I bought was $120 3inch 200 cell.

Good info. I found some 100 cells mil. sec cats for $179 that apparently is big in the Mitsubishi evo world. Contemplating if $400 is worth not having the smell.

what smell do you speak of, I am catless and do not have a smell...

I am going to be in the process of taking off my Cargraphics HJS 200 cell cats and then replacing them with Speedtechs 100 cell cats very soon,,,,thats if I do not sell this Cargraphics system.I hope this fabrication will be worth it,,,but I am also going to change my stock factory headers with another set that I ported/grinded down.I did not coat them because all the coaters were honset with me,,,They agreed with me also,that since they have short pipes,stainless and double walled,,,,,that they retain heat quite well AND on boost the hot flowing exhaust gases will not cool down enough to make a difference inour set-ups......So,take that for whats its worth.....

Blockhead your making my day, if I can go without spending the cash on 100 cells all the better.
What did you use to remove the cats, how long did it take? Any tips you found?
You removed all four correct? Not just the two small ones at the o2 sensors?

Still looking for advice of which way to go. Sixty percent removing all four and running that way.
Need someone to push me over the top.

Gas smell. I had this on my catless Subaru STI. I hated it. On that car it was only about a 10hp difference and definitely not worth it.

I do hear that sti's and mitsubishi evo's are bad for stinky exhausts if catless. I have only received info from one 996 turbo guy who says it's nothing to worry about. And I just need a couple more Porsche guys to come forward. I am surprised there aren't more speaking there mind on the subject.
On these cars with a flash running a free flowing exhaust makes a big difference,
But thank you for the info

I think most of them are running with at least a 100 cel cat. That is what I will be doing. I have the EPL flash and waiting for delivery of my 3" speedtech cross flow. It has 100 cell cats.

How can you run catless and not have a stinky exhaust? Smells like rotten eggs. Unless electric car lol.

Had a 66 chevelle, 67 barracuda, 72 vette, 89 z28(I gutted cats) all these cars were pre cat and I gutted the camaro's myself. I didn't notice any bad smell with any of them. Atleast what you would except nothing hideous. No rotten egg smell as some guys say. But I want to make sure with this exhaust before I weld it all together than spending the 400 for 100 cell cats is justifiable.
I would rather avoid this extra expense if possible but don't want to drive around in something that the vapors will give me a headache and be forced to redo the welding work again to install two small cats.
That's all anybody back me up here?

I don't see how one car can smell and another one doesn't but who knows. I have buddies who say they don't notice the smell either. To me it was horrible I have to stay in my car for little while after pulling in garage, and forget about driving in traffic with windows down lol. I was actually surprised at how well my vibrant race cat cut down on the smell being its such high flow. 75% less smelly and no rotten eggs smell.

I had no cats on my 996tt before my current exhaust and never noticed any smells at all. I did a few long trips with the family and my (overly) sensitive wife never mentioned anything either. I plan to go catless again if I downsize my piping to make up for the loss.

On the other hand, I have a 1977 bronco without cats. On that truck, you can smell it from 15 ft away.

I think it has a lot to do with the fuel conditions. The 996tt is a very very efficient running car and I doubt the ECU ever dumps much more fuel than will be burnt in the engine. The older carb'ed cars are more of the "close-enough" mentality.

Every performance car Ive owned has been catless and I never had a problem with smell. A proper tune key. Boosted car+catless=great.

You guys rock!!!! Decision made, gutting everything!!
Will post seat of the pants results and what method I use. Contemplating just cutting it open and pulling the cats rather than drilling it all out. Unless you guys point me in another direction.
Will be comparing a full 3 inch straight thru system a fabspeed gen3 twin can and this soon to be gutted stock exhaust.
It won't be dyno numbers as I dont have access to one but I know what an aftermarket and stock exhausts feel like with a epl stage 2 flash on a k16 car. Hope to reclaim some of that quicker spool I lost while maintaining a quiet interior. Plus my costs, so far $150 for a used stock system. Keeping my original stock incase this all goes sideways.
Will keep you all posted
Thanks

Nothing like the smell of catless and race gas :D

HI Gearhead,I am glad you have your mind made up now.....But..I have a stupid question,since performance is an issue.....How do you know how much you are going to gain taking all the cats out because you said you were not going to do the hole saw trick for the mufflers......Thus ,that might be your weak link,the mufflers,no matter if you take all the cats out or not...Just my thoughts to help you out.....

a little late to the discussion but if i were and i was doing all the welding work I would flange the exhaust, it will probably take you an extra hour or so now but it gives you the option of changing out the cats to cat delete of 100 cell cats. Also, a better option for exhaust might be keeping the primaries and deleting the rest of the muffler, my friend ran this set up with good results.

The info I collected states that running a stock exhaust minus cats only you will gain back 80% of what any aftermarket will give you but still sound stock both outside and inside.
This info was validated from a tuner who runs this on his personal car as he too wanted to be able to make phone calls while driving.
I rather not say who as he himself is partnered up with other exhaust company's and I don't want to remove food off anybody's plate if you get my drift.
But he is very well respected and has the ability to test his results so I take his word like it's carved in stone.
Granted this is only 80% which is more than enough of a difference for me to buy a second oem exhaust and see for myself.
As far as running only the primary cats which would help emissions and any possible smell, a highly respected exhaust manufacture I have been speaking with who has helped me alot with ideas said that the removal of all cats is by far the best as any cat works like a bottle neck impeding flow.
It maybe the best of both worlds for guys wanting quite and flow. And for $150 it's worth a shot.
Will post anything I figure out.

Can you take some pictures when you open up the OEM exhaust and some pics of the work you will do on it. I'm curious what the inside of it looks like....

Of course, I will share what I learn.

By tune you mean pulling fuel down low rite? I tune only ant WOT not under 30% throttle so maybe that why mine more stinky under regular driving. If you pull fuel everywhere in the tune it eliminates the smell but also you gonn run hotter all the time as a result rite? Cause from the lean gets rid of odor but runs hotter? You are rite catless better for turbo! On my car a high flow cat only costs 5hp I know someone who dynoed before and after. So not a great loss of hp trade off.

I would atleast replace with straight pipe to increase flow.

That would be insanely loud. Something I am trying to avoid, most aftermarket systems I found to loud, for me.
This is not the case for everyone but I don't think I am alone.
On a 911 the exhaust tube for straight pipes is like zoomies on a dragster they can shot flames as well if that's your thing, but teeth rattling loud. I agree it's hard to beat that level if flow but the volume is not worth it.

You will notice the lost mid range torque. At least off boost. My car was measured at 3 PSI back-pressure at full tilt by GIAC running 1.3 bar sustained to redline. It's an off the shelf 70mm (European size) Remus exhaust with 100 cell cats. Cat-less and even cats with straight pipes require a compromise that isn't worth it on street car. You need the pipe length to get proper scavenging to get the torque.

IMO not worth the benefit to the environment or loss in torque to run cat-less or cats and straight pipes.

I am not running straight pipes, i am assuming your comment is pointed at roll racer posts in this thread. There will be some restriction being a stock exhaust which will help torque, running catless will just allow the flow to not bottle neck. The advice I have received from the pros I trust, if they say it works, it works.
But I do agree with you if this was a straight pipe application(which it isn't) you would lose any savaging as the pipe would be to short.
But thank you for advice, I will always appreciate the help. The more we share the more we will all learn.

I just meant a straight pipe in that section instead of the round part of the cat where gutted. I know what you mean about exhuast scavenging but I figured that was for naturally aspirated small engines. With turbo engines I always thought better flow yields better turbo spool etc. Better turbo spool means better low end rite?

Yes,you do need to put a pipe in that area,a perferated pipe from a muffler/auto parts store would be ideal, which will help for performance and a non boat sound,,as I mentioned earlier here........

Funny, a Europipe 60mm stage 1 exhaust with 100 cell cats has 2.65 PSI maximum backpressure at 1.3 bar near redline.
Our 70mm stage 2 quiet with 100 cell cats has 2.2 PSI max. backpressure.

Hi ,,,Have you read,,,Modified OEM thread,04-01-2007 and Does anyonehave a dynofor a k24/18 without an exhaust thread,01-11-2011.......It may help,,,Good Luck

I have read the massive drill hole mod post. Some guys i talked to are still running it but some either welded the holes back shut or moved on to an aftermarket system.
Don't think I am going to bother, dont love or hate the factory sound just want to try this first.
I hear there is some hp gain, but most guys did it for the sound change.

Like I said, just trying to help you, with info I remembered....BUT that one thread has Dyno sheets from A/B comparisons and the other is got that guy Porsche Ken that does this for a living......Really,I just wanted to confuse you more........lol........

Ha, thanks your doing a good job i'm more confused than ever;) but I do appreciate the help with ideas, keep 'em coming.

I know you said you are already doing it, personally I would not. I have 100 cel cats currently and can almost not handle the smell. On an older catless car that I had (mustang 5.0) I would smell like a mechanic whenever I got out of the car from the exhaust smell. Or like I just finished mowing the lawn. I do have a very sensitive nose though and if you haven't noticed it on other cars, you probably won't on this one.

Thanks for the input, yes I have already started, one side gutted. Was this on a 996tt the smell you complain about?
Just about all the exhaust manufactures make a catless system or at the very least sell cat bypass pipes and the ones I spoke with said it's not an issue. Maybe you do have a sensitive nose? I hear alot of cars have this problem but many say these cars don't, so for me I have gone to far to go back so fingers crossed.