Dropping my engine next week and seriously considering deleting my cats. Tig welding in a test pipe in each cats location. I know the gains are very substantial…but just how stinky will it be? Will it smell in the car? I had a carbed mustang and that was aweful…but it was carbureted.
Is it worth it? My wife just doesnt want any stench in the car (neither do i)
The removal of the cats will yield a big hp and Tq gain. That is one of the single biggest holdups next to the ECU restriction. So your on the right path. You don’t really need to worry about putting in a test pipe. S4 owners from the B5 all the way to the B8 S4 have just removed the cats with great reward without the need to put in a test pipe.
Being that the S6 and S8 are wideban fuel injected cars your going to have a much better experience than with the old carb cars. Even my fox body with headers and fuel injecton still gave off a smell but wide ban cars are much much better at fuel control.
Keep in mind that the ECU uses a very rich mixture to warm up the motor. So on cold days the car is by design going to run rich. In these cases you stand a good chance of smelling something.
Your best bet to cut down on all of this is to get a JHM tune and request that they give you a cat delete cat warm up delete file. This stops the over rich condition and helps reduce out as much of that rich condition as possible. Add in that and the huge jump in power from the JHM tune and the fact that you would have a CEL if you didn’t get the JHM tune its a home run in side of a home run with a grand slam.
Do it. Everything Justin said especially the tune to take advantage of the cat removal.
I believe just cat removal is probably good for 30+ whp. Throw in the tune and it will feel like a new car.
What else are you doing service wise while the motor is dropped?
Sidetracking the conversation slightly, what would be the ideal exhaust system? With my limited knowledge and just off of researching some, isn’t a catless setup illegal and will cause the car to more likely fail an inspection (unless you go to John Doe’s under the table shop for a freebe pass)?
We all KNOW the JHM exhaust is the way to go, but would you guys do any modifications to that exhaust system? Like cat delete, or going from S/S-line duel exhaust tailpipes to RS duel oval tailpipes? And if you did go the RS route, would you stick to 2.75in or 3in oval tailpipes?
I’m just asking because I’m not quite there YET for the exhaust, but it would be potentially on the to-do list come December. It would be sweet to add in the RS6 diffuser and have 3in RS oval tailpipes, but does that sacrifice performance?? ???
Thanks. Tune will come eventually but cant swing the extra 700 right now. I already have a CEL due to the change over flaps being inoperative in the IM (and it is gutted…). Eventually will put a new manifold on it, but its quick and easy to do in vehicle. Good idea about the cat warmup delete tune. Only reason the cars run like crap on cold start is due to rich mixture and very retarded timing solely for quick cat light off.
I mentioned before by current boss was one of the developers on the V10 developers for the s6 and s8. First question he asked me when he realized I have the S8, “Did you punch the cats out yet?” The primaries are ridiculously restrictive. Necessary for such a high cell count due to the engine running very dirty thanks to the primitive low pressure DI system. It would not meet emission specs without those restrictive cats.
Regarding removing the cats, just physically punch them out? My only concern is going from small pipe to huge opening back to small pipe. Hence why I wanted to weld in a test pipe, but i thought about the isolite heat shielding, and i dont want to be ruining that…
Reason for engine drop is oil leak…and mysterious coolant leak.
Oil leak are rear timing covers dropping oil onto exhaust. Also the upper cam carrier leaks oil between it and the lower cam carrier into the spark plug tubes. Timing system has to come apart for that.
Also have dissapearing coolant.
I did a pressure leak down test on the system. Held 1 bar for an hour just fine. Also did a block tester (checks for CO2 in the coolant), that also passed. I see no coolant residue anywhere else.
I used accustrip test strip to check for glycol in ATF and engine oil. Those strips are useless…the scale is completely indistinguishable for a positive test if the oil is dirty. I called accustrip before and they assured me it is fine for engine oil…the positive test for glycol in oil is supposed to turn the strip brown…the oil is brown…
Anyway. also my little flex joints are leaking exhaust, so welding new ones in, and finally will be deleting the 4 resonators. Im still on the fence about deleting the cats, but…well see.
GRS, tailpipes have nothing to do with exhaust performance unless they are teeny tiny (undersized).
NA engine power comes from good breathing in and out. Equal length long tube headers that are properly designed for good scavenging and velocity are the big ticket. Following that is a good Midpipe which allows further scavenging (X pipe for example), then an nonrestrictive pipe diameter into a non restrictive muffler/tailpipe. yes being catless is illegal, however since I and alot of others are in MI, no inspections or testing are required.
I’m in the planning stages to remove my engine as well. I have no caveat about gutting the cats or removing them.
In other cars I’ve gutted the cat an welded a straight pipe through it for a visual pass in our cars only the bottom cats are visible so I may do that there. We don’t have emission testing where I am.
i am just hoping I can move the O2 sensors somewhere more accessible, but main goal is to just ditch the restriction in the cats. I’d love to do the $7k headers, but can’t justify that at this time. Canadian dollar is taking a beating so some of my plans have changed.
Yep on point there - i replied to this but it didnt make it through…Cats are literally big sponges on the back of your motor, designed to block the flow of “bader” air stuff coming out. Any race system always dumps this instantly, bikes incl. you can punch the insides of cats out but by the looks of the std ones and the fact your dropping the motor, cutting and welding in new pipe to match the old size will make a large difference.
You shouldnt notice any major smell change aside from the cold start as justin said. The old carbed muzzy was probably poorly tuned/out causing the smell.
Do it!
It was well worth it for me.
https://www.dropbox.com/sh/sgft6l1ldb0vq8j/AADD1MxEeHuIT1zTYowfCk0ra?dl=0
you have real headers though 
That whole exhaust system is pure porn.
As mentioned by Greg and others. The best formula for the best performance for a NA is
Equal length headers with a proper collector with a collection chamber to a long stretch after the collector slightly necking down. From there stepping up to the max size the 5.2 would need for transitions in acceleration and that’s 2.75. A proper X in the right spot depending on the overall size of the vehicle usually around the end of the transmission is good and from there flowing back to the bumper to non restrictive mufflers.
As for gutting the cats. Its a good safe bet for more power. Its hard to say how much. I looked back over a few posts and I’ll have to post a link one member dropped out the cats and picked up almost 70hp. That’s serious.
To take out the cat you just use an air hammer and a long extension. The cats latterly fall apart in about 10 min. make sure to have a lot of water and spray the cats with about a 1/2 gallon of water before you start. This cuts down on the dust that is toxic.
There hasn’t been extensive research to show that the empty cat can in place causes any major loss in power but… If you r able to common sense tells you to remove it and add in a straight pipe for better un interrupted flow. The best part about gutting the cats is that you can leave the can in place and you will never have any issues on a visual inspection as the cat can is still there.
When it comes to exhaust there is a lot going on right now and its exciting. We know full length headers built right with a functional merge collector and all supporting parts are good for about 100whp and 130+wtq that’s monster. There is build going on with unequal length headers with what looks like a simple collector non full merge and to be honest I think biased off what we have seen in the past I think you can get close to those same results with gutting the cats and putting in a straight pipe.
It will be interesting to see how everything unfolds but the great thing is this forum is full of excited knowledgeable guys willing to share and work together. That says great things for everyone.
Those power gains are nuts. Simply nuts. But picking up 7+mph in the 1/4 is proof in the pudding.
I agree. Dyno numbers are easy to flub even if your not trying. You can’t fool the clocks and the speed of the track.
Posting the vid from AR in the Audi Performance group on FB caused a total shit storm yesterday lol… So many people complaining how the gains quotes in the video were impossible. We defended the honor of CV though lol
Just joined to raise hell with y’all, ha.
What FB group?
If you want to go somewhere where there are a buntch of ass hurt loosers who have been kicked off most forums the FB groups are a great place to go.
CV has helped lots of companies make sure seriously fast cars and the same loosers on FB said that the 4.2 would never get into the 12s on all motor and they also said the 5.2 would never get into the 12s on all motor. Its a laugh fest.
Overall on the 4.2 I can look for the dyno sheets but the JHM headers add 50whp and 50wtq with nothing added. Take the 5.2 and do the same thing your going to get the same great results. like i said the other day. One guy on here had a gain of over 60whp just from pulling the cats.
So in closing FB is where people who dont know about cars and have been kicked off the main forums go… Its a bit of a waste of time.
JHM provided those RS style exhaust tips? I always liked the JHM logo welded on to the exhaust tailpipes in that video, lol.
This is spot on.
So I ran JHM straight pipes for almost 3 years; until literally no shop within 25mile radius would pass me, and car nearly was towed, so I had to put cats back on :’(
I would get occasional rich fuel smell in the cabin, and it was loud AF!
Just like Justin mentioned above on cold days I would get a CEL and misfires occasionally within 5min of starting up the car. I really should have got it tuned by JHM after the straight pipes.
Because my 4.2 is supercharged the notice from reverting back to cats was very noticeable drop in power, especially after 3500 RPMs
Costed me 18 shop hours / $2300 final bill to swap the cats back. Shop could not separate the sub frame and hang the motor because of the supercharger left only 1 available spot to hang from instead of 4, so they had to drop the tranny to get to the pipes. And living in UT all the nuts and bolts had rusted over.
I would only reinstall the straight pipes again if I could get it tuned by JHM and not ever worry about emissions again ( I would need to change my address to park city / st george UT)
Good info bigserve. First hand experience is always a good add.