b6 b7 s4 pre cat removal the must do mod.

B6/7 S4/RS4
#1

Being a b6/7 s4 (A6 and allroad guys to) owner you only really have two major worries. 1 is the timng components and 2 is the scored cylinder walls that come from a warped block or uneven piston expansion. What we have found out over the years is that the precats hold in way too much heat and some of that heat makes it way back into the cylinders causing uneven expansion of the piston or worse the warping of the block. The good news here is you can help reduce motor issues by removing the precat. Even if it’s too late and you already have a damaged block the removal of the precat will stop it from being worse and in some cases help reduce the amount of oil consumption.

The b6/7 s4 has the precat right at the exit point of the exhaust manifold. The precat is within 4 inches of the cylinder exhaust port. Not only is this terrible for power it’s a huge hindering to the long term health of the motor. To give you an idea on just how close the precat is here is a picture of the precat with the red arrow. The blue arrow is the exhaust manifold

http://audirevolution.net/addons/albums/images/948991046.jpg

If you go back just before 2001 there was no use of precats. A federal law passed in the late 90s required new cold start emissions laws. For the first few years after this law was passed company’s put the precats right up on top of the exhaust manifold just like the S4. Since then most car manufacturers have moved the precat further back away from the exhaust manifold giving the motors a better chance at breathing. A combination of that and better cat technology have removed most of the issues that are occurring with the S4.

For those of you who are never going to modify your car or worry about the possibility of pollution. You still should remove your precats. Stock cars are still effected with scored cylinder walls and warped blocks due to the issues with the precats. Even if you remove the precat there is still the main cats in place. The main cats are more then enough to keep the air clean. As a matter of fact no car with the precats removed that is still using the main cats has ever failed emissions due to the main cat not being able to handle the emissions load.

In this picture you can see what the main cat looks like.

http://audirevolution.net/addons/albums/images/283023169.jpg

The main cats are actually very good and can stand the test of even the highest requirements of emissions standards

Now that you can see just how close the precat is and just how harmful it is you can see why removal is a good idea. Even if your stock. In the end removing the cat helps keep your motor happy and adds a few more hp. It’s a us do can’t go wrong mod

#2

Thumbs up! Do you have more info or tips about removing them from the car and clearing them out?

#3

I think he means PRE Cat instead of MAIN CAT in the last paragraph.

#4

Fixed thank you for pointing that out.

I’m going to try and make something today. I know you were going to try this today as well so maybe you can add some pictures and thoughts

#5

I got the whole job done in about 6 hours. Pulled the downpipes, gutted them, reinstalled them and put the magnaflow exhaust kit on. The DIYs that Justincredible has written are very useful, and I really only would add two things to them.

  1. If you’re installing an exhaust at the same time as you’re doing your downpipes, leave the bolts on the DP connection to the manifold a little loose so they can move around. To get the whole system to align well, it’s helpful to have a little bit of flex there. Then don’t forget to tighten them when you’re done.

  2. The DP mounts to the transmission with the springs are a little troublesome. If I had it to do over, I would take pictures of them before so I made sure they aligned the same way. The tab that bolts to them could probably go on either side of the flange if you leave the sleeve out, which will skew the way they sit in there.

I cleared out the precats using a speedbore 12" extension and a 1.5" spade bit and it only took a couple minutes.

As for the worthiness of the mod, I’d say do it. It’s hard for me to qualify the change with just the downpipes, since I also installed the exhaust, but either way it seems to rev faster and has a good sound to it. Seeing as it’s a labor-only kind of a mod, you can’t go wrong. The magnaflow setup might be a little quiet… I’ll need to live with it for a couple weeks and see if I need to remove the resonators or not. New exhausts take a while to settle in anyway.

Also, I put O2 sensor spacers on that I got on ebay and the CEL hasn’t turned on yet. I will be doing the JHM ECU Flash this week once the cable comes in from datadatum. I have high hopes for that flash.

Hope this helps a little.

#6

its been my experience that magnaflow gets louder as it gets older.

#7

I have been considering this mod but am a bit confused as to where the o2 sensors go after gutting the pre cats, do you leave them as is, or relocate them?

#8

If your plans are to get a jhm tune just do nothing and get there tune. That will eliminate any issues and give you a ton of power and excitement. If your not going to get a tune you need to take the rear 02 and place it just after the main cat vs just before it.

The magnaflow is mostly glass pack so after a while the glass pack gets more and more packed down. That effects the sound and tone. If you drive like an old lady the sound will never change. But if your like me and your an on ramp warrior it’s not going to take long for the sound to change and the exhaust to get louder.

#9

Thanks!

#10

Whats the book time to remove/install DP on a B7 S4 Tip?

#11

Sorry just saw this. I think. Book time is 4 to 6 hours. The tip is a little harder to pull the pipes on due to the limited room of the bigger transmission

#12

Ya. My local shop struggled some at getting em out past my tip in the allroad. Think it was still about 6 maybe 7 hours total

#13

Ya. My local shop struggled some at getting em out past my tip in the allroad. Think it was still about 6 maybe 7 hours total

#14

Department of redundancy department. ;D

#15

Echo

Echo

#16

Thank goodness somebody is listening. :wink: