How many cats to lose?

B6/7 S4/RS4
#1

So I’ve now sourced an airbox from a member, taking care of 1/2 of my previous owners mods.

Next up is swapping GIAC tune for JHM.

While the ECU is out, I was going to gut my DPs to kill 2 birds with 1 stone. The car (B6 S4 6MT) has about 95k on it, so it’s likely those cats are well below efficient. I did some searching on AZ, but got lots of bad or non-info, so wanted to come here for help.

My question is: is there any real/tangible evidence to gutting pre-cats AND cats, or do you actually lose backpressure/scavenging going full flow? I know smaller/FI engines sometimes need backpressure, but don’t know the nuances of the 4.2 well enough yet. I don’t have the funds right now for a catback (unless the guy selling his FI non-res wants to drop price a lot!) so would I be doing more harm than good gutting everything?

It looks like the job isnt hard, just time consuming getting the DPs out and back in. And since I don’t have a lift, I dont wanna pull the pipes TWICE with jackstands if I don’t have to.

Thanks

#2

Oh, also, while I have the car up on stands and waiting for the ECU, would it be any easier to tear off the front end and replace the snub mount? It’s a mod on my list, but I heard its a bitch to mess with. I figure if the car is out of commission for a few days, I can take my time with that. I’m not ASE or anything, but I know my way around a wrench…although apparently the worst part is getting the bumper aligned properly (HATE bodywork)

#4

Before I went headers I ran around with fully gutted oem downpipes for a while. There’s a significance bump in performance over stock, and IIRC a little bit of a gain over piggies. Your car is a B6 right? Its pretty easy to replace the snub in these cars. It takes ~25 mins to pull the bumper and drop the rad support enough to reach the mount. Generally if you’re going to drop the front you might as well swap out the pulley/serp belt and maybe idler pulley/tensioner depending on your miles. So if you’re not ready to pull the trigger on a pulley and other odds and ends you might want to consider waiting so you don’t have to go through the process twice.

#5

Emissions here is OBD/visual only, no sniffer. So with the JHM tune I’d be fine catless (correct?)

And I guess what I was looking for was gutting piggies to save money for catback. I could go 2.5 catless DPs, but that’d drain most of my funds for catback, and honestly I think I’d see better gains with full gutted stock DPs and a catback than catless 2.5s and stock catback, correct?

I wasn’t planning on doing a pulley or belt overhaul, at least not anytime soon. Is that something I should reconsider given that the car has almost 100k? Only reason I was going to go with the snub is its cheap and there is minor slop in the driveline when engine braking or getting on/off gas in gear.

#6

If cost is a factor, gut the stock DPs and get a good exhaust.

In regards to the belt/pulley, definitely change it since you will have the car in service mode. The parts are not bad in regards to price and it is fairly easy to change them out.

#7

There is no loss in power for removal of both the precat and the post cat. There is no loss in power in removal of the precat and not the post cat (over stock)

Keeping the precat in and removal of the main cat is a bad idea as it really helps nothing.

#8

Here’s my only concern with that: If I replace anything that has a performance alternative, I’d always opt for the performance version. I’m not sure what a stock pulley goes for, but JHM wants 350$ for a LWCP. My current CP/belt system has no signs of needing replaced, other than preventative maintenance as the car is getting up there in miles. By the time I got a belt, tensioner and that pulley, I’d be 500$ in the hole for something that’s “not broken” to begin with. I agree it would be convenient, having the front of the car off and whatnot…I just can’t afford it.

So maybe I just do the tune and gutting, and leave the front end alone for a few months to save up for a LWCP (and then do the snub while I have the front end apart then). I don’t mind wrenching on the car and dis/reassembling stuff.

I was never planning to keep the pre-cat; I am gutting the pre-cat no matter what, I was just curious about the benefits of gutting BOTH cats versus the potential loss of scavenging/backpressure. But thank you for the reply.

#9

Taking the front off isn’t all that hard, but is a bit time consuming. I think you would be fine just waiting a bit and knock them all out at once.

You will be better off with gutted piggies + catback vs. 2.5" dp’s + stock catback. If anything you’ll likely see a very slight gain gutting all 4 vs just pre-cats.

good: piggies + 2.5" catback
better: 2.5" catless dp’s + 2.5" catback
best: headers + 2.5" catback

You can find 2.5" catless dp’s for $500-ish if you can swing it.

#10

My concern with waiting is that the bill will be tremendous (550$ for tune, 350$ LWCP, 180$ belt/tensioner, 40$ snub mount = 1100$ +tax/shipping). I guess thats not much for some people, but between car payments and insurance, GAS!, residual hospital bills from last year, etc…that’s about 4-6 months of saving for me. Meanwhile, I’d be riding around with 100k mile clogged cats and a piss-poor GIAC tune.

I guess I’m just curious if the LWCP is worth it if my current one has no issues? $500+ gets me well towards a catback, and while I’m not denying the gains from less rotational mass on the engine, I think I’d enjoy the catback’s performance/sound a lot more, at least from a bang/buck ratio. The current belt seems sound/tight and I’ve no reason to suspect the CP is on the way out, is there any reason to replace it as of now?

I wholeheartedly agree that, if money were not an issue for me, I’d swap pulley/belt out, as well as snub, trans and diff mounts while I was gutting my cats and getting the ECU flashed. It’s more efficient and less downtime for the car. But I’d rather not take on credit card debt or be strapped for cash for mods/repairs that aren’t warranted. (And yes, I know ALL mods aren’t NEEDED, but in this case I’m referring to fixing an improper tune, and a free exhaust mod).

#11

I hear ya. I’m all about bang for the buck.

In your shoes I would do this.

  1. Piggies (free if you diy) + catback ($700-ish if you do magnaflow or piece together something custom)
  2. Tune ($500-ish)
  3. add then add lightness/stiffness

The single biggest/cheapest n/a gains you will get from this car is getting rid of the pre-cats, getting a good catback, and JHM tune. Your car will be a different animal. It’s a bit of diminishing returns from there.

#12

second…piggies vs. fully gutted stockers isn’t a big enough gap that is worthy of failing emissions

piggies (gutted precats) or 2.5 catless or headers

#13

@Slideways, not the crank pulley, the idler pulley. The idler pulley and the tensioner are known to get noisey over time. If the belt et. al. was done recently, then don’t worry about it.

#14

OOOOOH.

I was (obviously) confused. I’m all for less mass, and have a clutch for when I save up enough/need a new one and add a LWFW and LWCP all at once. I was confused as to how that would have much to do with the belt at the time.

JHM has the tensioners for like 120ish I think, but yeah I’m good for the moment (or so it seems; no slack, squeal, etc)

#15

I have (2) sets of Downpipes

  • 1 fully catless milltek set
  • 1 set of piggies w/ main cats still in place (I can pass emissions with these)
#16

Piggies and then sell for catless 2.5" if you want to go catless. Once you fully gut the oem dp’s, they’re not worth much, but some people are willing to pay decent money for piggies to avoid doing the job themselves.

#17

I’m one of those guys. I’d be worried of messing up the job, so I bought secondhand piggies (saki’s) so I knew they worked :wink: http://youtu.be/DMQwSUM5Osc

#18

I’m def in the “DIY to keep costs down” camp.

Will be tearing into this next time I get a day or 2 off of work or can find a car to borrow.

Will pull the ECU, send it out for tune. Hopefully have it back in like 3-4 days?

While that’s out I’m planning to gut pre-cats and leave main cats in (main cats have 95k miles, but I don’t want to get that awful smell) and leave myself enough time to take my time with the R/I of the DPs.

I’ll worry about the snub when I change the tensioner down the road; everything is smooth with shifting, just some minor slop for now.

I figure I’ll have JHM send the snub and a VAG cable at the same time tho, to have on hand. Total cost, under 600$.

Thanks for the advice.