sakimano

So, I'm going to get 2.5" catless downpipes cooking soon (pronto) but I had a few questions

Question I - emissions- I know that in Ontario we have Californiaesque emissions standards, but I wanted to know...how do you actually fail emissions? From where I sit, there are a couple of ways for them to check
a) visual inspection - see if you have cats. Pretty simple.
b) CEL check - also simple, and avoided with my JHM tune (which deletes this)
c) OBDII scan for codes - again, JHM tune deletes
d) proper emissions test on your car on a rolling road - most test places here DO NOT have an AWD dyno, so they can't do this to my car

So, assuming this is true, I need to worry about inspection only, right? I wonder how realistic that is...I mean it's got to be pretty frigging hard to see where cats would be on an S4, doesn't it?

Here are OEM downpipes...from the looks of it, I bet the inspector can only see from the flex joint down to the main cats. i.e. he can't see where pre-cats would be (which could arguably be your main cats).


Question II - neckdown on my catback - I have a Fast Intentions 2.5" resonated X pipe catback (mouthful) but I opted to have it necked down to 2.25 or whatever it is to mate with the OEM downpipes (I have piggies).
Anyone got an idea about how much work it will be (cost, time) for an installer to fab up a replacement for the neck down piece? Anyone know how big the necked down section is? Anyone have any tips for potential problems with that part of the job?

Anyone here passed (or failed) emissions without cats? If so, please share your experiences.

Thanks.

mikeh

I too am in the process of getting catless 2.5" downpipes and am very interested in the response to this thread.

mikeh

BTW Sak who is doing your 2.5" downpipes?

sakimano

fab shop through a friend...

Mof

Interested to hear the answer to this too...
From a non-emmisions angle, do you worry about carbon monoxide exposure? Other gases? Clearly, garage idling is out, but what about people standing/kids playing near an idling car in the driveway?

mikeh

Who cares if you kill a few people...... its worth it

midrange

I once upon a time was a rebel and thought, my car will have so much more HP with no cat. Well the .1 of hp gain I got made me realize its not worth it. The only people that really get any advantage with no cat are people with Boost. Normally asperated cars depend on back pressure and that cat helps provide some of that back pressure.

I just stick with the rules and leave the cat on now...

sakimano

this not a legitimate concern in open air, unless the kids are wrapping their lips around the tailpipe...in that case, Darwin's theory would take care of them sooner or later. (kidding...awful thought). They are primarily to catch elemental carbon and to convert some CO to CO2 but again in an open environment this is not a concern. On your car this may require an engine pull unfortunately...the design is really weird.
p.s. if you want to test your launch control let me know...

not sure how to answer this tactfully. Basically this is entirely untrue as it pertains to the B6/7 S4. Sorry. I'm speaking from experience and from the p.o.v. of someone who has extensively researched this platform during my 3 years of ownership of it. Everything you posted used to be quoted in days gone by by the naysayers and 'experts'. It has all been proven to be hand-me-down myths that don't apply to this platform.

"the car needs backpressure" - old wives tale
a 2.5" free flowing catless exhaust with an x-pipe for exhaust scavenging and a tune that takes advantage of the flow will outperform a smaller diameter cat laden stock exhaust dramatically. If you like dynos, you're looking at about 30whp on this car. If you like street races, you're looking at 2-3 car lengths. If you like quartermile times, you're talking a couple of tenths off ET and picking up a couple of MPH. This has been proven time and time again in the b6/7 S4 community countless times over the past 2-3 years in all 3 metrics. Adding turbulence and restriction (i.e. cats) to exhaust flow certainly does not help in any fashion.

"you'll gain 0.1 hp"
On your car, maybe that was the case. I can't speak to that. What I DO know about is the B6/7 S4 and I can tell you that just dropping the pre-cat on the b6/7 S4 OEM downpipes made a substantial change to my car. Going with a 2.5" catless downpipe+exhaust+JHM tune as mentioned above will yield gains of approximately 30whp. This is not a guess...this is something we've seen time and time again.

Here's a dyno day summary for you to give you an idea what I'm talking about:
The 'Enhanced Street Perf.' dyno day (bottom one) had 4 B6/7 S4s there including a bone stock car. You can see the difference between the stock exhaust/tune S4 (hifitodd 264WHP/247WTQ) vs. a 2.5" catless downpipes + 2.5" x-pipe catback + JHM 93 tune (loucura 296WHP/280WTQ). The difference from those three mods alone was 32 WHP and 33 WTQ.

If you look at the 'superavant' numbers, he had a 2.25" downpipes with cats + 2.25" catback (there's your 'backpressure' model) and an APR tune...he dyno'd only 7WHP more than stock and made LESS torque! That's what happens when you listen to old wives' tales, and when you don't get a tune that is built for a freer-flowing exhaust.

'beemercer' had a 2.5" catless exhaust + JHM93 tune + JHM intake manifold. His car made 63 whp and 52 wtq more than stock...because the modified JHM intake manifold, coupled with the 2.5" catless fullback opens things up even more.

midrange

I see you trust yourself and that makes me smile.

As said, Normally asperated cars depend on back pressure due all the design and engineering of emissions and fuel economy.. On my DD, I am not trying to build a race car to gain tenths of a second.

You gain 30 whp by removing your cat. I don't think so, I don't believe it.

cheeba

Midrange, saki knows what he's talking about - he even provided you with way more of an explanation that I would have bothered to. He also never said that he gained 30wHP by removing only his cat. If you're going to challenge someone on this board, at least have the courtesy not to falsify their arguments.


What do you mean by "depend"? Are you saying that fuel economy and emissions will be negatively affected by less back-pressure in the exhaust system? If so, I'd like to see some kind of information to support this.

You're not entirely incorrect in saying that cars 'need' back-pressure in the exhaust, but in all practicality, you are wrong. If you have tubing running the entire length of the car - as nearly all passenger vehicles do - there is a point where having the tubing too large can reduce the velocity of the exhaust gases and have negative affects on performance; but, this is theoretical, and in reality you're not going to encounter this point.