I bought a nice low mileage 08 coupe a few months ago...of course got the bug for more grunt even though the car is insanely fast already. I found a set of slightly used ARH headers off a ZO6...1 7/8's with cats.
We installed them last week and the car is really a rocket now...esp top end and mid range...problem is ...even though we tuned it properly and the car runs very smooth...there is a terrible catless smell coming into the cockpit...happens almost constantly even if I don't get on it...idleing , cruiseing , etc. Closing the windows and running recirc helps a little, but the smell is there even with that. We verified there are no leaks. We are running zo6 npp takeoffs at the rear and running the front O2 sensors. As I said the tune is within 1% of ideal throughout.

Anyone else had this result? I got the cats because I want the car to be pleasant to drive...could they be defective? BTW they were run only a few K miles on the previous car and it was an NA car. ARH wants 675 for a replacement x pipe

 

I will reply to my own post....just read the thread on longtube zo6 headers...looks like you still get smell with aftermarket cats. I am wondering why they come into my cockpit so quickly...I am thinking maybe the hatch seal is leaking or maybe there is a hole somewhere in the firewall. Hoping to solve this problem. My last c5 vert had the stock zo6 manifolds and exhaust system installed with the cats and there was no smell...so I may have to go that route. Performance gain is still decent.

 

Just because AFR is right on doesn't mean the tune is perfect. That's just one aspect.

What other mods do you have, if any?

Was injection timing changed at all? Transient fueling?

How did you verify it wasn't leaking?

 

No other mods. Don't know about inj timing or transient timing.

Went under car while running to check leaks.

 

echelonphoto, it sounds to me that your issue is with the tune. If the cats weren't subjected to some extreme condition that may have damaged the chemical composition of the substrate, they run clean enough to pass emissions. If the tune is right you shouldn't get much if any smell. Before making an investment in a new catted X, I'd look into that first. Let us know what you find. Thanks.

Nick

 

The headers will move the high flow cats down line away from the heads, making the location of the cats themselves less effective than the stock location (less heat to the cats). Next is that high flow cats are not as effective as the OEM cats.


But lets back it up here, since in closed loop and idle, the car should be wanting to AFR to 14.7.

Once you are in the throttle on the tune, then it just depends what the AFR was set to instead.

So keep in mind that fuel is cooling, and you have less effective cats, and in a less effective location farther away from the head heat source as well. For this reason, the cats are installed in the X tube about 4" or closer to the collectors of the headers to get them closer to the heat source.

So if your closed loop into throttle is say less than 12.5 or lower, then you dumping so much burnt fuel into the cats that it cooling them down to much to burn off the excess fuel until they heat back up again, which could explain the smell.

Simply, look at the tune to make sure that the car is going into closed loop and going to 14.7AFR at idle, and during throttle use, your not dipping down below 12.5AFR. Lastly, check the location of the cats in the X pipe, and make sure that they are not so carbon soot'd that they are not working at all instead.

Hence the cats work by the honey combs in them getting hot, so any un-burnt fuel in the exhaust it burnt as it passes through the combs. It too much unburnt fuel is pushed though the cats, then it cools the honey combs off, and until they can get hot enough again, will not be burning off the un-burnt fuel through them.



To add, during the the tune, if the AFR reading is taken after the cats (tail pipe), then it screws up the real value of the AFR out the heads instead.

 

Extra fuel and the "cooling" factor is part of the story.. a lack of oxygen from being richer than stoich is the other part.

Normally in closed loop the oscillations from rich to lean puts a little bit of fuel towards the cats, then a little bit of oxygen, rinse/repeat

 

I just installed used ARH catted 1-3/4 on my 08 base with all o2 sensors hooked up. I have about 20 miles on it and I don't smell any gas even when idling in the garage. I have not had the car tuned yet, it is going in tomorrow.

 

May be a stupid question but doesnt hurt to ask. Seeing as they are used there is a small chance the previous owner may have gutted the cats. Did you visually check to see if the cat material is still inside there? Does it smell catless from the back of the car?

 

Thanks, all....the cats are indeed intact....according to my wideband guage...fuel was at about 14.7. My son has a similar setup on his g8 and no smell at all

 

Compare injection timing between the two cars

 

Still going crazy...this smell is ruining my car and my health....there are no leaks...we have spent a whole day tuning to get this car as close to ideal as possible...the car runs like a scalded cat...but I think the inline catalytics are not working at all. I am now thinking of going to the kooks green cats. I love the extra power of the headers...but not worth enduring this smell. BTW ....arh has been very standoffish about this problem. Why the heck does it come into the cabin so much anyhow?

 

It's not his tune. I tuned his car and the STFTs are slightly negative, -2%. LTFTs are -0.8 to 0. Wideband is oscillating correctly between 14.6 and 14.8 perfectly. A C6 with headers as the only mod is not rocket science to tune. Why would I mess with delaying injection timing when it's already pretty delayed from the factory in the Gen4 pcm? Shouldn't have to do that at all on the stock camshaft anyway.

I had a catless C5 and his car smells exactly the same as that.

My cammed G8 with ARH catted headers smells much cleaner. My cammed C5 when I added cats smelled much cleaner as well.

The smell he has is everywhere though. Idle and rolling. If the windows are down at 40 mph, you still smell it. We were thinking maybe the exhaust is coming in through the trunk seal somehow or there is a low pressure area around the car and the smell just goes there. I remember my catless c5 would do the same thing.

We are thinking of just welding in new cats. Can anyone make a recommendation? We were thinking the random tech cats. Kooks greens are just too expensive.

 

Exactly, because even on a stock tune given the stock camshaft, catted headers will not generate any extra smell. The trims will still be able to take you back to stoich. The smell is not really a rich or lean condition anyway, as many think. If the car is rich, it is a very different smell. More like a carbed muscle car. We can tell the difference because after I flash a tune, the gen4 PCM goes into super rich mode for 5-10 minutes right after flashing and it smells like a 60s mustang, which is actually a much better smell, ironically.

 

It was only an idea since clearly you two haven't been able to figure it out otherwise.

 

The stock G8 settings are a bit more aggressive than a stock C6 in terms of spraying the injector later, so I suppose I could try that.

But I never heard of anyone even touching those tables on the stock cam.

 

It isn't all that common.. but it is done, especially on forced induction cars, by some tuners anyways

I can't explain why.. I really wish I could. I swear I picked up some torque on the low end by making some changes on mine that another tuner recommended.

Whenever I get it on the dyno I'm going try changing it back and forth to see what happens just to see whether I was imagining things or not

Is it possible the cats are just bad on this car? I know that sounds like a dumb question.. but could it be that simple?

 

Yeah we are both thinking they are bad, but wanted to hear other's opinions first. They were a used set, so who knows what the previous owner did with them.

But we want to make sure before spending $500-$600 on getting new cats welded in. Maybe we should pay for a sniffer test lol.

FYI, here is a good EOIT calculator from dsteck.

http://www.hptuners.com/forum/showth...l=1#post311902

 

Even with no cats, you shouldn't smell exhaust while driving. I don't smell anything idling either and I have a big cam - it'll make your eyes water if you stand behind the car, especially on a cold start, but that doesn't get into the car with the windows up.

Something else is going on.

I'd look into that rear hatch seal. I'm thinking you have a leak somewhere either in the exhaust or at the rear of the car. Exhaust leaks at joints manifest themselves by soot at the seams as well...look for that up on top as well. Your storage cubbies weren't cut out were they? A smoke machine would help in this analysis if you have one at your disposal...

I'm drawing a blank on location, or if I'm getting my cars confused, but isn't there a port on the rear pan with a flap on it, to allow air out when you shut the hatch...if that flap is missing for some reason, it'll let air back in that shouldn't get back in if my memory is correct...

 

The shifter boot was leaking on mine.. Previous owner did a bad job cutting the boot when installing the mgw shifter so it was a pretty big leak right from under the car