What cats?

[Jethrow]

Hi all,

so I recently took my car in for an emissions test and the results were not too good, I was too high on NOx. Everything else we pretty bang on, just high on NOx. Basically suggesting my cats are not doing their job. The cats I used are just the ones FFR supplied with the kit, and I am guessing they are prolly not the greatest sets of cats to ever grace a cobra.

So for all you guys who have passed emissions, ideally with an LS3 or similar, what cats did you use? And were they good enough that you were happy to leave them in afterwards?

Thanks

Tim

[vector1]

Take your pic, most don't give a hoot.

[KirkV]

lmao I was thinking it!

[Rog246]

PMSL................oh yeah !!
I was gunna say ***** CATS, but THAT most obviously has been done
Not much else can be said

[d488y2]

Tim, high NOx can be caused by the Cats but there are things that can cause high NOx as well, I'm sure you're done a google search?
I don't know if you have had your car to a good tuner? I hear Active Automotive in WA are the gun Tuners in WA.
Good luck with it.

[Treeve]

Cats don't cause NOx, high combustion temperatures cause NOx. Running lean will do this.

Best cats are OEM spec for your engine, so OEM commodore ones would be a good first choice, second on the list are XR6T cats, and finally, suitably sized ceramic core 400 cpsi (cells per square inch) cats from ebay and the like. Don't go for second hand - once a car has reached 160,000km it is considered that the cats would be fully depleted / damaged beyond useful life.

Incorrect fueling will damage cats very quickly, so as I've mentioned a few times in the past to pass emissions you need 14.7:1 for a cat to function effectively, and also the rapid fluctuation from rich to lean offered by a narrow band sensor. This replenishes the cat of both oxygen and fuel to aid the combustion and conversion process.

Treeve

[albanycobra]

totally agree with Treeve....I was thinking it just couldnt be arsed typing it. Also if you are getting your car tested at the Jag place then dont bother going back, they all fail there. Mine also failed first time there with high Nox, problem with my car was it was running hot due to revs at stationary testing with no additional cooling. I changed to OE Holden Cats and went to Xtreme Tune in Midland. Much better service and Simon knows his stuff so can help you out with whats required, nothing like a big dyno fan to keep your motor cool. PM me if you want to talk more about it.

[Jethrow]

Thanks everyone.

Yes I was tested at the Jag place, I know what you are thinking about them ...

Anyway, so during the test My AFR was 14.9 (according to their sensor), so perhaps very slightly lean. But I would think it would be around the right number. My NOx registered at 140 odd ppm, whilst the Hexane was just 3 ppm. Also I understand what was said about cats not causing NOx above, and whilst yes that is true, it is also true that the job of the cats is to remove the NOx. All engines make NOx, the cats are there to scrub it. But yes, a poorly tuned engine will make too much NOx, more than the cats can handle.

Albany, which OE cats did you fit, ie from which model of Holden?

Thanks

[albanycobra]

cant remember was awhile ago....I think VE. There are two types, big ones and small ones. I used the small type but the large ones are even better.

[Treeve]

As I said, it's the cycling from rich to lean that you need; constantly on the lean side will do exactly as you have said.

[Jethrow]

Thanks for the reply Treeve. So can you elaborate on the cycling a bit? I do run the standard GM ECU with GM supplied (presumably) narrow band O2 sensors, so does my ECU do that cycling?

The test we do over here, called a 5 gas test, is just done with car in neutral and engine revving at 3000 RPM. Will the engine exhibit the traits you suggest in this test mode?

Also I wondered if running a touch richer might help reduce the NOx (at the expense of some Hexane) but then again, the engine is prolly running closed loop at the test point so I may not be able to effect much of a change.

Thanks again.

[Treeve]

OK so the cycling bit:

With a narrow band, they are normally set at 14.7:1 as the changeover point. Some are not - the furthest from this mark that I've found are mid-late '90's Range Rovers, which have the change over point at 12.3:1.

That is all beside the point. What is important is the cycling from rich to lean. A Narrow Band sensor is so called because it changes voltage over a very narrow band at the change over point, from 0-5V, hence all it is doing is saying to the ECU "Too rich, make it leaner" or "too lean, make it richer". Therefore every time the O2 sensor takes a reading (roughly every 4 cycles / 8 revs I've been told, depending on revs) it will then make an adjustment to the fueling.

So, lets pretend you're really lean. 4 cycles go by, the O2 sensors reads that it is still too lean, so it puts a few more points of fuel in. Next O2 sensor cycle it makes another adjustment, and then overshoots. You're now running too rich. So then it makes another adjustment, and you're too lean.

These adjustments aren't much, but they are designed to keep you at 14.7:1 because this is where the lowest of all three measured emissions are. A 5 gas test looks for CO2, CO, THC (Total Hydro Carbons, measured in free hexane), O2 and NOx. I've got one home here, and I use it on my customers cars as well as for basic setup of my own stuff before I head out on the road for the first time with a new tune.

Here's a picture that I've put on a few different forums (normally linked from wherever google shows me first). It's from Toyota, and it's accurate for getting the idea across that rich or lean is bad when it comes to emissions. You need 14.7:1 as this is where the total of the 3 polluting gas components (THC, NOx and CO) are the lowest in total.



The cycling from rich to lean is necessary for the cat to replenish O2 to be able to 'burn' the fuel completely, hence converting the CO to CO2, removing more of the THCs and converting the NOx molecules (not 100% of the chemistry of what they get turned into).

Does this help? I've put heaps more info on OzClubbies (and there is even more information from a handful of guys who have either made extraordinary amounts of notes from their IM240 testing in Vic or from their works as emissions engineers working for Holden.

Cheers,

Treeve

[Treeve]

-double post, please delete-

[Jethrow]

Very interesting Treeve, thanks for the share.

So I understand the closed loop bit, which you explain where the ECU and sensors work together to target the 14.7 AFR. But to go further, does the ECU then deliberately move the target plus and minus around the 14.7 to achieve the cycling? Would I see that on a trace if I was logging AFR whilst say holding the revs steady?

Also, if I may, for my test where I registered low hexane (3ppm) but high NOx (140ppm) do you think if I enrichened up the car it could bring the NOx down to an acceptable level with only minor change to the hexane? As indicated by your graph?

Tho to do that, I might have to give the ECU a new closed loop richer target if that is at all possible.

Thanks again.

[Treeve]

G'day Tim,

The ECU tries to hold 14.7:1 all the time in closed loop. However, as your engine's cylinders do not burn perfectly exactly the same each and every ignition event, then there will always be some change in the output. Therefore the ECU aims at 14.7:1 and will constantly overshoot or undershoot a little bit. And it is a little bit - I'm not talking +/- 0.2, it's in the region of +/- 0.01 AFR.

If your car was running at 14.9:1 as you have suggested, then it is running too lean, and there isn't enough fuel to burn cleanly in the cats - if you consider a cat to be a fireplace which can only burn on what comes out of your engine, then it will help to understand. This fire needs fuel (rich) and oxygen (lean). You need this to run cleanly too, so not too rich and not too lean. Now, imagine you can only feed one or the other of these at a time - fuel or oxygen. This means you have to cycle between the two to get your fire to burn efficiently. Therefore you use a narrowband sensor which does this.

Your car running at 14.9:1 is putting heaps of oxygen in, but not enough fuel and hence you are getting low fuel readings, but the lean flame is hot, and hence you aren't breaking down the NOx molecules.

If you go too lean, you get lean misfires and then you get high NOx and high THCs - but normally in spikes which can be picked from the results chart depending on how the load is applied to the engine.

Now, this is an analogy and it doesn't hold up to detailed inspection, but it does get the idea across simply enough I think.

Does this help?

Cheers,

Treeve

[Jethrow]

Yes, helps a lot really.

Tho I do question the 14.9 AFR reading for two reasons, the first being that the closed loop should be forcing it to 14.7, and second I have heard from other operators that the testers gear does tend to read leaner than it really is.

That said, if it is too lean, can I tell the ECU it is getting the closed loop wrong? I have scanned the ECU for errors and none are being reported. The O2 sensors fitted are also new items from GM.

Thanks

[Treeve]

It depends - to diagnose this kind of thing properly you need to be getting gas readings pre-cat, and I'm guessing your dyno guy has done it up the exhaust, post cat, which would make the tuning a bit tough to pinpoint properly.

[Gaz64]

Here's a good read on catalytic converter operation if you're interested:

http://www.bearriverconverters.com/data/CatOpp.pdf