blown 87

The test is run after the cats, to set the CO you test it before the cats.

Nox is with in range, I see nothing wrong with those numbers.

Hilton

Yes, the CO is set correctly using the test pipes not the final exhaust. The NOx is within the permissable range; however, the CO/NOx results indicate the car is running lean still.

A quick check of emissions threads indicates its still on the high side, and in a non-cat car the tune is what you get - no O2 sensor to adjust mixture, which means the car is running leaner across the whole fuel map, as the LH assumes a certain baseline under no load at idle rpm's.

From reading, seems like NOx of 300 and CO < 0.1 is pretty easily attainable for a car with a good MAF. Some 928 pass results are better than that.

My own results are in g/km and not sure on conversion to PPM, so not sure how it compares to you US guys (although the test used here is IM240 which I believe is the same one used in CA?)

blown 87

Any test done after the converter is not going to give a very valid indicator of engine tune.

I have seen cars look ok on every thing but Nox, put a new converter on it and the numbers be as close to 0 as you are going to get them.

I guess what I am trying to say is a converter can work good for Hc's and not for Nox, they are 3 way converters.

From what I understand he is using old converters, no way you can get a handle on the state of tune when the numbers are as good as his are.

I do agree he needs to get his O2 sensors working and it is more than likely lean, but there is no good way to tell for sure, with out going pre cat.

Sailmed

They have a different section for us on the group W bench...
and I think there are enough of us to warrant a short bus.

Mike - I have a spare cat if you would like it - I just happen to be passing by soon.

dprantl

Well, the results that were posted from the station (after cats) showed 0.01 CO and NOx in the 800 range. Yes, this passes (in GA), but is still very high. If the car were tested in a state that has more stringent limits, the car would still fail. A properly running 928 should not show such high NOx numbers. The numbers indicate the car is running lean, and could be tuned to run much better than it is currently. That's all.

Dan
'91 928GT S/C 475hp/460lb.ft

Hilton

Completely agree Greg - you mentioned earlier in the thread that you have a CO meter? Would be good to connect to his test pipes on the ends of the airbox and measure CO directly to adjust to 1.2-1.3% - easier than removing the cats if they're welded on. (WSM calls for 1%with a .5% margin - as MAF ages it gets leaner so >1% gives a longer period between adjustment)

Actually on this point I disagree - getting an O2 sensor working is going to be painful unless he runs with the air pump disconnected (which may be an issue with emissions compliance in some states and hence a reason for insurance companies to cause headaches in event of an accident).

Non-cat S4's have a hideous air-pump arrangement that pumps into the exhaust before anywhere you can fit an O2 sensor, thus screwing the O2 readings. Removing the air pump manifold requires removal of air pump, PS pump and oil filter just to get at the damn banjo bolts in the heads by the exhaust ports. Taking the piping off requires cutting it in several very hard to reach places.

See this post earlier in the thread for a picture from PET of the non-cat air-pump piping.

Porsche specs say a non-cat car tuned to spec gives same power output as a car with an O2 sensor, so simply adjusting the CO to correct value is the easiest path to a well-running S4

Note the CO meter calibration is important - here in Oz most garages don't use 'em much anymore, and the first place I went to showed 1% on their machine but was still very lean. Second place was a motorsport crowd who had a recently calibrated sensor and it was actually at 0.4%

blown 87

And all I am trying to say is that a marginal cat can skew the numbers, do nothing to the tune, but put new factory cats on it and the numbers will be a lot different.

New cats could bring the Nox down well below 50 PPM and the CO stayed the same, and nothing would have changed with the tune.

I have seen many times where the cats would light up and still do nothing for the Nox.

I do agree it is most likely lean for a couple of reasons, he adjusted the CO screw and the O2 sensors are off line.

I am not trying to argue, I am just saying that from what I have seen working on cars and getting many hundreds of them to pass, checking what comes out the tail pipes is just that, checking what comes out the tail pipes.

Greg

blown 87

I agree the machine has to be right, that is why I offered to get a new sensor for mine if he needed to use it.

dprantl

No argument here. If his cats are bad, then that's the issue. I guess you could try to tune to the ear based on comparison with a well running car, but without measurement before the cats it may be difficult. And yes, the reading at the tailpipe is hugely different from what the O2 sees. I did a dyno run before I installed a wbO2 sensor and was freaking out that my AFR was over 15:1 at WOT after 5,500RPM. Then I installed a wbO2 (of course before the cats) and it shows me never going above 13:1.

Dan
'91 928GT S/C 475hp/460lb.ft

blown 87

Mine will run about 14.5-14.6 at cruise, WOT it gets pretty fat.

I bet you did freak thinking it was that lean under boost.

I am not saying that his cats are bad, just saying they tend not to work as well when they get older and have some mileage on them.

mcholdfast

I think I'll be taking you up on your generous offer, Greg. I'll send you a PM to set it up. Thanks.

The cats aren't very old. I have the receipt from the PO showing they were installed in April of 2006 and they have less than 6000 miles on them. The receipt doesn't give any info regarding the brand or type of cats. The price was $1465.00 including $750.00 for parts and $677.00 for labor including "seal up air tubes not used on this vehicle". Thanks to the POs for not sparing any expense in maintaining this beauty. There's so many maintenance receipts, it took me a while just to find this one.

I'm going to turn the pot in a couple more turns for now since it seems it is still running a little lean but I hope to get it spot on soon at Greg's shop. I'll post a follow up once we're done.

First I gotta go pay the tax man and get another sticker. I don't need no stinkin stickers.