Exhaust effect on A/F ?

[Krissrock]

Has anyone experienced a big fueling (A/F) change after they installed a free flowing / muffler-less exhaust?

Is the 1.8T that sensitive to you exhaust pressure?

I had a custom cat back installed- 3 in to 2x2.5 inch free flowing exhaust and I've been running rich since. (-9, -2)

Shops keep on pointing to the exhaust stating the lack of back pressure can cause the change

I've not heard of anyone else with this problem with an open exhaust

all other facters have been tested - new MAF, O2, Coils, plugs, TB, Tstat, CTS, Suction Jet Pump, PCV,

I would really appreciate anyone's input as I'm running out of money and ideas.

[Vince@R/TTuning]

big exaust can cause rich conditions on turbo cars....


basically ur exaust is more free flowing...spooling the turbo more, which causes the intake side of the turbo to draw more air with raises ur MAF voltage, yet your throttle body is only allowing the same air in at idle or low cruise conditions as before. if you log MAF voltage i bet u will find that it is higher then spec or higher then it was before

[Elliott18t]

would that not cause a code?

I only ask because (not to thread jack) I have a huge crack in my downpipe I just discovered (http://www.tristatetuners.com/forum/...t=66620&page=3)
and its basically free flowing now.... its right at the weld of the flange connecting to the turbo. yet no CEL..

[Vince@R/TTuning]

it depends if it is rich enough to throw a code...and some times if the crack is before the upstream O2 sensor it might throw a lean code....

[Krissrock]

Originally Posted by Vince@R/TTuning

big exaust can cause rich conditions on turbo cars....


basically ur exaust is more free flowing...spooling the turbo more, which causes the intake side of the turbo to draw more air with raises ur MAF voltage, yet your throttle body is only allowing the same air in at idle or low cruise conditions as before. if you log MAF voltage i bet u will find that it is higher then spec or higher then it was before

lol hey vince. thanks. I'm not sure if can log that..i'll have to see if my old laptop is up to it...

I understand what you're saying. But if more air is being pulled in at the maf, doesn't that air have to go somewhere? There cannot be more air coming in at the maf, and it not be going into the engine...?

[superskaterxes]

when r u running rich? at idle or cruise? is -9,-2 your LTFT's?

[Krissrock]

yes, only at idle. The second number varies between +0.78 and -2 .... which is fine

[superskaterxes]

wait so only at idle your rich? and your LTFT is -9 or -2? whats your STFT?

[Krissrock]

those are my block 32 numbers -9 (idle) -2 (driving) -
Ardmore in the house! Tha's my hometown!

[superskaterxes]

the idle would indicate u have some kind of vacuum leak. u should pressure test your system and check for leaks. your exhaust has nothing to do with your idle AFR because the amount of backpressure at idle between a 2" and 4" exhaust is negligible. if u find u have no leaks do u have the ability to change your MAF scaling?

[Krissrock]

i've pressure checked it plenty of times. I did find a leak early on, but i've been dealing with this december, and multiple checks since have been good. Am I incorrect in understanding that vac leaks cause lean conditions?

Plus for the exhaust, I've had my stock exhaust break apart the mid pipe. So I had to drive with the exhaust dumping straight under the car, no muffler. I never got a code from that

[Krissrock]

also the exhaust is just a cat back...
Vince do you have any more suggestions? you recommended before changing the MAF housing...

I'm about to throw in the towel and hook the AFC back up, but I don't want to circumvent the problem if there actually is something bad.

[Vince@R/TTuning]

Originally Posted by Krissrock

lol hey vince. thanks. I'm not sure if can log that..i'll have to see if my old laptop is up to it...

I understand what you're saying. But if more air is being pulled in at the maf, doesn't that air have to go somewhere? There cannot be more air coming in at the maf, and it not be going into the engine...?

the MAF is on the suction side of the turbo, so its possible to pull more air on the suction side and then build more pressure on the compressor side in the system. its real common on Big turbo MAF cars cause they flow so much more air and there is now where for it to go.


-9 fuel trims arent super bad it seems like its just enough to effect things...essentially it just sounds like you need your car tuned to your specific setup

[gdmbat85]

why not just block your exhaust with your shoe or something and see if it changes your A/F ratios?

[Krissrock]

My friend who's an audi tech did that and he said that it did bring it down some. to like -6. -9 is the highest available value for the idle trim. Vince, i gotta give it up to you. I took a log of my g/s at idle and they avg'd 4.5 g/s which is higher than what it should be. From what i've read, it should beween 2 and 4. I can post the log, but it's text, not a graph.

so vince, tune or bigger maf?

Also on a side note. My AFC list the airflow in a %age. This is about 4 ish at idle, and goes down when driving....so when accelerate, it goes down to like 1 - 2 % ... is that right? Shouldn't the airflow %age go up?

[Vince@R/TTuning]

Originally Posted by Krissrock

My friend who's an audi tech did that and he said that it did bring it down some. to like -6. -9 is the highest available value for the idle trim. Vince, i gotta give it up to you. I took a log of my g/s at idle and they avg'd 4.5 g/s which is higher than what it should be. From what i've read, it should beween 2 and 4. I can post the log, but it's text, not a graph.

so vince, tune or bigger maf?

Also on a side note. My AFC list the airflow in a %age. This is about 4 ish at idle, and goes down when driving....so when accelerate, it goes down to like 1 - 2 % ... is that right? Shouldn't the airflow %age go up?

Its up to u whether ya wanna try a bigger MAF but in my opinion u should have it tuned anyway. Your set up is pretty unique and i dont know if anyone is gonna have a compatible Flash for that.

As far as the AFC airflow usage, they are a lil funky..thats why they have all of those settings in them to get the right calibration. That was never designed for your motor, so it prob still works but when it comes to the numerical values I would be that concerned.

[Krissrock]

The AFC Select just turned out to have a setting incorrect.
The AFC Select is designed for Euro cars and even had instruction for my car in the manual. I've had it tuned it before but had since disconnected it cause of a MAF CEL. Turned out it was another fuse that was causing the issue.

I guess i'll have to reconnect it and let you guys tune it. I have no idea what corrections were made from when it was tuned previously.