Exhaust Not Worth It

[Che'_Moderator]

LOL. See post 3-5. I do not waste time on this anymore. Half of you are trying to talk way over your pay grade. Stick to bolts on or general wrenching and stay away from sensors and theory.

[Fishey]

So ummmm I suppose changing the diameter of the tube the MAF sits in has no effect either?

Wait, WHAT!?

So, your trying to apply the same theory about a MASS AIR FLOW sensor to a Oxygen Sensor? ROFLMAO

1. Mass Air Flow is dependent on knowing how much air (Flow) and MASS! (Why most MAF sensors are what we call "Hot wire") So if you change the pipe size the ability to accurately messure is lost since the sensor has to be calibrated to a certain size of pipe for many reasons.

2. Oxygen Sensor is not at all in any single way without any doubt on this earth known to man related to pipe size. This is for a billion reasons but mostly because oxygen sensors do not read dependent of flow in any way what so ever. They simply read a sample of there enviroment and compare oxygen levels to normal atmospheric conditions and since regardless if you have a 1" pipe or 4" pipe the flow of the pipe is not important or even factored into its function. An oxygen sensor will read 100% accurate if no flow is present hince why with many wide bands calibration is done before startup with the car off to get a baseline. The problem is that you think because you space out a 02 sensor it will not read correctly but if you put an o2 sensor out of the flow but in a 2" diameter bung it will read fine. When you space out a oxygen sensor you make it hard for the sensor to be contaminated by outside air so the sample it test isn't accurate to its surroundings. The most important part of an oxgen sensor is what heat levels its designed to function within that is why distance from engine is the most imporant factor (EGT) when installing a oxgen sensor.

-Your Pal

Fishey

[flaco]

You now what they say... arguing on the internet is like winning the 'olympics'

If che thinks of an O2 as a MASS sensor (LOL), no argument will ever win that...

DING DING DING we have a winner!!!

[Che'_Moderator]

Hook an oscilloscope to a o2 and watch the read out. A pile of things effect the voltage and the waveform. Larger pipes run cooler as stated. But as you pointed out there is also the issue of density. You can change the wave form just by putting your hand over the exhaust exit and restricting it a little. The vent/s in the sensor do not work as intended and you start to see voltage drop. These are all small changes but the effect of the pipe does have a direct effect on the sensors readings.

[Fishey]

Hook an oscilloscope to a o2 and watch the read out. A pile of things effect the voltage and the waveform. Larger pipes run cooler as stated. But as you pointed out there is also the issue of density. You can change the wave form just by putting your hand over the exhaust exit and restricting it a little. The vent/s in the sensor do not work as intended and you start to see voltage drop. These are all small changes but the effect of the pipe does have a direct effect on the sensors readings.

A larger pipe you can move placement closer for temp reasons but density of the AIR is not a factor in how an oxygen sensor reads. The sensor itself needs to be at a certain heat range not the air. We know this because heated oxygen sensors can work and read correct without any exhaust temp or air temp so long as the sensors heat circuit has the sensor itself up to temp.

-Your Pal

Fishey

[Reximus]

A larger pipe you can move placement closer for temp reasons but density of the AIR is not a factor in how an oxygen sensor reads. The sensor itself needs to be at a certain heat range not the air. We know this because heated oxygen sensors can work and read correct without any exhaust temp or air temp so long as the sensors heat circuit has the sensor itself up to temp.

-Your Pal

Fishey

i second the theory

later model cars with the angle flange turbo has their o2 sensor close to the turbo only about 8 inch away instead of the usual 18 in on the old model 850 turbo's. personally i've had a custom 3" downpipe that i got from someone else, every once in a while it will tell me my o2 sensor response is slow, for me it's mainly due to exhaust heating effect on the o2 sensor. once i swapped back to original exhaust with small diameter, the codes never came back... if a custom exhaust wasn't built with enough research done, it will cause more problem than performance :lol:

[Che'_Moderator]

A larger pipe you can move placement closer for temp reasons but density of the AIR is not a factor in how an oxygen sensor reads. The sensor itself needs to be at a certain heat range not the air. We know this because heated oxygen sensors can work and read correct without any exhaust temp or air temp so long as the sensors heat circuit has the sensor itself up to temp.

-Your Pal

Fishey

Yes we all know why there is a 4th wire and the car runs open loop till the sensors hit 300 degrees. Air density plays a huge effect on the voltage output though. Run your exhaust through a 6" pipe that ends at the front fender and you think its still gonna read the same at idle?

[Fishey]

Yes we all know why there is a 4th wire and the car runs open loop till the sensors hit 300 degrees. Air density plays a huge effect on the voltage output though. Run your exhaust through a 6" pipe that ends at the front fender and you think its still gonna read the same at idle?

Air Density inside of the exhaust doesn't effect an oxygen sensor its that simple as well as has no effect on the % of oxygen present inside of the exhaust.

I guess your trying to say that Air Density effects % of oxygen inside our atmosphere (Here on earth) and that is very true but the inside of the exhaust has its own atmosphere and no matter what you try to do air will have the same % of oxygen regardless of back pressure inside of the pipe. This is because its basically sealed the only air inside the pipe is from the cars exhaust. So, the only way you would get false readings is if somehow you had the oxygen sensor so close to the end of the exhaust that it might get hit with some sort of scavenging. However, that being said 2-3ft inside the pipe that is simply not possible.

-Your Pal

Fishey

[troyhyde]

Hey guys, if flow doesn't make any difference why do people put anti-foulers as spacers in the rear O2 running catless. Typically this results in an instantaneous change in the O2's reading, pulling it out of the exhaust stream clicks off that CEL indicating poor catalytic converter function.

Discuss....

[TRACStar]

Hey guys, if flow doesn't make any difference why do people put anti-foulers as spacers in the rear O2 running catless. Typically this results in an instantaneous change in the O2's reading, pulling it out of the exhaust stream clicks off that CEL indicating poor catalytic converter function.

Discuss....

Are you serious? There is nothing to discuss there.

Its a totally different situation, it takes the sensor out of the exhaust flow so it sees a lower reading and thinks the cat is still there.

[bergmjs]

Air Density inside of the exhaust doesn't effect an oxygen sensor its that simple as well as has no effect on the % of oxygen present inside of the exhaust.

I guess your trying to say that Air Density effects % of oxygen inside our atmosphere (Here on earth) and that is very true but the inside of the exhaust has its own atmosphere and no matter what you try to do air will have the same % of oxygen regardless of back pressure inside of the pipe. This is because its basically sealed the only air inside the pipe is from the cars exhaust. So, the only way you would get false readings is if somehow you had the oxygen sensor so close to the end of the exhaust that it might get hit with some sort of scavenging. However, that being said 2-3ft inside the pipe that is simply not possible.

-Your Pal

Fishey

Density = (mass unit)/(volume unit)

Changing the volume unit means changing the density. Bigger pipe means less dense fluid, this means the oxygen is spread out over a wider cross-sectional area while the the oxygen sensor profile remains constant, thus changing the amount of oxygen passing over the sensor. This is very simple physics people.

[troyhyde]

Are you serious? There is nothing to discuss there.

Its a totally different situation, it takes the sensor out of the exhaust flow so it sees a lower reading and thinks the cat is still there.

I think you've just made my point for me, thank you.

The argument some are making is that flow doesn't make a difference, whether it be 2.25" or 3". Pulling the sensor out of the direct exhaust flow is no different than increasing the pipe size, it is seeing the same gases and concentration of oxygen vs. exhaust but in reduced volume. It isn't seeing special air in its little nook.

[Reximus]

Hey guys, if flow doesn't make any difference why do people put anti-foulers as spacers in the rear O2 running catless. Typically this results in an instantaneous change in the O2's reading, pulling it out of the exhaust stream clicks off that CEL indicating poor catalytic converter function.

Discuss....

sorry for the lengthy post in advance guys, just trying to make some points here please ignore most of my post if you don't have time and skip right to the last paragraph in bold text:lol:

if you want to go very technical with things like this... although i'm not a fluid/air flow dynamic specialist, but i do know that engineers put the oxygen sensors in it's respective position for a good reason.

knowing the pipe diameter, knowing the amount of back pressure presented in the exhaust system, know the air flow through the engine under all conditions, they determine where the oxygen sensor should go and at what angle. mainly for the reason of engine management software calibration with the respective running condition.

let's just put things into numbers to put things into perspective. if you have a stock pipe which is around 2.25" inside diameter, it will have an flow area of roughly 4" square. and if you have a 2.8" inside diameter (roughly the size of a 3" piping) you will have an flow area of roughly 6" square. so we are talking about 50% more flow when comparing the sizes... this to put the word "free flow" into perspective.

let's say the 2.5" pipe are capable of flowing 120 cfm (cubic feet per minute) of free flow air. then in theory the 3" pipe will have a free flow rate of 180 cfm. higher flow again, but we are not talking about air density right now, that will get too technical when you introduce back pressure into the equation, which i am not educated enough to fully explain this

now talking about the placement of oxygen sensor. oem sensor on the 850 turbo is 18" away from the turbo, manifold and turbine housing aside, that equal-ate to 72 ci of volume to fill with exhaust gases before the sensor will read the exhaust gases. if you were to put the oxygen sensor at the stock mounting position on a 3" pipe, that same distance will equal-ate to 108 ci of volume to fill with exhaust gases before it will response. that is 50% slower response rate if at that setup. engine management system is programed to run on millisecond scales. it will adjust the fuel trim almost instantaneously with the input from the oxygen sensor. with the 50% delay, this will set off the diagnostic function of the ecu and put the system into false loop with fuel trim function turned off. with my personal case, with the badly designed "custom" 3" pipe by some "performance shop", this was the case... cel left and right all the time. slow first o2 sensor, slow second o2 sensor, this and that... not to mention the lost of power as well.

i then swapped back the original pipe back onto the car, with still the same oxygen sensor that set off the code on the 3" piping. recorded the oxygen sensor reading on the oscilloscope for both the oem downpipe and 3" downpipe. not to my surprise the 3" downpipe has a graph mostly fluctuating on the rich side, where as the oem downpipe has a perfect graph average on the 14.7:1 ratio. later found out that the 3" was doing this because by the time the oxygen sensor sensed the 1st batch of exhaust gases from the first fuel trim adjustment and made adjustment accordingly, it is on a 50% delay already. when the fuel trim is adjusted for the second batch, the fuel trim has been trimmed way too rich already. (we are talking about within the milliseconds) fuel trim was on the rich side (112) for the 3" pipe, and almost spot on (101) for oem pipe. the ecm was adjusting for a batch of exhaust gases that's consider "history" by the pace of ecm programing.

i personally conclude by this test i've conducted, was that exhaust gas density doesn't come into effect with the amount of upsizing we are talking about. response time is a key factor though for engine management. where as the oxygen sensor defouler is a whole different game. it is actually putting the oxygen sensor in a dead end street (defouler or spacer) off the main street (exhaust pipe) so it will get less traffic (exhaust gases). hope i havn't bored every on here yet :lol:

[TRACStar]

It isn't seeing special air in its little nook.

Your right, it must be black magic that makes the CEL go away. :arob:

[Spanky]

Density = (mass unit)/(volume unit)

Changing the volume unit means changing the density. Bigger pipe means less dense fluid, this means the oxygen is spread out over a wider cross-sectional area while the the oxygen sensor profile remains constant, thus changing the amount of oxygen passing over the sensor. This is very simple physics people.

Your getting closer!! . I think everyone else is just lost

The gas is going through very rapid cooling and changes in gas density/volume/temperature/flow would occur. There would be a specific upstream volume which would be large enough to affect lambda accuracy due to lowering temperature, density increase (because of rapid cooling), volume diminishes in which gas velocity takes a nose dive. Hence the reason why you should see more fluctuations of lambda and less accuracy compared to a smaller diameter pipe throughout the rpm range. I am sure there is a breaking point at which diameter/distance henders accuracy, do I know it? No