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O2 sensors after a resonator?.....LS3

gt1guy

gt1guy

Apparently a racist
Joined
May 19, 2020
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New Iberia, La.
Building a set of headers for my LS3 powered pile. To be honest, I completely forgot about the O2 sensors.

So I have two, well three actually, choices for mounting the bungs.

First choice is to shoehorn it in front of the resonator, right next to the front driveshaft. It's tight a fuck and exactly where the balance weights are on the shaft. I don't like that spot.

Second choice is right behind the resonator. Lots of room and away from the spinning things.

Third choice is to break it all apart and move the resonator downstream so I have more room in front of it. Trying to avoid the third choice if I can.

2022-11-05 17.25.07.jpg


1st choice puts it 1.5" after the collector.

2nd choice puts it 11" after the collector.

Does it matter if it's after the resonator?
 
That's pretty much what I was thinking. I know you wouldn't want to run it behind a cat, but a straight through resonator I can't see how that would make any difference.
 
You can run it after a cat too.

Ever heard about a sniffer pipe ?
 
Bebop said:
You can run it after a cat too.

Ever heard about a sniffer pipe ?
Click to expand...

I don't think that's for the sensor being used to drive the tune.

I know most vehicles have O2 sensors before and after the cat. The one before the cat is the one the ECU uses for the engine tune. The one after the cat is used to monitor the condition of the cat and doesn't come into play with the tune. That's how I understand it anyway. The second sensor is looking for a drop in O2.

I was reading last night about installing wideband sensors. After the cat via swapping out the factory sensor or using a sniffer tube will always read a certain % off because of the cat. It will read richer than it really is.

Lucky for me I'm not dealing with a cat, just a straight through resonator. I want to make sure I don't kill a brand new crate engine.........cause that would suck.
 
gt1guy said:
I don't think that's for the sensor being used to drive the tune.

I know most vehicles have O2 sensors before and after the cat. The one before the cat is the one the ECU uses for the engine tune. The one after the cat is used to monitor the condition of the cat and doesn't come into play with the tune. That's how I understand it anyway. The second sensor is looking for a drop in O2.

I was reading last night about installing wideband sensors. After the cat via swapping out the factory sensor or using a sniffer tube will always read a certain % off because of the cat. It will read richer than it really is.

Lucky for me I'm not dealing with a cat, just a straight through resonator. I want to make sure I don't kill a brand new crate engine.........cause that would suck.
Click to expand...

When people put a cam in a corvette and bring it to their guy for a tune adjustment, where do you think they they put the wideband for the tuning process ?
 
Bebop said:
When people put a cam in a corvette and bring it to their guy for a tune adjustment, where do you think they they put the wideband for the tuning process ?
Click to expand...

Sniffer tube, unless they have a unused bung.

Are the wideband sensors used in the sniffer calibrated for after cat use?


I went ahead and went for choice #3 today. Added a 3" straight section between the v-band and the bend before the resonator.

IMG_20221106_153125[1].jpg





Passenger side is a straight shot so no issues there.


IMG_20221106_153233[1].jpg
 
gt1guy said:
Sniffer tube, unless they have a unused bung.
Click to expand...
Exactly
gt1guy said:
Are the wideband sensors used in the sniffer calibrated for after cat use?
Click to expand...

No.

My tuner posted something on fb about people getting all worked up because he was using a sniffer pipe on customers cars. So he put 4 sensors along the exhaust and ended up showing the AFR is constant through the hole exhaust.
 
Bebop said:
Exactly


No.

My tuner posted something on fb about people getting all worked up because he was using a sniffer pipe on customers cars. So he put 4 sensors along the exhaust and ended up showing the AFR is constant through the hole exhaust.
Click to expand...

Found the problem:flipoff2:


From all the reading I've done in the past few days, I've come to a couple conclusions. A wideband in a sniffer tube, or anywhere behind a cat will read slightly rich compared to one installed before the cat. This is due to how the cat works by using O2 to convert the nasty stuff to less nasty stuff.

Dyno tuning is done a WOT which is where the cat is less effective, so any difference in readings between before/after sensors will be less and could actually be zero.

A wideband (sniffer tube) in the end of the tailpipe is the worst possible place to install a sensor. But because on a dyno you're only logging at WOT................is doesn't matter. Nobody runs a sensor at the very end of the tailpipe on a daily driver.

After cat sensors on new cars are only there to monitor the health of the cat. The ECU is expecting a difference in readings between the before/after sensors. Which proves that during regular driving, there's less O2 making it out of the cat than there is coming into it.

The narrowband sensor the ECU uses for the tune is always before the cat. I never read anything anywhere that even mentioned using one after the cat. In fact, the determining factor is proximity to the most upstream location that it can read the entire bank of cylinders.
 
No, I just found it interesting that something like the different views on sniffer tubes and cats can all be right at the same time.

It's the nuance of all these rules of thumbs that interests me when you dig a little deeper.

I don't have a dyno guy. Never ran a vehicle on a dyno. I do need to find one whenever I get this pile finished. I always thought/assumed/read that the vehicle gets strapped down, put in the closest gear to 1:1 and the peddle got smashed. The dyno operator/machine ran the engine through the rpm range through the brake on the dyno.

How bad did I miss the mark? Right now I'm thinking I've just made an idiot of myself.
 
Soooooo, I'm just saying that if you had your O2 sensor after the resonator, you'd be fine.

If you can put it before that's cool, but I promise you, the .1 value of AFR that may or may not be impacted by the location is far less important than what the magic laptop man (tuner) will do when it's time to get it working right.
 
Why didn't you say that from the start?:flipoff2:

Adding that bit of straight tube does allow me to install the sensor in a nice open space.

Now I just need to snake the rest of the exhaust past the links so it can dump at least near the rear tires.
 
Anyone who's factually done any tuning will tell you idle tuning and in gear, idle is the hardest, especially with serious cams. That is why you want the upstream o2 sensor(s) close to the engine.
You will see people fighting a tune with needed smooth idle with longtubes with sensor one down at the collector. If your not driving at near an idle, then don't worry about it or just go speed density.
 
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