3800sc 02 sensors needed? (Page 1/3)
ttt123 DEC 09, 10:26 AM
#1 do I have to use 02 sensors if i'm getting a tune? is there a benefit to having them vs not having them.

#2 If I do use them where should I weld in the bungs for them on a custom exhaust set up with true duals with an x-pipe and equal length headers. (mounted longitudinally, not transverse)

Bonus question and a chance at the prize behind door number 3... where should I weld in a bung for a wide band if I was going to use that?

Thanks guys for any and all input

olejoedad DEC 09, 01:35 PM
Yes, you must use O² sensors.
OntarioKev DEC 09, 02:16 PM
You need one O2 sensor near the factory location. The PCM isn't setup for dual bank O2 sensors, so you can't run a true dual exhaust on a factory PCM and expect it to run well.

Full standalone that supports 2 banks would do the trick for you.

In either case you don't need the second sensor after the cat as that one is for emissions and can be shut off in the tune.
ttt123 DEC 09, 04:30 PM
I have standalone set up but using factory pcm that has desktop tune already. So if i only used one o2 where should i put it? Or do i need to rethink this? exhaust is already built like mentioned.

Any thoughts
Initial.F DEC 11, 02:22 PM
Only 1 O2 sensor can be used, your gonna have to use both, or get your pcm programmed, or use the zzp o2 sensor precat delete plug in chip
OntarioKev DEC 11, 11:07 PM
The stock PCM Is set for only one O2 sensor to make fueling adjustments on. If you have a true dual setup you will be only measuring the fuel mixture from a single bank of sensors, so it will be unknown how the other bank is mixed which could cause major issues.

What "standalone" do you have that is using the factory PCM? I suspect you are describing just a tune on the factory PCM which can't be adjusted (as far as I know) for a two bank fueling system like the V8 PCMs are.

Can you fully describe this "standalone" system you have.

Your typical O2 sensor placement is in the collector for the header.
ttt123 DEC 12, 07:03 AM

quote
Originally posted by Initial.F:

Only 1 O2 sensor can be used, your gonna have to use both, or get your pcm programmed, or use the zzp o2 sensor precat delete plug in chip



i'll look into this. thanks

"The stock PCM Is set for only one O2 sensor to make fueling adjustments on. If you have a true dual setup you will be only measuring the fuel mixture from a single bank of sensors, so it will be unknown how the other bank is mixed which could cause major issues.

What "standalone" do you have that is using the factory PCM? I suspect you are describing just a tune on the factory PCM which can't be adjusted (as far as I know) for a two bank fueling system like the V8 PCMs are.

Can you fully describe this "standalone" system you have.

Your typical O2 sensor placement is in the collector for the header. "

sorry my knowledge is poor with the electrical type stuff.. I have a stand alone wire harness and a tuned stock pcm. I do have a true dual set up with x pipe. after I get the swap running the vehicle will be going in for a dyno tune.

So can the o2 just be tuned out? Can an aftermarket wide band set up replace the need for an o2 for tuning? This is a track car with lite road use(occasional weekend cruise) but it is built with all the typical 3.8sc stuff like xp cam, rockers, rollors, intercooler, pully, ported charger to heads etc.

thanks for the input guys, I really appreciate it. Pennocks is the place for gurus on this motor now. All the others have faded away.




[This message has been edited by ttt123 (edited 12-12-2019).]

OntarioKev DEC 12, 01:17 PM
So the O2 sensor is used for fueling outside of Power Enrichment mode.

It adjusts the fuel up or down (called fuel trims) to a maximum value (30% I believe either way) to maintain stoichiometric (roughly 14.7:1) air to fuel ratio.

On many V8 applications for example, there are two primary O2 sensors (in each exhaust manifold/header) and the fueling requirements are changed individually for each bank (bank 1 or bank 2).

On the 3800 PCM you have it is setup to have the O2 sensor on the rear exhaust manifold, which is connected to the front exhaust manifold via the crossover pipe. It uses this to maintain its desired A/F ratio.

On your dual setup, you will only have readings from half the cylinders, so it may not run that well.

On the plus side for a track car, you can make the adjustments typically have no affect on the power enrichment tables (wide open throttle basically). If your fuel trims are positive (adding fuel to maintain stoich) when you enter power enrichment mode, that same percentage of fuel will be added to what your tune is setup for. Now if the fuel trims are at 0, or negative, the fueling when you enter power enrichment will just use your tune settings. So aim to have your tune be as close to zero fuel trims, but negative instead of positive when you tune it.

If you want to watch out for issues with your system, get two Wideband sensors/gauges and put one in each pipe to monitor both banks for peace of mind. This could help you spot an issue such as an injector failing open on the bank you don't have the O2 sensor in.

In my experience its best to keep the factory narrowband O2 for running the PCM, and used your wideband for tuning/monitoring your operation. Wideband O2 sensors are far more prone to failing than the narrowband, and can leave the car undriveable/running very poorly when it happens. Most wideband gauges do have a 0-1V output that you can patch into the PCM, but I would recommend against it.
ttt123 DEC 12, 01:32 PM

quote
Originally posted by OntarioKev:

So the O2 sensor is used for fueling outside of Power Enrichment mode.

It adjusts the fuel up or down (called fuel trims) to a maximum value (30% I believe either way) to maintain stoichiometric (roughly 14.7:1) air to fuel ratio.

On many V8 applications for example, there are two primary O2 sensors (in each exhaust manifold/header) and the fueling requirements are changed individually for each bank (bank 1 or bank 2).

On the 3800 PCM you have it is setup to have the O2 sensor on the rear exhaust manifold, which is connected to the front exhaust manifold via the crossover pipe. It uses this to maintain its desired A/F ratio.

On your dual setup, you will only have readings from half the cylinders, so it may not run that well.

On the plus side for a track car, you can make the adjustments typically have no affect on the power enrichment tables (wide open throttle basically). If your fuel trims are positive (adding fuel to maintain stoich) when you enter power enrichment mode, that same percentage of fuel will be added to what your tune is setup for. Now if the fuel trims are at 0, or negative, the fueling when you enter power enrichment will just use your tune settings. So aim to have your tune be as close to zero fuel trims, but negative instead of positive when you tune it.

If you want to watch out for issues with your system, get two Wideband sensors/gauges and put one in each pipe to monitor both banks for peace of mind. This could help you spot an issue such as an injector failing open on the bank you don't have the O2 sensor in.

In my experience its best to keep the factory narrowband O2 for running the PCM, and used your wideband for tuning/monitoring your operation. Wideband O2 sensors are far more prone to failing than the narrowband, and can leave the car undriveable/running very poorly when it happens. Most wideband gauges do have a 0-1V output that you can patch into the PCM, but I would recommend against it.


OntarioKev, great post. thank you tons for sharing that info. I love building motors but for some of us the technical stuff just doesn't come easy so again thank you thank you thank you.

so quickly off that same thougth.. my exhaust is a very short system. headers to X to dynomax bullits then out. would it be logical to just put the one o2 sensor in the X pipe or would that be to far down the line from the engine. it would roughly be 27" inches to the X pipe from the heads.
olejoedad DEC 12, 02:35 PM
27" from the O² to the exhaust port or to the collector?