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Longtubes without tune?

11K views 27 replies 10 participants last post by  JBert 
#1 ·
Hey I recently installed JBA longtubes and was wondering if its ok to run the car without a tune for them for the time being. I have a dyno tune in 24 days but I dont know if they will still be open by then with how this pandemic is going. I had the car tuned for no cats already so I dont think it should run too lean, but id appreciate some feedback as there seems to be varied opinions.
 
#3 ·
yeah, I don't think it will be a big problem . . . the actual fuel mixture probably will be off a bit, because the signals coming from the O2 sensors to the computer will be off a bit, I'm not sure how much . . . but probably not enough to cause any damage

I'd go easy until I got the tune; just in case it is running lean.

BTW, the rear O2's have no affect on the air/fuel mix. The rear O2's, which you removed along with the cats, only tell the computer whether the cats are working. The front O2 sensors, which are just downstream of the exhaust manifolds or headers, do provide a signal to the computer to adjust the air/fuel mix. The headers affect the readings from the front O2 sensors (because the sensors are now further downstream than before) and that's why a tune is needed.
 
#13 ·
I'd go easy until I got the tune; just in case it is running lean.

BTW, the rear O2's have no affect on the air/fuel mix. The rear O2's, which you removed along with the cats, only tell the computer whether the cats are working. The front O2 sensors, which are just downstream of the exhaust manifolds or headers, do provide a signal to the computer to adjust the air/fuel mix. The headers affect the readings from the front O2 sensors (because the sensors are now further downstream than before) and that's why a tune is needed.
Well, not really...as far as the A/F ratio, that will only take a few minutes to adjust as the ECM can account automatically for up to a 25%+ fuel mixture adjustment (per the Ford engineer and fr altitudes above 8K feet they have a special program). As far as the absence of the 2nd set (downstream cats), that is an issue as.... although in simple terms the O2 sensor's are in essence monitoring and sending signals to the ecm specific to AF mixture, the downstream O2's are not just monitoring the cats as many "experts" state but the ECM software algorithm combined with the "pre-determined modeling" is used for comparative analysis to determine the final A/F ratio and when the downstream O2's are eliminated, unlike the early gen ecm software, the ecm WILL make erroneous decisions (short term/initial at the start of each cycle) which in short creates a slight over-rich/then lean mixture.... this slowly over time (I'm talking years/high mileage) induces/creates valve tuliping (in layman's terms, warped valves)…...ANY machinist will have these laying around the shop to show customers what happens when to defeat a critical component of an extremely advanced software program. I know you have posted before that this is untrue, except both myself and the a senior software engineer (who designed this software at Ford) disagrees with you (who BTW is my neighbor and just in his passing conversations has taught me more about this system that I can possibly ever convey on SMS).


My neighbor is a really nice guy who has told me he just doesn't try or even let people know about his background.... he likes the car shows and meets but when he hears someone talking about something like this that IS eventually going to cause damage, he just doesn't even bother trying to "educate" most because "Everybody is an expert" except him, one of the engineers who wrote the program...…


Now there is 1 potential issue and that is the specific location of the O2 sensors on the headers and downstream of the cats..... you can only hope that the mfg did some testing to ID the best location, otherwise, yes, you might need a tune or need to relocate the O2 sensor locations, but you're not going to know that until you fire "her" up and drive for 50-100 miles....
 
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#4 ·
You'll most likely get a "slow response" code since the upstream O2's are further down and take a bit longer to warm up to go in to "closed loop". I believe a tune adjusts for a delayed response.
 
#6 ·
This.
Consequently the engine might run lean and hesitate slightly during the short transitional period between the ECU going into closed loop and the front O2 sensors being fully warmed up. This becomes almost a non-issue as the weather gets warmer.
 
#7 ·
I've been a Member of a Corvette Forum for over 7 years :( . I have read several people have added long tube headers on their Vette's without a tune with no problems. I had a tune on my 2011 v6 before I added my long tubes with a catted x pipe and ran it that way for over a month without any problems. Extra power everywhere and no check engine light. I paid $100 to have a new tune made (MPT) for the addition of my long tube headers. No difference from my old tune performance wise when compared to my old tune :( .

I would give it a try. If anything is off too bad you will get a check engine light...……..
 
#8 ·
Well the check engine light did come on after a couple minutes of idling but it sounds just fine. I havent driven yet because I snapped the dipstick tube flush with the block so I just threaded a bolt partially into it. Hopefully that will get me to the shop to extract it without popping out, then I can see how well it actually runs.
 
#9 ·
These cars are real quick to tell you when they're running lean. Believe me. Don't worry about that.
Good luck with the rest of your snafu.
 
#10 ·
Much depends on how much further back the upstream O2 sensors have to be moved from their original position. The 3/4 length Kooks, Pypes, and MAC long tube headers don't have that issue but full length headers such as BBK, JBA, American Racing, and Stainless Works might do unless the upstream O2 sensors are close to the collectors.

The ECU doesn't need to be retuned for the increased airflow requirements of long tube headers because the MAF sensor can adapt to this. The same goes if you port the heads (as long as your injectors aren't maxed out).
 
#11 ·
O2 Transport Delay

There is a short period of time between when combustion occurs in the chamber, and when the O2 sensor measures the oxygen content in the exhaust. This short delay is called the O2 transport delay. There is a table in the tune that describes how long this delay is, and it varies based mainly on RPM. The faster the engine is spinning, the faster these exhaust pulses go down the exhaust pipe and the shorter the O2 transport delay is.

As the O2 sensor wears out naturally over time, it is less sensitive and takes longer to react to the combustion event. And so the O2 transport delay increases with O2 sensor age. Because of this, the PCM has to learn what the new transport delay is. So the table in the PCM describes the original transport delay, and the PCM starts with that. But over time, it learns what the delay is based on how well the O2 feedback controls the air/fuel ratio. And so there is a table in the Keep Alive Memory that's identical to the table in the calibration. And that KAM table starts out identical to the calibration table, but over time that table increases to match the wear of the O2 sensor, and it now reflects the real amount of time the O2 sensor delay is.

When you relocate the primary O2 sensors further back due to long tube headers, this O2 transport delay must increase. How much? We don't really know exactly. But the further back it is, the bigger our first guess will be. Perhaps 30% at idle speeds. Anyway, if the PCM learns this over time anyway, why do we need to tune for it?

Because the PCM learns slowly. And in the mean time, we want it to be closer to accurate. And we don't want to set a code for slow response. And so we jack up that table using our best guess. That'll do, for now. The PCM learns the rest anyway.

In a perfect world, the custom would drive that car for a few months, without ever clearing KAM or having a dead or disconnected battery, so that table can learn real good. And then the tuner could read the leaned values from this KAM table, and copy and paste them into the actual calibration table. That way, the car immediately knows the right answer, even if you disconnect the battery. This isn't really necessary, but it is ideal.

That never happens in the real world. But a sharp tuner might eyeball a few of these types of tables over time, and he will get a better feel for what those numbers look like with various types of headers. And his best guess will be that much better next time.

There are plenty of other "trims" in the computer which could also be learned from over time. Returnless fuel system cars in particular would benefit from learning these real world values and then punching them into the tune.

I don't believe the car would really have a major air/fuel ratio problem or run lean because of this table. It might be a little off, but it learns quick enough to not really be an issue. Things like this, and fuel trims, are why a car will end up with better gas mileage after you put some miles on it (after tuning or battery disconnect).
 
#12 ·
^ awesome explanation, thanks for that, I had a vague clue about how this all works but you just filled a lot of understanding gaps for me!

and it made me wonder -- should the O2 sensors be replaced at some point? my car has over 100,000 miles on it; I'm wondering if there would be an benefit to "refreshing" the O2 sensors?

also a good reason to avoid disconnecting the battery, it possible
 
#16 ·
Alright well i ran the car a little today and it seemed all well besides a little popping when shifting gears. I dont know if thats just from the headers and no mufflers or not. I then stopped at my parents house for a little bit and when i started the car again it seemed like the timing was off, it had very poor acceleration and popped non stop when i hit the gas. So i stopped at a gas station to get more gas and when i started it after that it ran just fine for 15 miles all the way home. Is there any reason why the car would do that? I thought it might be the ecu trying to relearn but im honestly not sure.
 
#18 ·
On many Fords, there is a system called FAOSC. Fore-Aft Oxygen Sensor Control. When equipped, this system does actually use the rear O2 sensors for adjusting fuel trims. But I don't think it's about valve warping, or at least I've never heard that.

As usual, it's all about emissions perfection. Emissions is always the #1 goal at all the OEMs. So the way it works is that the front O2 sensors adjust the engine to a nearly perfect air/fuel ratio (stoichiometry). Then the cats clean up any leftover emissions, and finally the rear O2 sensors monitor the final exhaust output. The primary reason for the rear O2s is to determine catalyst efficiency, and to alert the owner when the cats are no longer doing an acceptable job of this.

But with FAOSC, this goes a step further. The goal is to get to a precise 14.64:1 air/fuel ratio (stoichiometry if using non-ethanol gas). By monitoring the final post-cat exhaust readings, they can use this to fine-tune that last percent or two of perfection. So they let the rear O2s do just a tiny amount of final fuel trimming.

Would this be enough to cause engine damage if it didn't occur? No, I don't believe that it would. It's all about perfecting emissions, that's all. Most OEMs don't even do this. Ford didn't for many years, and doesn't always use this system now. FAOSC is just an attempt to squeeze out a tiny bit more emissions and gas mileage. The heavy lifting is done by the front O2 sensors, which are important.

As for replacing oxygen sensors, yes. O2 sensors are a wear item. They have a slower response time as they age, which is why the O2 transport delay table is stored in KAM. They may not be labeled "bad" in a test or with a trouble code, but that doesn't mean they're up to new standards. I remember replacing O2 sensors that tested perfectly fine in my 300ZX twin turbo, and I picked up 1.5 mpg per side. Went from 16 to 19 mpg total, and I was quite impressed with that considering I didn't think they were "bad".

Ask any tuner, and he will probably tell you that it is not a bad idea to buy 2 new front O2 sensors on an older car. And ALWAYS use the OEM Ford/Motorcraft sensor. They just always work better for some reason than aftermarket. On a scantool, you can see the difference in the switch rate. If you datalog front O2 sensor voltages, you'll see them constantly sweep from lean to rich, 0 to 1 volt (approximately). The amount of time it takes for the sensor to detect a change in air/fuel ratio and turn around and start putting out a voltage in the other direction is directly relate to sensor age, and also somewhat related to how good the tuner has the tune. We want to see as fast of a signal as possible. It looks like this: /\/\/\/\/\/\/\/\/\/\/\.


When you start to see it look more like / \ / \ / \ / \, it's a slow response. Halfway up that hill is the 450 mv mark, or the switch point. We want to see the least amount of time between switch points as possible, for best gas mileage and emissions. The $100 you spend on two OEM oxygen sensors can probably pay for themselves in gas mileage in less than a year.
 
#19 ·
The tank was a quarter full, so not that low. It just seems really odd how that problem was brought upon and fixed by turning the car off and on again. I didnt mention it but i actually went for a five minute drive the day before to see if that nut in the dipstick tube would hold up and it drove fine then too.
 
#22 ·
It's possible that the O2 sensors are cooling down quicker than the engine so when you start the engine warm, the ECU is still in closed loop operation while the O2 sensors aren't yet ready. This could cause the engine to run lean for a short period until the sensors are warmed up. As long as you go easy there shouldn't be any issue but ultimately, you'll need the tune to be revised to keep the ECU in open loop for longer to correct for the O2 sensor delay.
 
#25 ·
I found out after spending $500 at the tuner that bank 1 was one tooth off and thats why i was having problems. I retimed the motor and it obviously runs 10 times better. I know i shouldnt be hammering on it without a tune for the headers but i just cant resist. My next tune is 20 days away so i think it should be fine until then.
 
#26 ·
Had you recently done anything that would have caused the timing to be thrown off such as cams or phasers/chains, or oil pump?
 
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