Why disable LTFTs with open loop idle?

[Red01Z]

I tuned a car with a healthy camshaft this weekend and decided to use OL to control fueling between 0-1200 RPMs. Afterwords, I began researching a solution for P0131 and P0151 and noticed many people are using the same method to tame camshafts with large overlap, but they ultimately do 2 things I didn't. I didn't disable the DTCs and I left LTFTs on. I accept that there simply isn't a way within the controls of the factory PCM to get away from disabling the DTCs. But I'm puzzled why people are disabling LTFTs.

With the car in OL, LTFTs aren't used. From what I've read, the further away from idle, the less of an affect overlap has on 02 readings. I could see the LTFTs creating an overly rich mixture right off idle (1201-2k?), but it seems like we're cutting off the nose in spite of the face. Is this done because our high gear cruising is generally 1500-2000 RPMs, which may still be in a the window where overlap is skewing the fuel trims? If not, can anybody explain why LTFTs are generally disabled in this scenario?

Thanks!

PS. I'm not looking for responses like.. "LTFTs are junk. I've had my disabled for 10 years" without an explanation of _why_ leaving them on is detrimental.

[lwrs10]

LTFT are generally disabled on most modified vehicles, not ones with just big cams. Biggest reason is the farther away from factory you get the fuel trims will be very hard to dial in perfect. A bone stock motor it is very easy to get the fuel trims damn near zero under any condition. Once you start adding a cam, ported heads, bigger injectors, intake, and so on, it is almost impossible to get trims under any condition perfect, specifically transients. You can get damn close, but will never get it "perfect". Since LTFT are just an average of STFT, and respond pretty slow to change, AND groups together a pretty large group of VE cells per fuel trim cell, it is not ideal.

Now STFT react almost instantly to change. If you run only STFT, you will only have a split second of wrong AFR before it reels it in. And while STFT also follows the fuel trim cell model, as said before correction is instant.

Another problem with LTFT is if they are positive, they will add fuel when in PE WOT. Most people do not want trims affecting your full throttle AFR.

You can also look at it this way. People who tune for a living just do not have the time to work on your cars for days to get everything 100% perfect. A 4 hour dyno tune and a quick around the block test are not enough to get everything perfect. They would go broke quick if they did spend the time. So most get it really close and using STFT only makes up for the countless hours it would take.

I am mostly taking about part throttle stuff. WOT tuning can be spot on with 4 hours of dyno time.

If I am mistaken here please correct me.

[NewbieTuner]

I actually saw this thread and registered to reply. I'm extremely novice tuner, however: @lwrs10 "LTFT are generally disabled on most modified vehicles", I don't find that statement true from my limited experience. Just because LTFT is on, doesn't mean STFT is now ignored, or off. Those instant adjustments will still be made. I don't see how turning off LTFT affects STFT, or benefits it. I'm not arguing against turning it off, I just don't understand why you would.

I'm also curious what some expert tuners would have to say on this subject.

[Red01Z]

LTFT are generally disabled on most modified vehicles, not ones with just big cams. Biggest reason is the farther away from factory you get the fuel trims will be very hard to dial in perfect. A bone stock motor it is very easy to get the fuel trims damn near zero under any condition. Once you start adding a cam, ported heads, bigger injectors, intake, and so on, it is almost impossible to get trims under any condition perfect, specifically transients. You can get damn close, but will never get it "perfect". Since LTFT are just an average of STFT, and respond pretty slow to change, AND groups together a pretty large group of VE cells per fuel trim cell, it is not ideal.

Now STFT react almost instantly to change. If you run only STFT, you will only have a split second of wrong AFR before it reels it in. And while STFT also follows the fuel trim cell model, as said before correction is instant.

Another problem with LTFT is if they are positive, they will add fuel when in PE WOT. Most people do not want trims affecting your full throttle AFR.

You can also look at it this way. People who tune for a living just do not have the time to work on your cars for days to get everything 100% perfect. A 4 hour dyno tune and a quick around the block test are not enough to get everything perfect. They would go broke quick if they did spend the time. So most get it really close and using STFT only makes up for the countless hours it would take.

I am mostly taking about part throttle stuff. WOT tuning can be spot on with 4 hours of dyno time.

If I am mistaken here please correct me.

Thanks for the reply. However there are a couple of statements that just don't sit well with me. "Perfect" LTFTs have generally been regarded as those between 0 and -5 (closer to 0 the better). I haven't had a problem tuning normally aspirated modified LS1 into this range in driving cells. I'm under the impression that the most critical time for LTFTs to be negative (or 0) is the second the vehicle enters PE. This usually is just above idle or midway in the RPM band from a roll. Both of which are easily tuned using conventional methods with LTFTs enabled.

STFT's ability to make quick corrections is not hindered my LTFTs, unless the LTFTs are skewed by false 02 sensor readings (like large overlap cams *at idle*). In fact, STFTs should actually make corrections faster with the use of LTFTs.

I will agree that often times, shops don't have the time (or the customer isn't willing to pay the money) to fine tune things. However, the statements I've read on this forum I've read haven't had an asterisk by them indicating it's only for people looking for a shortcut.

[Kyne]

Biggest reason, they can add fuel to PE when you don't want it to. Some turn them off some don't.

The real fix to bad o2 readings on a large cam is injector timing not fuel trims.

[5_Liter_Eater]

I don't see LTFT's doing anything that STFTs won't. Once you're in closed loop the trims are going to do what they think is right to keep it stoich. Whether you turn off LTFTs and have STFTs do that or leave LTFTs on and let them both work it out doesn't make any difference in regards to what you're talking about where the cam is still causing wonkyness below ~2000 RPMs.

People disable LTFTs because then they don't have to worry about positive LTFTs being tacked onto PE and making it richer. Thats really the only reason. STFTs are held in memory between ignition cycles just like LTFTs are so as long as you're within +-25% then there is really no need for LTFTs. But again, if you're in closed loop where the cam is still causing wonky trims it doesn't matter whether LTFTs are enabled or not.

[Red01Z]

So, the consensus is that turning off LTFTs has nothing to do with having an OL idle? I'm aware of the implicit adder during PE when the LTFTs are positive, but to me, that seems like a safety measure that doesn't need to be disabled for this vehicle. I noticed the few threads talking about OL idle mention turning off LTFTs which caused me to question how they were related.

Thanks!

[lwrs10]

Thanks for the corrections to my statement. I still have lots of learning to do!

[NewbieTuner]

I'm going to give this a bump, I'm interested in hearing more opinions.

[MMGT1]

What if the vehicle is exposed to 50*C temp changes in the run of a just a couple of days? It happens here, believe me...lol Would you not want LTFT in play if the vehicle really does need a boost in fuel? For example; when one day it's your perfect 13.0AFR with ambient temps of lets say 15*C and 2 days later its -35*C. Logic tells me it needs more fuel for the increase in oxygen content. If we only have STFT in play would this not result in an unwanted lean condition and possible damage to your engine?

[roastin300]

Assuming a correctly tuned VE and MAF table, would it not compensate for the more dense air charge (higher air mass flow) as a result of the lower intake temperature and maintain the targetted Equivalence Ratio specified in the PE Table?

[danieloneil01]

I'm by no means an expert at tuning. But I have a blower and SD tuned. I keep my STFTs on with LTFTs. My car was dyno tuned during the summer with no meth. My tuner did say I didn't need the STFTs or LTFTs on at all but I keep the STFTs on just incase.