Is adjustment of LTFTs practical without altering MAF table?

[JohnL]

I'm doing my first tune on a friend's LS1 VT Commodore A4 using EfiLive and Flashscan.

The car has a set of Pacemaker tri-Y's and a Unifilter pod filter and inlet. It pulls about 165 rwkW on an old Vane dyno. The LTFT's are between 10 and 20% positive and the AFR's on my wideband show mostly 10.9 to 11.5 at WOT. The headers go from blue to red very nicely :shock: .

If this was an early SD Delco, I would adjust the VE's appropriately and it would be good. But I've read conflicting accounts of how the VE inputs are a low priority part of the fuelling filter on an LS1, which purportedly relies mostly on MAF inputs to set the injector pulse width. On this theory, altering the VE's is not the quick way to zero LTFT's, and may not even have much effect.

The EFILive note on the MAF table says something like "don't change this table unless the MAF is modified from stock"- and it's not. So if the VE's aren't worth changing, and you shouldn't modify the MAF table on a stock MAF, where to from here?

I'm planning to adjust the VE's to take account of the LTFT data, scan the PCM again, and then modify the MAF table until I had close to zero LTFT's. But a post from GMPX says + or - 5 is okay.

I'd appreciate some enlightenment please, Oracles :)

John

[dfe1]

For optimum fuel economy and emissions control, actual air/fuel ratio is supposed to be 14.7:1. Long term fuel trim is a response by the PCM when air/fuel ratio should be 14.7:1, but isn't. Its first response is to alter short term trims and if they move too far, or stay there too long, then the long term trims are moved. In essence, short term trim ratchets long term trim until short term can return to or near zero. If the long term trim amount is excessive, then the fuel system is either delivering too much fuel or not enough for a given operating condition. Since fuel flow is the problem (in most cases) adjustment of fuel flow is the most logical answer. Rather than screwing with the MAF and VE tales, alter the Injector Flow Rate. It doesn't take much of a change to have a noticeable effect, and since the LS1 PCMs have a table, rather than a single entry, the fuel delivery curve can be tailored as required. There's no need for trims to be zero at all times, and you find that even with a stock engine/stock tune they move around a bit. So long as trim amounts are in the 3-5% range all is well, but keep in mind, they should be slightly negative to prevent the PCM from adding fuel at wide open throttle. On the other hand, if major modifications have been made to an engine, then VEs have in fact been altered, so the VE tables should be changed to more accurately reflect an engine's volumetric efficiency after modification.

[dfe1]

Just had another thought-- if you don't disable converter overtemp, you'll never get the WOT air/fuel where it should be. If you log the dyno run, you'll probably see commanded air/fuel take a noticeable dip at some point, which is where overtepm protection kicks in.

[JohnL]

...Rather than screwing with the MAF and VE tales, alter the Injector Flow Rate. It doesn't take much of a change to have a noticeable effect, and since the LS1 PCMs have a table, rather than a single entry, the fuel delivery curve can be tailored as required. I understand what LTFT's and STFT's do, but it sounds like you're suggesting I fix a VE/MAF issue by a kludge based on the IFR vs pressure table. This might help a bit (because virtually all the LTFT's are > 10% and I could take this or better off every point in the LTFT table by selectively adjusting points in the IFR vs MAP table), but it sounds like a waste of a great PCM :) .

I'd like to fix the LTFT problem the "best" way (maybe I'll try the easiest way second ;) ), but I'm not clear on how the PCM filters VE and MAF- based calculations to give the WOT AFR.

The other thing I don't get is how the car ends up with AFR's around high tens, low elevens, when the maximum EQ is 1.13, and I have 14.6 AFR in cruise (non-PE) mode. It's like it's adding the 20% LTFT correction (required to reach the 14.6 in the first place) twice, then factoring by 1.13 for PE mode :? .

John

[dfe1]

I don't think you have a VE/MAF issue, I think you have a fuel issue. If the only modifications are the ones you listed in your first post, the VE shouldn't be all that different from a stock engine. Besides, altering the VE or MAF tables results in fuel flow being altered. If you increase a value in the ve table, that tells the PCM the engine is using more air and consequently needs more fuel. If you alter the MAF table, you're fudging the frequency/air flow relationship-- again the cause the PCM to alter fuel flow. If your LTFTs are off by more than 10%, either your IFR table is wacked, or the injectors aren't flowing what they should. Either way, it's a fuel issue. The engine is extremely rich at WOT because your LTFTs are positive. You have 14.6:1 AFR in cruise because the PCM is adding the amount of fuel required to get you there.

[JohnL]

1)
I don't think you have a VE/MAF issue, I think you have a fuel issue. That's "statin' the bleedin' obvious" but thanks for staying with me on this. This post is not attracting much other attention is it?- must be either too complicated for most folks to answer, or too obvious :wink: . What else can the issue be but VE or MAF settings (unless it's the code) if the injectors are being held on longer than the stock tune requires at cruise, but less than the stock tune requires at WOT (because of the low AFR's).

2)
If the only modifications are the ones you listed in your first post, the VE shouldn't be all that different from a stock engine. Besides, altering the VE or MAF tables results in fuel flow being altered. If you increase a value in the ve table, that tells the PCM the engine is using more air and consequently needs more fuel. The LTFT's are correcting what the PCM senses as low VE's on the basis of the O2 results. 100 MAP VE's are at worst 10% low- that seems about right for the modifications. The LTFT's are correct because I'm getting AFRs of 14.6-odd everywhere in cruise mode. I realise that increasing the VE or MAF values adds more fuel, but how can it be that the LTFT's add fuel at cruise and I need to pull fuel out at WOT?

3)
If you alter the MAF table, you're fudging the frequency/air flow relationship-- again the cause the PCM to alter fuel flow. It's certainly possible that the MAF characteristics have been altered by different flow patterns in the new pod filter and MAF pipe- that might justify the MAF table recalibration (but the MAF itself is stock, and the Flashscan note on the MAF table says don't change it) :? .

4)
If your LTFTs are off by more than 10%, either your IFR table is wacked, or the injectors aren't flowing what they should. Either way, it's a fuel issue. How could this be the case if I get 14.6 AFR's at cruise?

5)
The engine is extremely rich at WOT because your LTFTs are positive. You have 14.6:1 AFR in cruise because the PCM is adding the amount of fuel required to get you there.How can it be extremely rich at WOT when the WOT fuelling is calculated as a maximum of 1.13*injector pulsewidth at cruise (which should mean about 12.9 AFR)?

Thanks again for keeping up the dialog.

My most basic problems are that I haven't read anything that definitively indicates how the overlapping roles of the VE and MAF tables are handled, and I'm confused at how the PE fuelling gets to be so much richer than cruise.

John

[The Alchemist]

What I have learnt is that the MAF table has the main effect on fueling and that the VE table is a backup for whern & if the MAF fails. I have tuned 2 cars with modified MAf's and it took me a wee while to get the MAF table where it needed to be to get the AFR's that I wanted. The VE table wasn't changed at all as I have not done a MAFLESS tune on either of these vehicles. Doing so would take a very long time to get right, even on my dyno, and the customers would never pay for the time :)

However in your case the MAF is standard and "yes" I have seen the warning too about not changing the MAF table if it is standard....however I fail to see how changing the Injector table as being your best choice to patch up your readings ( for a number of reasons ) and agree with you that this would be a waste of a great PCM. Where to from here? Well my suggestion would be to recalibrate the MAf table with the help of a dyno operator. This must be done very carefully, obviously, but I think this will give you the results you seek.

regards, Mike.

[Bruce Melton]

I started with new larger injectors applied to a good custom tune done before my new Flashscan.
I applied a calculated correction factorto IFR and went way rich (+) on my LTFTs; all of them.
By scaling back the correction factor by almost 50% I have achieved >-5<0 LTFTs in all but the very low end and idle. In that this is not an issue for PE I intend to reduce those low end values individually on the IFR table.
I invite your comments as I see having a relatively straight line IFR fuel advance graph with ~~0 LTFTs and a stock MAF as the best and most direct approach?