Magnus

iTrader: (4)

First, I have a 3.8 V6... but I have done work on friends LS1's as well and I am always trying to learn a little more on how our PCM's work.

I read in some of these posts, and on how-to pages about the MAFT that you should try to get a 0% LTFT at cruise before you begin to modify the WOT settings.

Now, doesn't a 0% LT at cruise mean that your MAF reads just like a stock maf does? What if you change your MAF itself?

Whats wrong with having 5% LT's or -5% LT's at cruise? Does it really make a difference? The PCM will correct the air/fuel ratio anyway.

Lets say I have .890 O2's at WOT but my cruise LT's are +5% and my timing advance is good. Why change?

The reason I ask these questions is because I did some work to my MAF on my v6 car and now the sensor picks up more air than it would stock. I get good timing advance and run a little rich at WOT and idle. Not worried about idle cause I don't race at idle and I can always fix WOT with the MAFT. But why bother using the MAFT to make the PCM think my MAF is stock? Doesn't make sense?

So whats the reasoning behind finding 0% cruise LT's.

I also have noticed, the bigger your changes are with the MAFT, the more innacurate your readings become.

Pro Stock John

iTrader: (34)

I was told a few years ago that the GM considers +/-7% Ltrims to be reasonable. So I have always tried to keep within that.

I have the base 2% rich because my part throttle/cruise Ltrims were like 10-15%.

Matt98SS

0 is really just a "hopeful" # because the PCM does not have to work overtime adding or subtracting fuel. I don't think having them "zeroed" out so to speak with custom tuning for all but WOT fuel cells is a good idea.

if you are around + 5% then you are good to go IMO. You will see when it switches to different fuel cells it will go up or down a little bit. And that is what EFI does constantly changes its tune due to what load, conditions, fuel, the engine sees

Magnus

iTrader: (4)

Our PCM's work very fast so even with a +20LT I still believe the calculations will be done instantly.

Still not really any solid reasoning as to why 0% LT's at cruise are ideal.

Lets look at a scenario.
Stock LS1, no mods, readings at cruise.
15lb/hr MAF, 0% LT, 14.7:1 A/F (15lb/hr actual)

Now, do something funky with the maf so it picks up less air.

10lb/hr MAF, 5% LT, 14.7:1 A/F (15lb/hr actual)

See what I'm getting at here? Even with a cruise LT of 5%, the PCM will correct the LT's to be factord with the MAF readings to achieve the ideal A/F ratio.

I believe the PCM uses the LT's as a factor with the MAF readings to create the pulse width as the result.

If intake air is increased and MAF accurately reads that increase, LT's will remain at 0 and A/F will be ideal.

If intake air is increased and MAF does not pick up all of it, LT's will fall into the + #'s to produce an ideal A/F.

If intake air is increased and MAF picks up too much of it, LT's will fall into the - #'s to produce an ideal A/F.

The PCM will change the LT's to its likings to achieve its ideal A/F.

So, I am still not sure why 0% is ideal. Can anyone help me here?

[ December 13, 2001: Message edited by: Magnus ]</p>

SteveD

I had nothing but trouble with my car after I adjusted my Lt's to 0. I had alot of converter lockup problems. When the car shifted into 3rd at the tract I would get a bad case of TM that lasted for about 1.5 seconds. I have since raised my Lt's to about 8 and all of my problems seem to be gone. Boy I can't wait for LS1 Edit.

Steve <img src="graemlins/burnout.gif" border="0" alt="[Burnout]" />

[ December 13, 2001: Message edited by: steve350 ]</p>

kewlbrz

Fuel trims are frozen from being updated during fuel enrichment mode (WOT). The PCM takes a "snap shot" of where it was at just before going to fuel enrichemnt and freezing the trims.

In most cases more consistant tuning is achieved during WOT, when the trims are close to Zero.

For purposes of tuning, slightly positive skewed trims will achieve a bit of timing advance.

Try to think of trims as not adding the the base pulse width fuel equation, but as how the pcm perceives load.

for more information see my web site.

Magnus

iTrader: (4)

I'm still not really clear on why 0% LT's at cruise are ideal. Thats just a near stock #.

I'm guessing 0% LT's are not ideal at cruise. They are just acceptable, as are 5% and 10%.

Can anyone make this a little clearer for me or is the ideal 0% LT cruise just a common misconception?

JAS BLK 98 TA A4

Something that is never mentioned is that if you have an A4 and reduce you calculated load, you also reduce your commanded transmission pressure. A lot of guys are quick to blame torque management...

As Shawn is trying to explain, understand the system. To many people are using the MAFT as a bandaide...

Magnus

iTrader: (4)

Thats exactly what I'm trying to do here. Understand the system. I only use my MAFT+ to fine tune the WOT air/fuel mixture and bump up timing a little (LT1's, L36's and L67's have that capability with an ICM).

I understand that the PCM will always correct for a 14.7:1 ratio at cruise which is why I wondered why even bother trying to get those LT's to 0. As long as they are within range, your still going to have the same air/fuel mixture as soon as the PCM corrects for your change.

[quote]The general tuning rule for WOT where it's in speed density mode, and fuel trims are frozen, a near 0 value (when it was in closed loop non-wot mode) is useful to tune WOT a/f ratios, and keep a handle on KR.<hr></blockquote>

I'm not really sure I understand that.. I'm thinking.. It works to have a 0% LT @ cruise because the PCM would think the load is normal/stock and would not increase the timing that much at WOT and produce any KR, therefor its a good starting place to tune the WOT o2's using a MAFT.....

But, if you can have an increased LT and show the PCM there is less load, therefor having increased timing and still recieve no KR at WOT.. then that is even better (if your looking for more timing) to work with than shooting for 0% LT's.

To put it simpler... if you have 10% LT's at cruise and .890 O2's and no KR and good timing advance, you should NOT play with the base MAFT settings to reduce your LT's (Unless you want less timing).

One thing I have noticed though with the MAFT (I believe I already said this but I'll say it again), As you change your base settings further from 0%, the readings become less consistant. The bigger change you make with your base, the more variance you may have with your WOT O2 #'s. Its simple math, the bigger the factor, the greater variance the end result will have.

One more thing about load, advance, and PCM's. I notice with the LS1 that lower MAF readings and increased LT's usually result in more timing advanced. With my PCM (97 3800 Series II V6) I notice the opposite. I see 5 more degrees of advance with a -10% LT than I do with a +10 LT. Just some FYI.

Now basically I see the base setting on the MAFT to be used to help cold startup and idle qualities. (Other than trying to get some more advance out of your car)

- Keith

[ December 14, 2001: Message edited by: Magnus ]</p>

kewlbrz

[quote]Originally posted by Magnus:
<strong>I'm still not really clear on why 0% LT's at cruise are ideal. Thats just a near stock #.

I'm guessing 0% LT's are not ideal at cruise. They are just acceptable, as are 5% and 10%.

Can anyone make this a little clearer for me or is the ideal 0% LT cruise just a common misconception?</strong><hr></blockquote>

I basically said it above, but The reason 0 is where the PCM likes to be, is because, again that number reflects the pcms perceived engine load. Thus the primary factor it affects is timing advance and to a degree, WOT. Remember that during normal cruize the PCM corrects to 14.7:1 stociometric air/fuel ratio, no matter what.
It does this by moving the fuel trim's values in a postive or negative direction. Wheter they are +15 or -15, the output is still 14.7:1 air/fuel ratio at non WOT. You cannot by this function physically run rich of 14.7:1 or lean of 14.7:1 a/f. So to say, "im running rich or lean" is a misnomer during Non-WOT. The thing that changes however, is timing advance. Positive skewed values in the trims indicate a less perceived load is occuring and thus timing will be advanced. The converse is true for negative values. The general tuning rule for WOT where it's in speed density mode, and fuel trims are frozen, a near 0 value (when it was in closed loop non-wot mode) is useful to tune WOT a/f ratios, and keep a handle on KR.

[ December 14, 2001: Message edited by: kewlbrz ]</p>