Another important thing to consider with excess trims (in either direction) is that they are used as a correction factor on the fueling end result, not the initial calculation. The desired injector pulse width is calculated off of the engine RPM and calculated mass of air per cylinder (based on MAF, MAP, ECT and IAT input), with the assumption that the injector flow rates are accurate. Simultaneously, the PCM uses that same calculated air mass value to reference spark timing (in addition to torque calculations mentioned by 12secSS).

When the injector flow rate table represents the ACTUAL flow of fuel and fuel trims go out of whack, you absolutely will see incorrect air mass calculation (assuming properly working O2 sensor(s)). Correction of this condition will yield spark reference and torque calculation that is much closer to actual vehicle conditions.

To confirm this, take a look at two similarly setup vehicles with largely differing trims. You'll see that the dyncylair value will vary considerably between the two, and identical spark tables will yield differing final spark advance numbers.

 

 

Dang... and the past 3 years I was soooo worried about tuning my VE table. All I had to do was enable my fuel trims. Hell, tuning cams will be easy now.

 

L-trims are just that. If you want to spend every last breathe getting to Zero, then go for it. But sometimes you have to accept the fact that you are only going to get a few percent one direction or the other. It wont hurt. The things that should matter to you is how consistant they are when you enter WOT.

Other tricks you can look for are when you have big cams, its nice to have the trims alittle on the positive side under high map conditions because it helps with the reversion at times. Something to think about.

To help out DC-Justin, the thing that mis respresents your timing tables is the MAF. The more you mess with the MAF, the more inaccurate your tables become. The best **** on the car is your IFR table.

Rick

 

Fuel trims are pretty meaningless except as a diagnostic. They are affected by the air quality, by the air temp, by the engine temp, by how much you've lied to the PCM in MAF table, by how much you've lied to the PCM in IFR table, etc. A very very tight tune is +/- 4 LTFT. Why? What's 0 today may be +6 tomorrow with no changes to the tune. The GM standard is +/-10...

Tweaking IFR to get fuel trims to zero or slightly rich is a practice that developed so you could tune WOT that tuning session. The reasoning is a 0 LTFT or a rich LTFT means the PCM adds no fuel when you are entering WOT, so you can accurately set your PE.

The ubiquity of widebands, a better understanding of the VE table and the role of the MAF, and learning how to easily disable the MAF for tuning purposes has lead to much more effective tuning methods, namely, developing a custom VE table and controlling fuel that way for WOT tuning. Why is it better? Your maf table is accurate. Your IFR is accurate. Your VE table is accurate. There are no lies in the machine.

There's a saying from computer science that every DIY tuner should chant to himself as he falls asleep every night: "Garbage in, garbage out."

 

Tweeking the MAF table should be the last thing you do. IT has nothing to do with just setting up for PE.

People who think that trims change within days like that must not drive long enough drives to make sure the trims are where they belong. Things like heat in the pipes, fuel warming up versus being cold all effect your trims, thats why time is what you need. And if you think an SD tune does not change, you are wrong again. Again, fuel temps and such effect the AFR rate as well.

You will also notice that by changing IFR rates, it has no direct effect on the timing table and such other things used by the PCM. Changing or altering the MAF table changes that and other things as well. The IFR table is there to change rate of flow. It does not know what injector you have. The MAF is a calibration the entire PCM uses and is based off of. You will notice the IFR rate changes when your O2's are in different spots, MAF is closer or further away from the TB, ect. So again, there is no one true spot for the IFR table for any injector.

I have pretty much tuned cars in every fasion from open loop, to closed loop, to 1 bar-3 bar. MAf, no MAF, ect. I have found that in time, simple is more.


Rick

 

I think what I was trying to get at came across wrong, as I would have to disagree with this statement. For a technically accurate tune, the IFR table is one of the only tables that you absolutely should not be tweaking. Set it to the accurate flow rate of the injectors, at whatever fuel rail pressure is being used, and be done with it. Assuming that it is correct and fueling correction needs to be made, then make it to the VE table and/or MAF as needed. Yes, it may result in an air mass calculation that is quite a bit different than before, and yes it may require changes to your spark table to compensate, but those changes are necessary to bring everything in line. If done properly, the resulting tune will be as close to accurate as possible.

 

Curious myself.

 

good stuff here!

 

It has the same effect on LTFT as tweaking the IFR. It's just the other side of the same air-fuel coin.

STFT change every second. LTFT change based on how much data goes into the LTFT calculation. Using the "clear your LTFT's" tuning method, they change very quickly. As I said above, GM says the variance on LTFT's is +/-10...

I totally agree SD changes. That's why we have MAF's to begin with to avoid having to retune the VE for season and altitude changes. That has nothing to do with IFRs or the MAF table however.

I've also tuned cars in all those methods and would add in hacking at MAF and fuel tables using hex editor with a PROM burner in 1995 I taught myself to tune these new fancy 'control everything' PCMs in HPT on a 10.5:1 FI motor at 10psi in Florida heat (102* the month I started) and on the street. I've never been lean enough to hurt anything (on any of the vehicles).

I started with the tuning method d'jour : IFR tweaking and LTFT obsession. It was a horrible experience. No one knew why changes weren't stable, no one knew why we saw such funny side effects. No one knew why it worked in some RPM ranges and didn't in others.

What I've discovered is that it's important to give the computer an accurate picture of what's happening (because otherwise you are spot tuning and it *will* side effect your PCMs functioning in other 'spots'). Your IFR is affected by one thing. Fuel pressure. Injectors don't know anything about O2s or heat or anything but the size of the nozzle and the amount of fuel pressure driving fuel through that nozzle and the PCM never learns IFRs. Injector Duty Cycle (or more accurately the commanded open time of the injector) is effected by the things you mention, PLUS the MAF, IFR, and the VE tables.

Here's what putting in an incorrect IFR does. Let's assume for a minute that you tell the PCM the injectors are 10% bigger than they really are.
1) The MAF reads x amount of air and the PCM looks at the VE, MAP, etc. and calculates it needs y amount of fuel (gross simplification).
2) The PCM looks at the IFR and determines to get y fuel in the chambers it needs to open the injectors for z time
3) The PCM opens them for z time, but the injectors are actually 10% smaller than the PCM thinks resulting in less fuel in the chambers than it thinks it has to provide based on the calculations
4) The PCM looks at the O2s to see if it's successful (comparing the o2 mVs to what it expects)
5) Based on the comparison in 5, it records an STFT and adjusts injector timing for the next cycle (After the appropriate time period, the STFTs for the period are averaged into the LTFT data)

Virtually the same thing happens when you put in incorrect MAF (or VE) values. Only 1 and 3 are different.
1) The MAF *incorrectly* reads x amount of air... (or calculates airmass based on an incorrect VE reading)
3) The PCM opens the injectors for z time and puts y amount of fuel in the chambers, but because the MAF incorrectly metered the air (ve had the wrong value to start the calc from) there is 10% less fuel than is needed.

Here's the rub. The VE table is an *approximation* in the stock tune. It's a generalization that GM hopes is "close enough" for all cars in all settings (but in reality is car and environment specific), and if not we have the MAF to help (there's a reason why the PCM ignores the VE over 4000rpm!). The MAF table is ussually *accurate* for the stock MAF, but may get out of whack a bit because of gunk on the senor, moving the MAF, etc. The IFRs are *assumed* to be accurate by GM in their calculations. Look at any stock tune...

There is no doubt you can cheat the PCM into doing what you want by many different methods (not the least of which is adding some electronics... Like a plug in resistance module on the MAF -- sound familiar hehe), but if you want the PCM to be accurate, you are better off tuning to the deficiency in the tune.