Another_User

I think I may have found SOME of what is causing the surge at cruising speeds, and while working on it I found a way to keep your revs from dropping too fast.

I was logging and noticing a large difference in spark advance on and off throttle, so I tweaked the Base Spark in Park and in Drive tables. I changed the values for 1200-2000 rpms x .08-.16 g/cyl to 34 (same as 2200 rpms+ in case the value is different on other years). The results, less lugging while transitioning between no throttle to partial throttle. A side effect was caused by the changes to Base Spark in Park (neutral), which I don't really need but may help others. I noticed that change cause revs to drop considerably slower. That may help some people who are having problems with revs dropping too quickly between shifts.

Any other thoughts?

I think the rest of the part-throttle low-rpm surge issue is being caused by a lean mixture, but I have not decided how to go about adding fuel while TPS is at 0 during cruise...

patSS/00

iTrader: (6)

Were you changing the values up or down?

Ragtop 99

iTrader: (7)

I notice there was a huge difference bewteen the high octane table and base spark in drive table and plan to try reducing the timing in the high octane table to see if that helps with my cam surge in that range.

Your sig does not indicate that you have a cam swap. Are you on the stock cam?

Another_User

Hotcam, 219/227 112 lobe sep .525 lift...I think those are the specs. I don't think reducing timing while driving will work, raising the "no throttle" in drive to come closer seems to help. Instead of being off like 12 degrees most of the time it is off by like 4. Seems to make a difference, but I think there is still a lean issue somewhere around there. I have started from scratch and taken my IFR back to normal...I have a thought that changing that isn't so good at low rpms...but I could be wrong. I got to thinking about those "global" values everyone changes, but everything is based off your injectors. So if you haven't really changed them, I am thinking it might be bad.

On a seperate subject, my test on modifying the VE table has directly affected my LTRIMS, but I am not positive if it was a 1 for 1 change. Pretty close though. Of course now that I am scaling my IFR back to stock, it is going to require some more changes.

Ragtop 99

iTrader: (7)

I bumped my VE table by 6% at 1600 and 2% at 2000 and it did not seem to help in terms of curing the surging.

If cutting the timing does not work, then I'll experiment going the other way. I was concerned that if I'm lean, adding timing would lead to knock. I haven't had transistion issues, just some surge in that general range, both on and off the gas at low TPS.

HumpinSS

iTrader: (1)

I used my ifr table to fix my ltrims issue. Some places are -10 but i will log that with Hptuners to get the histogram and see where i have to reduce in those spots. My ve table was calculated using a formula and not by my ltrims. I dont know if the ve table is a 1:1 relationship, i do know the IFR table is.

Another_User

One thing that makes me a little less concerned about it is since there is more overlap with the cam, it gives kind of an "EGR effect" at low RPMs, cooling the cylinders, and if I am not mistaken you will also end up with a lower effective compression ratio.

As for the 1 to 1 on the IFR, that would probably be ok, except you are lying to the computer about the injector flow. I am trying to figure out if that would negatively affect other fueling areas. For some reason my car always seems to run worse after I change the IFR, even after the computer compensates. Beyond LTRIMs, I am wondering if there is some multiplier the IFR is involved in that could mess things up.

HumpinSS

iTrader: (1)

Man this is confusing. At first tuners used the maf to correct for fueling. This went away fast because it was claimed to trick the computer into thinking more or less air was being metered by the sensor. So we moved on the the ifr table. same case withthe maf the injectors havent changed so why should we mess with that table to get things squared away. A little while ago i read not to use the ve table for closed loop ltrim fueling. Figure out what the engine needed and go to other tables to get your ltrims straigh. Is there some mysterious table that we arent seeing that will put all this to rest?

Ragtop 99

iTrader: (7)

Back from a test ride. Cutting the timing didn't help. I'll try your idea next.

Bink

Humpinss - how the PCM uses the MAF and MassAir Calc effects lower RPM/Part Throttle. From Gameover:

LS1 VCM Main Airmass Calculation
=================================

A very important calculation the VCM must make to ensure correct fuel mixtures under all driving conditions is the dynamic airmass calculation. This is the main calculation the VCM uses to determine how much air it should use for the Base Fuel Calculation (Inj PW). The airmass calculation is not simple and uses various combo's of MAF and SD inputs depending on engine operating conditions such as current engine RPM. It may also make decisions based on whether the engine is in a steady load state (steady MAP) or unsteady load state (Unsteady MAP). Note, that these thresholds change with RPM and MAP. eg. at higher RPM or high MAP readings you have more leeway before the VCM decides that you have an Unsteady MAP condition. This unsteady MAP defintion is basically there to decide if a throttle transient has occured (or other) ie. the MAF input is known not to be accurate under these operating conditions.

Under normal conditions (ie. all sensors working properly) in the code i'm looking at it is like this:
(caveat: many of these thresholds may vary between code revs and vehicle type)


RPM > 4000
----------
trust MAF completely and ignore SD calcs (apart from MAF sanity checking purposes)

RPM < 4000
----------
if RPM < 2400 and MAP < 84 kPa then
Steady MAP threshold = 0.0 kPa
else
Steady MAP threshold = 0.8 kPa

If (SteadyMAP) then
Calculate MAFAirmass/SDAirmass ratio (used for Unsteady MAP operation)
Correction Airmass = MAF Airmass (filtered)
else
Correction Airmass = SD Airmass x MAF/SD Airmass Ratio (calculated during Steady MAP conditions)

Transient Corrected Airmass = previous Final Airmass + proportion of Correction Airmass

Final Airmass = fn(MAF Airflow, previous MAF Airflow, prev 3 MAP readings, prev 3 TPS readings,
Transient Corrected Airmass)


There are 9 coefficents to this filter (and a total of up to 16 diffent sets of coefficients depending on operating conditions). It is worth noting that the previous value is weighted heaviest followed by the 2 MAF terms, so MAF dominates IMHO).

There are also a number of checks at the end to make sure things do not exceed certain limits


To summarise:
1. High RPM behavior is totally based on MAF
2. Mid RPM behaviour has an allowance on Steady MAP behaviour before it switches to Unsteady MAP
3. Low RPM behaviour (where the bulk of the fuel cells are) is dictated by unsteady MAP behaviour that is still mostly dominated by the MAF input with small tweaking from SD)

If the VCM decides that a throttle transient has occured (unsteady MAP), the airflow incorporates a "correction" from the SD calculations. This value is the SD calulated airmass multiplied by the previous ratio of measured MAF airmass to calculated SD airmass (this normalises it, since you are worried about the transient deviation from MAF airmass only). The way i understand it is this, imagine you are cruising (MAP is steady), you have a decent vacuum in the manifold and you change the throttle position quickly. Air rushes into the manifold to service the change in air demands from the engine itself but also to try and fill the vacuum. The result is that the MAF reads higher than it should at that point in time (spikes). This is more pronounced at low RPM where the engine airflow is smaller and the relative proportion of extra airflow due to filling vacuum is higher, also the MAF is known to be more inaccurate at lower RPM and more non uniform airflow. IMHO, the SD correction is to account for filling and emptying of the manifold during throttle transients and also to smooth the MAF's spikyness at lower RPMs.

The bottom line is that if the engine is at a steady load state or operating at high RPM then the airflow is 100% based on the MAF once you get thru all the filters and calculations. And the SD calcs only get used for transients and smoothing lower speed operation.

At no stage does the MAF get ignored completely in these calcs (the dominating terms of the main filter calc are always MAF based).

An interesting point to note is that removing the MAF basically bypasses the whole system and directly sets the Final Airmass value to be the result of the SD lookup (it also disables things like knock learn and a few other nice things). Most have taken to calling this "Backup SD Mode" which is as good a name as any i guess and i meant to allow the engine to run with a failed MAF (although it is quite possible to tune usuing this mode (eg. HSV GTS 300kw comes factory this way). There is another way to disable the MAF system completely (ie. without setting the DTC's) and by tuning of the thresholds and other flags you can get a fully functional SD tune happening, the so called "True Speed Density Mode"... maybe more on that later...

Since i haven't had time to step this out in painstaking detail and i haven't yet qualified for my guru pin, please feel free to correct any errors that are in here. i'm sure there are some.. as things have been a little busy around here lately

gameover...
__________________________________________________ __________________________________________________ _______________________________

And here is the Link to the Thread of Gamover:
http://www.hptuners.com/forum/YaBB.p...num=1078851555

Hope this helps some.
joel

patSS/00

iTrader: (6)

Very useful info, and it does make sense - during MAF "spikes" at lower rpm, a SD calculation is used briefly instead of the MAF input. Seems like there are some possibilities for this algorithm not to work too well under some conditions, though. For example, won't the SD calculation only be optimal for a given atmospheric pressure, meaning affected by weather and altitude?

Ragtop 99

iTrader: (7)

tested out your idea. bumped the timing up to 34* and no luck.

I may be stuck having to use more idle air.

Another_User

I don't get it. I still get the surges, but not the insane bucking you can get sometimes at low-rpm. I still think it is something running lean to finish cleaning it up...back to the drawing board.

Ragtop 99

iTrader: (7)

Yea, it is not insane bucking. At times when coasting, it felt pretty stable. I think I may try leaving the base table up but pulling the high octane table down a bit so that I don't use 40* of timing.

Another_User

I thought about that too. Now that the timing part is starting to fall into place I need to examine my fueling. I have started from scratch on my IFR (back to stock) and I am using my VE table I half-stole/half calculated. After I drive a little more I am going to start tuning and see if my lean spots have changed any.

Ragtop 99

iTrader: (7)

I loaded up a timing table for the high octane where I cut the timing from 2* - 4* to get some smoothing between the high octane table and the base spark table. I think in the effected area it keeps the timing to between 34 (base spark table) and a max of 38* (high octane table) but I would have to take a second look.