2001CamaroGuy

iTrader: (6)

388 all bore with 246/253 109LSA cam....


I have the engine idling (950RPM) "ok" (not "good" but I could slap the peddle to increase RPM and it would come back down and find its idle) in open loop (EGTs were in the 550-600 range) but as soon as it would kick over to closed loop, the LTrim would jump to +20%-+25%, the EGT temps would drop to 300-350 range, and the engine would flood out and die. It looks like the computer is reading the overlap as lean (lots of intake charge being dumped out the header) and dumping fuel to try and fix the "problem". As I see it I am going to need to just turn off closed loop (set the enable point at 250 degrees) and work on getting the car to idle and drive right in open loop first.



I decided to take another look at the VE table. I have already done some massive cutting on the 400-1600RPM range (45% cut at 400 and 85% at 1600) which seems to have helped a lot. While looking the table over, I noticed that the "best" VE point (where the engine would be most efficient) was in the 4500RPM range (which would match the stock cams torque curve). As I see it, I need to shift the table around (put the "best" VE at my expected peak torque/5500RPM area) and then blend/smooth the rest of the table out (no peaks/valleys) so the table matches what the engine should actually do.

Does what I said sound right?

Country Boy

iTrader: (24)

This is the only thing stopping me from tuning my own car.... this VE stuff is confusing!

HumpinSS

iTrader: (1)

Hvae you changecd cylidner displacement in your tuining program. That may be a dumb question but thought id ask

2001CamaroGuy

iTrader: (6)

yep......

monodax

Based on the analysis done by Chris @ HPTuners the PCM doesn't rely on the VE Table as much when the RPM's are > 4000. If you look at his pseudocode here you will see this. So in theory you need to have some basic VE info in these ranges but it is very likely that the PCM is going to use the MAF sensor for it's fueling calcs.



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...

Bink

The VE table still needs to be accurate for 4000 and above RPM. It of course also needs to be accurate below 4000 RPM. FWIW.
joel

Another_User

I think you pulled way too much off your low-end of the VE table. Thats probably why your LTFTs are so out of whack. The more you scale down the VE table, the leaner you run...but the computer always tries to put you back to the right air-fuel ratio. If you are seeing +25 the computer can't adjust. I have had alot of success with only scaling 400-1600 rpms by 85%. Try that, and additionally if you are seeing positive LTRIMs in RPMs beyond where you scaled down the VE table, then change your IFR to put you slightly negative. I am getting decent at molding the VE table...but there are still some things that elude me...

2001CamaroGuy

iTrader: (6)

LOL....well ignore everything I said in my first post....

It turns out my MAF table was WAYYYYYYYYYYYYYYYYYYY off. If I unplugged the MAF (to force speed density) and adjusted the VE table to idle right (16:1 A/F), when I plugged the MAF back in it went to 25:1 A/F and died. Off idle everything seems to work fine (MAF is correct) as I can enable closed loop and the computer takes care of everything. The only explanation I have thus far is that perhaps the 109LSA coupled with the duration is just making the "atmosphere" inside the intake/TB/MAF so turbulent that it can not read correctly (with the "stock" MAF table).


So in final, you will still have to scale the VE table for big CI motors. Sorry for the wrong information.