5.7RamJet

iTrader: (2)

Struggling with an issue where I'm maxing out the VE table at 2400 rpm under load just to get the fuel the engine needs.

I've check fuel lines, filter, pressure at the rail (43.5 psi with vacuum disconnected), timing, it's got the Walbro 255 pump. It's a Ramjet setup, and it's got the 24 lb/hr injectors, so this is with the injector constant set to 24.

Running a '727 ecu with $8D. Logging using TunerPro and WB. The WB (LC1) display agrees with values TunerPro is logging.

Fudging the injector constant helps but doesn't seem right.

Attached pic is with injector constant adjusted to 18 lb/hr.

I can provide any other details needed...

Any help greatly appreciated!

ULTM8Z

When you say "maxing out" out the VE, what is it you're chasing? High BLM's? Lean AFR?

5.7RamJet

iTrader: (2)

Lean AFR

ULTM8Z

you may be attacking the wrong thing with the VE table. If you're in Power Enrichment mode in this RPM and map, then modifying the VE table won't have any effect.

5.7RamJet

iTrader: (2)

Thanks for the replies.

Is it appropriate to temporarily disable PE while tuning this area?

ULTM8Z

You can. But you'd be looking at BLMs at that point. The ECM will be shooting for 14.7:1 AFR's, so you'll probably see the AFR bouncing around quite a bit. If you want lower AFR in that area with PE disabled, you'd have to shift the O2 thresholds upward.

However, if you're always going to be in power enrichment in that RPM and map, it may be more worthwhile to work the PE tables instead.

RBob

iTrader: (1)

It may be a lack of AE.

RBob.

5.7RamJet

iTrader: (2)

VE table is not used at all in PE mode?

Is PE included in commanded AFR?

ULTM8Z

No direct influence during PE. The only influence VE has on AE is the BLM that you're at just prior to entering into VE (somehwat indirect).

If your BLM is lean (i.e., >128 just prior to PE), your PE AFR will command richer than what you programmed into it. For instance, if your VE table is too lean and you're at 136 prior to going into PE, the commanded AFR will be increased by a factor of 136/128 = 1.06.

As such, I program my VE tables to deliver BLM's at or below 128 everywhere, that way the ECM is not screwing around with my commanded AFR's.

Once in PE, the ECM ignores the VE table completely. Commanded AFR is completely controlled by the PE tables (the coolant and RPM tables).

ULTM8Z

I might suspect that if the lean out was a transient condition, but if it's sustained lean, it seems like it's a PE AFR issue.

5.7RamJet

iTrader: (2)

I'm running open-loop right now, so not referencing BLMs (locked at 128). I've been trying to tune the VE table simply by looking at history tables of WB vs Commanded.

Thanks for the help so far!

ULTM8Z

The VE tables are ignored in open loop too, I believe.

IIRC, open loop is only a function of the stoich AFR and the Open Loop tables (AFR % change vs coolant and AFR % change vs map).

BTW, if you're running the S_AUJP adx and xdf files in TP, I'd recommend looking at the fuel items in the data list views... it'll tell you when you're going from open loop into PE (there's a parameter called "In PE").

MrWillys

During PE both the VE and PE % add is used. By reading the o2 volts you can tune from .860 to .900. The Target afr table is not accurate.

ULTM8Z

Actually the only things that are used to calculate PE AFR are..

1.) Stoich constant (typically 14.7:1 in the constants)
2.) PE % Change to AFR vs Coolant (where AFR is the stoich value)
3.) PE % Change to AFR vs RPM (again where AFR is the stoich value)

The way these are used is the following...

If item 2 above is A, and item 3 is B, then for a given RPM and coolant temp...

PE AFR = StoichAFR / (1 + A/100 + B/100)

So for example, in the attached picture (the PE tables from ANHT)...

At 80°C and 3600 rpm, I have values of 22.7 and 12.5 respectively...

Thus if I plug in the values I get

PE AFR = 14.7 / (1 + 22.7/100 + 12.5/100) = 10.87

I know this is correct because TP will read out the commanded AFR which matches the calculated values (again with the caveat that the BLM is less than or equal to 128 just prior to entering into PE).

Open loop works the same way and the equation is the, but using the Open Loop tables.

5.7RamJet

iTrader: (2)

ULTM8Z, in your example above, how does the ECM calculate an injector pulse width, knowing that it wants 10.87 AFR?

MrWillys

You should never want 10.87 afr and if tunerpro is calling for that it has detected a lean condition. The target afr in tp work inverse. Reading the o2 is the real way to tune wot. Lean best torque is 12.5:1 afr and 12 is safe.

Again, the pe % add table is in addition to the ve. Otherwise, why have ve beyond 91.9 kpa? The point at which pe kicks in?

ULTM8Z

The actual injector pulse width calc is something that someone like RBob probably can probably answer.

Yeah, the equation above is what you tell the ECM you want for AFR. Mind you, the actual AFR does not necessarily correspond to the commanded AFR. You have to get out with a wideband O2 and see what's actually occurring, and then adjust your table values accordingly.

Nevertheless, what I believe happens is that the ECM has to figure out how much air is in the cylinder in order to determine what amount of fuel to put in (since it's not directly sampling the airflow like a MAF car)....

engine displacement
air temperature
map

In this sense, MrWilly's may be correct in that it could be using the VE table to help determine the actual amount of air as part of the calculated injector pulse width.

But again, that calculation is still over my head...

5.7RamJet

iTrader: (2)

That's what I was trying to get at. I understand about the PE AFR, but at some point it needs to determine BPW and I just am not sure how it does that without using the VE table as a base.

If that's true then I would think that I should be able to attain commanded AFR (whether it's in PE or not) by adjusting VE table, or am I still missing something?

I used to be way more in tune with this stuff (pun intended), but I haven't driven the car in about 10 years. I'm finishing up the resto that I started in 2015, and decided to start tuning this weekend since it was nice out and the wife is out of town. The truth is that I probably drove the car more this weekend than I have in the past 15 years combined. So thanks for your patience.

Thanks again.

ULTM8Z

GM is notoriously rich in their PE. So I'm not surprised it's that rich.

Agreed that AFR's in the 12's are best for power. I seem to be at around 12.75 for best seat of the pants feel.

Actually into PE entering in the first place is not just a function of map. It's also a function of %TPS. So there could potentially be a high load condition where you hit over 91 kPa map at a %TPS that's insufficient to trigger PE.

5.7RamJet

iTrader: (2)

Thanks MrWillys. I was just going with 10.87 in his example for the sake of argument.

5.7RamJet

iTrader: (2)

I've been using 18 lb/hr constant for today just to see where that gets me, and so far things are looking good. Have the tables tuned to about 3600 rpm under all load conditions, and up to 4400 rpm in certain areas.

Once I see where the VE peaks with this constant, I'll probably change the constant back closer to, if not at, 24 lb/hr and re-scale both VE tables.

ULTM8Z

I've only ever used the PE tables to tune PE, but again, using the WB to see if I'm getting the AFR I want. That method works. Even if the VE is used somehow in the calculation of injector pulsewidth, I'd prefer to modify as few tables as possible in PE to get the AFR I want.

Thus if I leave the coolant modifier table alone, I'm only modifying the RPM modifier table (just one table to change). And I simply leave the VE table for part throttle operation.

Fast355

iTrader: (2)

That may be why you have had so many issues with your tunes. Adjusting the PE multiplier to fix a **** poor VE table is a bad mistake. Tuning the VE table is a much better approach. VE table is absolutely used for WOT as well as Open loop.

ULTM8Z

Not necessarily... looking back, yeah I've had issues, but typically it's been due to me experimenting with various parameters and then getting an unexpected result.... lol... or accidentally changing something and not knowing what it was that changed...

But I don't think I've ever had an issue where the solution was to fix the VE table.

Any rate, I'm at a point now where the PE AFR's I'm getting (reported on the WB) are extremely consistent and where I want them to be.... and that's with just tuning the PE tables for PE AFR.

But I do have the BLM set to lock at 128 in PE, so maybe that's helping take the VE out of the equation? Not sure... all I know is, I have PE tuning down to tweaking just the one table. Maybe not the most elegant/correct approach based on what I'm reading here, but I certainly can't argue with the WB results.

ULTM8Z

This thread prompted me to do some searching....

https://www.thirdgen.org/forums/diy-...t-afr-how.html

Interesting read...

So, the VE is used in calculating the injector pulse width, but not used in calculating the numerical AFR that you want. Interesting indeed...

My MAP and TPS% thresholds for entering PE has been significantly reduced, and with my BLM locking to 128 in PE, I'm generally into PE mode significantly before 100 kPa on the VE table (which essentially gives me no BLM feedback at high map operation). So my VE tuning in that 80-100 kPa region for the most part has been an extrapolation from map values below that.

Hmmm....So I guess my VE tables were "unwittingly" set pretty good since my commanded AFR (via the PE tables) is only about 0.5 to 0.75 off from my actual AFR at any point in the RPM range, and I get a consistent 12.5 to 12.75:1 AFR everywhere in PE (i.e., I'm commanding at most 13.5:1 to get 12.75:1 at the WB, which to me anyway, seems pretty good).

5.7RamJet

iTrader: (2)

Funny you mention it, I was just looking at some of my saved notes from literally over 10 years ago, and I had this exact part of TRAXION's post written down:

GMS of Air / CYL = ((MAP - EGR part press) * CYL VOL)/((MAT + 233) * 128)) * VE

We can replace a large portion of this formula with INV MAT (explained later) to yield…

GMS of Air / CYL = ((MAP - EGR) * INV MAT) * VE