GREGGPENN

I think it's fairly common for people to start with a basic spark advance table, adjust it as needed, and zero the PE Mode Spark Advance table. Thinking about this, I've started to question PE mode's existence...especially spark. Why even have a PE mode?

Maybe you can argue having the ability to add fuel (customize fueling) in a quicker, more direct manner makes sense...even for the factory? But, the PE spark table may be easier to modify.

The PE Spark Advance table is a bit of a mystery to me. If you look at the main spark table, it's obvious there has to be some formula to adjust (lower) spark depending on row and/or cell. For example, the 9 degrees added at 3200 rpm can not be that high for rows above/below. It's even harder to "guess" what might be true for lower load rows.

And, that's all I can do...is guess. Maybe that's why everyone typically avoids using it?

SO WHY EVEN HAVE IT?

If you don't accelerate aggressively enough, the engine doesn't enter PE mode. OTOH, if you don't accelerate aggressively you could stay out of higher load rows!!! Why not just code the higher/highest rows of the spark table to do exactly what WE normally do as customizers?

Maybe the engineers were trying to have an "economy" power mode and a more aggressive "power" power mode? Without loading up tuning software and observing what it takes (in terms of accelerator pedal depression) to activate each? Is there a difference between say...half-throttle load up to higher rows versus the point where PE engages?

That's all I can consider....that it's a way to conserve a bit more fuel for EPA ratings?

I suppose this is a bit more interesting when you consider the hwy mode that wasn't activated in factory bins. I also think about the VW suit and cheating....Did GM THINK about altering tests?

It's a coincidence that I've read a bit more about engine theory while pulling out and doing a 5-yr review of what I tuned back in 2010-11. I understand a HAIR more about fueling/advance. But, I haven't found enough about differences/strategies that might be used to tune for economy versus power. Often, I'd think both would go hand-in-hand.

All this said, I was intrigued that I (we) might be missing something in the CODE that activates...not activates when PE tables are populated. The point of this thread is to discover anything I haven't thought of.

(When we look at tuning, we see the parameter values. It's rare that people talk about/show the actual instructions of how values are used).

tequilaboy

The main spark table has been optimized for operation at the stoichiometric AFR as a function of load and rpm.

PE mode generally enriches the AFR which slows combustion. Additional spark advance is therefore added in PE mode to compensate for the slower rate of combustion.

This method provides flexibility and nearly optimum ignition advance for all load/rpm/mixture conditions.

In reality, many choose to tune spark advance via the main spark table only and zero the PE advance table. This is convenient and helps to avoid confusion since what you see in the main table is what you get, but is a bit of a compromise for some rpm/load/mixture conditions.

cardo0

I think you have a misunderstanding here (or else I do). A denser fuel/air charge burns faster than a lean charge. It's the higher piston speed at the higher RPM that requires the increased ignition advance such that peak cylinder pressure occurs at the optimum piston position - something like 15 degrees ATDC.

Power Enrichment I believe is the same thing as Acceleration Enrichment yes? That is the compensation for the wet walls of the manifold/cylinder head runner walls drying out as the airflow increases with opening throttle. Called Tau. Also compensation for deceleration which has its own complications.
IMHO it allows the car manufacturer to use the same ECM/program but with a different tune for each different car/engine they use it in. And it allows us hobbiest's to tune AE for our modifications. AE can adjust for changes in intake temps which in turn greatly affects the amount of fuel on the manifold/cylinder runners walls.
This is the polishing of the tune for daily driving. Most 1 or 2 hour tune sessions won't get you there. I believe the car needs to be driven on the street or should be to develop this correctly.

bjankuski

In my opinion the reason the factory used PE mode was to try and get better gas mileage. The factory tune stayed in closed loop until 70% throttle which meant that the car was near max power (zero intake manifold vacuum) and still in closed loop. This combined with the high temperatures these car run means that to avoid detonation the timing needed to be very conservative. Once you pushed the throttle past 70% the car drops into PE and the additional fuel and timing are added in to get better performance. If you command PE to come in at 40% throttle and keep the temperatures cooler like 200 degrees you can add all the timing into the main table and zero out the PE table.

GREGGPENN

zero intake manifold? Is this a reference to vacuum?

Your explanation seems to scratch the surface best, but consider this....

At 3200 rpms, there is no additional fuel in PE by RPMs. More over, 3600 and above, GM SUBTRACTED fuel...making it leaner (like 7% leaner!). I suppose this is because long tubes "rob" the intake of air at higher rpms. It also doesn't quite fit with your explanation because 3200 rpms has a 9-deg bump in PE timing. IOW, the biggest change in timing doesn't correspond with that extra 6% added in the two rows lower than that.

As a side note, I assume that 9-deg bump drops to 7, then 5, then 3, then 1...since that's the "slope" it would take to merge into the upper/lower rows next to 3200.


For the past 5 years, I did as you suggested....I only used the base table and had a much smoother transitioning map that correlated to the general shape of the stock TPI. This weekend, I decided how I think PE "averaging" must work and tried the GM method. Basically, I made an assumption (from dynoing/configuration), that my piston config is 15-20% faster than stock. (That's how much lower my total timing is compared to stock).

I created a formula that worked pretty well to convert/try the stock approach.... ((stock timing-20)*.85)+20. This removes a percentage of rows esp above 20-deg. (I smoothed it a bit from there.)

The result has slightly more throttle response than I had before. Largely, this is due to the smoothing/curving I'd done in my attempt (back in 2010) to achieve best low-rpm driveability for a stick.)

I'm not sure it's any "faster" under the highest load, but my dash says I'm getting about 2 mpg (average/combined) better than before. And, really, I like having better off-idle response....for normal driving. The downside is it "hides" a bit of the higher-end performance by narrowing the difference.

bjankuski

Greg, once you exceed 70% throttle (in the stock tune), the car commands the PE AFR which is 12.0 in the stock tune, if 7% is being pulled from the PE table all that means is GM is adjusting from an overall rich condition to still hit around 12.0 AFR. You are no longer in closed loop and targeting 14.7 AFR. (More fuel and more timing)

tequilaboy

My responses so far have been geared specifically toward PE spark advance. I had assumed that the need for PE fuel was already universally understood.

In simple terms, additional fuel is required for component cooling (piston tops and exhaust valves) and as a method to reduce the detonation tendency. In carburetor terms think of running on the main jet and/or power valve (if present).

In the $32B and $6E bins, the PE AFR target was tuned to vary with rpm.

With typical factory PE fuel tuning, peak enrichment of 11.71 (+6.25/2 %) occurs from 2400-2800 rpm, it then leans out to 12.0 at 3200 rpm and finally to 12.40 (-7.82/2 %) above 3600 rpm (AFR targets for warm/hot coolant with 22.66% coolant multiplier and 14.73 stoichiometric ratio).

PE spark advance works in parallel with the PE fueling.

The 9 degree PE spark advance spike at 3200 rpm does seem to be excessive, but the only other thing changing in PE mode is the AFR target.

Mixture density is addressed by the load influence. Since increased density increases the rate of combustion, spark advance is correspondingly reduced with respect to load in the main table.

The 87-89 L98s were tuned to be quite rich near the torque peak and the extra PE spark advance appears to be an attempt to restore performance in spite of the rich mixture. They would likely benefit from both a leaner target and reduced spark advance in this region.

The over-fueling may be present to help preserve the catalytic converters. Hard to say now what the real reason was, but pretty clear that the spark advance is (crudely) linked to the fueling.

Note: The 1986 $32 bins typically had the PE AFR target fixed with respect to rpm at 12.0 (0%) with no PE spark advance table available.

Obviously both approaches can provide acceptable results with appropriate tuning.

cardo0

Well can you imagine how much wall wetting with fuel of the manifold and cylinder head runners downstream of the injectors is affected by batch fire injection? . Just some food for thought here. No I don't have all the answers but Tau is major player here.

bjankuski

Acceleration enrichment is not the same this as power enrichment. Acceleration enrichment is the additional fuel that is dumped into the air stream as soon as the throttle is snapped open. It is used to cover up any lean bog that may be present when the air speed is still low in the runners and the fuel has a chance to puddle on the manifold walls. This additional fuel will be decayed away quickly.

Power enrichment is the additional fuel that is added to the air stream to hold the engine at a predetermined AFR of lets say 12.5 to 1. This fuel will be added as long as the throttle is held in the power enrichment condition.

If we use the carb example listed before:

AE = The accelerator pump on a carb
PE = Main jets and power valve

GREGGPENN

Brian, I understand that. What my original question boils down to...and still escapes me, is GM's purpose for removing 9 degrees of timing, in the upper range of load rows, when PE isn't triggered?

Tboy detailed what WAS programmed into my 89 factory bin. I've been revisiting those details since you suggested I be sure my timing was LOW enough. (BTW...it was and I didn't like when I moved it lower...plus it didn't "cure" the idle heat issue where it has a slow creep. Not complaining about your advice here...just clarifying why I'm back into tuning strategy.)

Now that I'm comparing the nice "pretty" cure I've been using versus the stock AYPY bin (which there are different versions of online), I notice some trends that I previously ignored. The biggest might be the nuance of the base timing table. After identifying the closest 89 BIN that APPEARS to be written for a ZF6, I think there are some tweeks intending to make it "more driveable" at lower rpms. All I will say here...is there's a LOT of timing used to develop low rpm torque and make it easier to drive. I've also noticed the removal of timing ends up affecting how the IAC sets idle down. With less timing, the IAC doesn't act quite as aggressively...which tells me TIMING is (at least) part of the strategy for controlling this function. But, that's not related to my question in this thread.

Yes...I get side-tracked sometimes! LOL

So...you are cruising along and step on a STOCK 89. But, you don't hit 70% throttle. That 9% is NOT added into the base timing values -- centering on 3600 rpms where the stock intake creates it's BEST torque. OK...so you're still running in closed loop, commanding a 14.7AFR.

When I tried to wrap my head around this, I thought "HUH?" Are you telling me that 9 degree's LESS timing is necessary because of the increased flame-front speed of the burn?

OK...So, the mixture IS a large chunk rich in these rpms. As Tequilaboy points out, it's down close to 11.5% -- which I see at TOO rich. Recently, I pointed out (and asked about this discrepancy) w/o feeling satisfied with the answer(s). Maximum torque isn't supposed to be THAT rich. MULTIPLE sites say 15% is maximum torque -- which is 12.5:1.

One thing I wonder about is the intensity/pressure as the design of the long tubes build torque with pressure waves. Does it NEED more fuel because of pressures? Is it really a dumb methodology? How does this fit in with DROVES of posts saying MORE fuel (added by increasing regulator pressure) makes the car even faster.

Yes...lots of people THINK more fuel and more timing makes an L98 run even harder. From what I see in the "INTERNET" bins, it doesn't seem likely. This is part of what I "learned" and convinced myself back in 2010....That lots of these anecdotal posts are probably hooey.

But I also wonder about the BIN(s) themselves. And, PE is one of the biggest things that made me think WTF? That 9 deg flip-flop in fueling just seems odd? Maybe, I SHOULD be as baffled by the "commanded" 11.5% fueling in the mid-rpm range where they add 9 degrees to light it off. Could this be valid?

To be honest, there are a couple of (what I consider) glaring errors in the various versions of the 89 factory bin. I think it would have been better had I asked more questions about them back in 2010.

The point of this thread really is two-part. I think part 1 is answered by the posts that suggest FUEL SAVING is part of why they wrote a PE mode into the bins -- versus just building it into the BASE values.

The other part is the APPARENTLY goofy dumping of fuel COMBINED with large swing in timing. H$LL, anyone with half a mind has to wonder about getting too much fuel on the walls of a cylinder! (The guy above keeps eluding to it...even though while we are struggling with some definitions.) While that's PART of what I wonder, the rest is why GM would "spread out" the power using this PE technique?

Do you think it's really a "valid" fuel-savings technique? Could it be a lame attempt to create a top-end pedal "bump" that makes you think you hit turbo mode? LOL

And, I'm still wondering how much (if any) shooting for extra fuel BEYOND 12.5:1 is useful/valid? Maybe I SHOULD have tried some bins (on the dyno) where I used this technique WITH EXTRA timing in the max torque range?

So...was PE for extra fuel savings? Or...maybe extra power? Or...another reason to question the validity of the factory BINS?

GREGGPENN

Yeah...I think "all of us" probably shoot more for the 12:1 PE target and lose the PE function all together. Again (as posted above) that's why I question it's creation?

As you see in my post above, I also question the HUGE swing of 9 degrees -- presumably required to go from 14.7 to 11.5 AFR.

I guess part of my train of thought is to gain a better understanding of situations where designers went OUTSIDE the norm of common thinking. And, that "norm" is by declaring the range from .85 - .9 "lambda" to be where we all want to tune for power.

I would even question staying near stoichiometric in the upper load rows -- as a fuel-savings technique? Intuition might lead you to think best power is the most conservative. That's mostly because you can get OUT of that power-using mode faster and stop using fuel. IDK...

cardo0

That is not correct. Acceleration enrichment not only includes initial throttle opening (enriches to prevent stumble) but all the way to WOT max RPM fueling. I suspect Power Enrichment is unique to the tuning software you are using. All the tuning material I have refers to Acceleration Enrichment as does Charles Probst, and Greg Banish.

We can only speculate what the GM engineering required of fueling for power and emissions. I don't know what speculating of proprietary information would do to help anything other than waste time and effort. Time would be better spent on a Eddy current dyno with an accurate wide band as 12.5 AFR sounds way to rich to me let a lone 12.0. What I'm saying is you would have to prove to me that anything greater than 13.0 AFR makes any more power.

bjankuski

What you are referencing is the transient fueling that is in the tunes for the newer OBD2 cars like the LS1, those computers have tables that take into account the evaporation and fuel wetting of the intake and cylinders, as shown in the attached picture:




Our computers do not have those tables and the AE fueling is decayed away over time and is influenced by engine RPM, and temperature, as shown in the attached tables:




So I stand by my statement the PE = the desired AFR at WOT and AE is the additional fuel that is decayed away over time.

Also max power is usually located around 12.5 to 12.8 to one but each combination is slightly different and a dyno helps you locate the best for each combination.

bjankuski

Greg,
I cannot answer all your questions because I do not know all the reasons GM did what they did but this is my best answer for the questions above:

As far as I am concerned you can tune the entire car on the main timing table by using load values and determine what the best timing for each load position is, there is no need to complicate the issue with the PE table.

GREGGPENN

Looking at the chart you posted, makes me think of one reason GM might have opted for "excessive" fuel for WOT....

CO and HC increase with the extra fuel. But, the converters are always there to remove/convert it. OTOH, NOx isn't controlled during WOT because the EGR is disabled -- to prevent reduction in power.

Sounds like the designers were transferring the burden of emissions reduction to the converters during WOT operation.

For less than 70% throttle, economy (and emissions) was the goal. For anything over that, economy was less "important" but they still had to deal with all 3 aspects of emissions.


(Obviously, none of us care about emissions when adding HP, ripping off converters, getting rid of EGR, etc... Power is the goal. So coding the base table for maximum power is THE approach.)

cardo0

Well thank you for your efforts here. But those snapshots don't show the PE tables as you say that are inside the L98 ECM you have. And that's really in the tuning software used to display the ECM program. We are disagreeing on the effective range of each term here but I haven't seen separate tables for each. I guess different tuning software will use different features to accomplish the same thing as we witness in this thread itself the ability to zero the PE table.

As for max pwr fueling the point is that optimum AFR is closer to 13 than 12.5 and there are plenty of tables/charts that show this also. IMHO 12.5 AFR is nowhere to be but every engine is different and what's best for each engine can only be seen during power pulls with data log or at least a timing slip from the track.

That's enough for me, take it or leave it.

bjankuski

Please do not be offended by my posts, I am just trying to present information as I believe it to be true. If I am misunderstanding something then please point it out, I am always willing to learn and i am always willing to help.

I have attached shots of the tables available and the PE tables, i am just showing that indeed there are PE tables and AE tables.