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I've done enough tuning with Subaru to know my way around their computers quite well, but I bought a 2002 Corvette a few months ago so I've got some learning ahead of me. Right now I'm having a hard time getting my heard around fueling, especially Power Enrichment. So, can someone tell me if my thinking here is correct, or way off?
Assume for the sake of discussion that MAF and VE are already dialed in, the engine is warmed up (oil and coolant around 200F). Both Subaru and GM have intake air temp compensation tables that work the same way. Subaru uses grams-per-cylinder of air ("load") in places where GM uses manifold pressure, but those two are tightly coupled so they're conceptually pretty similar.
With Subaru, you do most of the AFR tuning with a table that has RPM on one axis, and load on the other axis. Here are a couple of examples:
But the coolant temp columns are all pretty similar for the normal operating temperature range (175-230 F or so), which means that once the engine is warmed up, enrichment is basically just a function of manifold pressure. Unless the driver opens the throttle enough for Power Enrichment to kick in - then enrichment is just based on RPM instead:
GM's approach seems to imply that (with coolant around 200F) you can just let AFR vary by manifold pressure and the engine will run just fine at any RPM. But if the driver wants power (as indicated by opening the throttle) then the PCM uses additional enrichment to get that power - and in this mode, manifold pressure isn't important and AFR only needs to vary with RPM.
Am I getting this right? Or am I way off base?
It makes sense not to use PE all the time, because it burns more fuel, and if the driver doesn't need the power then why waste the fuel... but after having tuned fuel for years using Subaru's approach, I'm sort of suspicious of the idea that you could just tune fuel based on manifold pressure (same enrichment for any RPM), and get decent results, even just for low power / cruise stuff. And I'm suspicious of the idea that you can tune fuel based on RPM (regardless of manifold pressure) and get decent results, even just for high throttle stuff. And yet, GM's PCM toggles between those two modes, and everybody seems to be happy...
I can sort of see how Power Enrichment based on RPM makes sense for WOT pulls, because at full throttle you can be fairly sure what your manifold pressure / load is going to be. But it seems really weird to have enrichment toggle like a switch from one mode to the other based on throttle position, and sample tunes that I'm looking at have PE switching at 30%-60% throttle. So I wonder if I'm missing a big part of the story here, or if this really is how people think of Power Enrichment.
I realize this was a massive wall of text. If you made it this far, thanks for bearing with me.
Personally i just disable PE entirely. i never understood the need for it.
I would love to hear what everyones thoughts on the necessity of it is.
That would seem to leave you with the same AFR at 2000 RPM + light throttle (just cruising along) and at 2000 RPM + full throttle. Presumably you'd want 14.7 at cruise, for fuel economy... Then the question is, how much power does that end up sacrificing at full throttle?
Would 12.5:1 give better power? Would 12.5:1 let you advance the spark timing to get more power?
If it's safe to run 14.7:1 up to, for example, 2500 RPM, with any throttle opening, then you could just declare < 2500 to be for cruise and economy, and > 2500 to be for power. Which is not unreasonable. Full throttle and 2000 RPM is doing it wrong anyway, downshifting will will be faster...
Just thinking out loud. I still kinda like the simplicity of disabling PE, even if I'm right about leaving some power on the table at low RPM.
I too would love to hear thoughts from other tuners about this.
PE is a function of RPM, TPS and KPA. It isn't a 2 dimensional table.
The Ramp In rate also changes the speed of the slope of PE activation, so its not going directly from one targer AFR to another.
PE is a function of RPM, TPS and KPA. It isn't a 2 dimensional table.
The Ramp In rate also changes the speed of the slope of PE activation, so its not going directly from one targer AFR to another.
Would it be accurate to say that enrichment at low TPS is a function of KPA, and enrichment at high TPS is a function of RPM?
(Where high TPS / low TPS basically just means above / below the TPS threshold for Power Enrichment.)
I realize it's not a 2D table for GM but coming from an ECU that does use a 2D table, I had to draw it as a 2D table in order to make sense of it. I'm kinda secretly hoping that someone tells me I'm looking at it all wrong, because it almost seems too simple to work properly. I'm used to working with a fuel table that looks more like this:
That would seem to leave you with the same AFR at 2000 RPM + light throttle (just cruising along) and at 2000 RPM + full throttle. Presumably you'd want 14.7 at cruise, for fuel economy... Then the question is, how much power does that end up sacrificing at full throttle?
Its been a really long time since i touched a maf or pe tune, mine is OLSD, so i can control afr based on map which is different at 2500rpm cruise compared to 2500rpm wot. it should be different when using a maf also because the motor will be flowing a lot more air at wot than at cruise even at the same rpm?
Would it be accurate to say that enrichment at low TPS is a function of KPA, and enrichment at high TPS is a function of RPM?
(Where high TPS / low TPS basically just means above / below the TPS threshold for Power Enrichment.)
I realize it's not a 2D table for GM but coming from an ECU that does use a 2D table, I had to draw it as a 2D table in order to make sense of it. I'm kinda secretly hoping that someone tells me I'm looking at it all wrong, because it almost seems too simple to work properly.
You can use either TPS or MAP, or both. In the past, I've used strictly KPA while tuning in the past. I'll set TPS activation to basically anything above 0, that way I know that the 80-100 kpa section when mapping target/actual AFR won't be effected intermittently by PE, it will always be a given AFR. Using KPA and TPS will cause certain cells to alternate the target AFR based on whether or not conditions are met.
If you set the KPA low, it basically becomes a 2D TPS vs RPM table. If you set the TPS low, it basically becomes a KPA vs RPM table.
PE in GM PCMs is references off of TPS, RPM, KPA or torque. some have a delay once the requirements are met before it will enable off a reference to RPM, time or both.
the enrichment rate is how fast the PE corrections are applied. it will enable off the highest value selected. example enable TPS of 20% may be achieved for most of the driving time, but if the enable MAP is set to 65KPA then it wont enable to then. delay modifications will also effect it and I personally delete them.
the commanded enrichment AFR is an odd number too. its stoich divided by AFR. example if you wanted a 13.0AFR in PE the 14.7/13.0 = 1.131. that is the command PE EQ value.
The factory PCM looks at the richer of "commanded fuel when in OL" and "PE modifier based on RPM". I can post table numbers if someone wants it for a specific year Fbody/Vette. Whichever is richer is what the PCM will look to as target AFR. I typically only use commanded fuel when in OL for cold starts and strictly use the PE for WOT. Commanded fuel when in OL is based on map, not RPM. I don't want 1 AFR target at WOT, I want it based on RPM. PE modifier based on RPM is based on....RPM
The factory PCM looks at the richer of "commanded fuel when in OL" and "PE modifier based on RPM".
That would change everything. How come nobody else knows this? Whoever tuned my car (at least two owners ago) didn't know this either, because the entire PE table is richer than the entire OL table.
Using one table to enrich by RPM and the other to enrich by MAP isn't as good as the 2-axis table approach but it seems far better than alternating between an RPM-based table and a MAP-based table.
GM's airflow tables are all determined referencing lambda (stoich for whichever fuel you are using). It then uses a mode called power enrichment as a multiplier (in this case you are multiplying by a reciprocal) which modifies lambda/stoich by this multiplier. It assumes that the tables are all reflecting 1.00 lambda, and multiplies that cell by the power enrichment multiplier. The power enrichment mode is triggered by certain conditions, such as RPM, KPA, TPS, in some cases torque too. Once in power enrichment, the PCM enters open loop (no feedback from the narrowband sensors) and references ALL of the open loop tables. If the open loop multiplier table commands a richer lambda (mixture) than that of PE, then it will always reference the table that commands the richest mixture. It is also important to note that if your LTFT (long term fuel trims) are positive then when going into PE, the strategy of the PCM is to ADD that percentage of LTFT into the final mixture. So if your LTFT are 6%, and you command 12.8, the 6% would be added to the base fuel table, during PE.
That would change everything. How come nobody else knows this? Whoever tuned my car (at least two owners ago) didn't know this either, because the entire PE table is richer than the entire OL table.
Using one table to enrich by RPM and the other to enrich by MAP isn't as good as the 2-axis table approach but it seems far better than alternating between an RPM-based table and a MAP-based table.
I learned this from asking the right people on EFI Live lol. Honestly I think what I listed in my last post is the right way to do it. When in OL below PE TPS, it's usually cold start and warm up driving, so use the commanded fuel in OL. When you want to go WOT, you don't want 1 AFR for all the RPM range, so exclusively use PE by RPM table for that. This way the 2 tables never meet as theyre used for different things.
I should also clarify that OL part throttle the PCM will only look at "commanded fuel in OL" until the TPS % exceeds the enter PE TPS% that you choose, then it will look at the richer of PE by RPM and commanded fuel in OL.
Originally Posted by subeone
....It is also important to note that if your LTFT (long term fuel trims) are positive then when going into PE, the strategy of the PCM is to ADD that percentage of LTFT into the final mixture. So if your LTFT are 6%, and you command 12.8, the 6% would be added to the base fuel table, during PE.
And if they're negative, it doesn't pull that percentage.
That would change everything. How come nobody else knows this? Whoever tuned my car (at least two owners ago) didn't know this either, because the entire PE table is richer than the entire OL table.
The information is out there. I have included this in my training materials and classes for a long time. That doesn't mean that people pay attention or that some "professional tuners" have taken the time to learn how the systems really work for themselves. There is a LOT of village wisdom being passed around, so it's sometimes difficult to know who to believe, even from some "professional trainers".
The information is out there. I have included this in my training materials and classes for a long time. That doesn't mean that people pay attention or that some "professional tuners" have taken the time to learn how the systems really work for themselves. There is a LOT of village wisdom being passed around, so it's sometimes difficult to know who to believe, even from some "professional trainers".
Learning from your stuff it's almost embarrassing the tunes I've had to fix from "top tuners in the country".
Subaru does the same kind of thing with closed-loop fueling trims remaining in effect during open loop, so that part is familiar.
So, this is how the OL fuel table and PE fuel table interact while PE is enabled? These are small versions of the OL and PE tables, with values that may not be realistic, but just for the sake of discussion...
Also just for the sake of that screenshot, assume that PE is always enabled. When disabled, fueling would just follow the OL table.
If that's how it works, then I can see why people opt to leave PE enabled pretty much all the time.
07-10-2018, 03:01 AM
Trying to make sense of Power Enrichment 
