Piggy Back Computer

[bratcop]

I just finished my stroker and it is running great without a piggy back; the OBDII seems to be doing fine in open and closed loop.

I have, however, purchased a PSC1-003 for installation. I did not initially install it because I wanted to limit my areas of potential failure in the case of the stroker not starting or running correctly. Now that it runs fine, I am considering installing it, but I have a few theoretical questions concerning it....

I understand that my 01 Laredo has and OBDII system that uses a map sensor and a rpm to determine AFR at idle and wot (open loop) and under normal driving conditions (closed loop) the ECU monitors the 4 O2 sensors, two up stream and two down stream, as well as MAP sensor and TPS to determine proper AFR. It might even throw in another sensor to further evaluate, I'm not positive.

My questions are this. I understand how the Piggy back would work on the open loop. It intercepts the MAP signal, adjusts it per your map, and sends it, altered, to the ecu. This,in effect' tricks the ecu into thinking the MAP is reading different than it actually is, causing an altered fuel mixture. You, of course, have to be monitoring your mixture with a wide band O2 sensor of your choice. This would enable you to adjust your fuel map accordingly to produce the desired AFR.

Now how could it possibly work in closed loop? The ECU is watching AFR via the factory narrow band O2 sensors and using them to adjust the AFR via the ECU. So if I adjust the MAP sensor reading via the Piggyback to trick the computer, it will still see the actual O2 reading and try to keep it at 14.7:1. Am I missing something?

I have seen something about trying to get the fuel trims to 0, which I assume you do by tweaking the piggyback computer to change the MAP reading. So if I get all the trims to 0, the ecu has to make no adjustments as the AFR is good without adjustment? How does that benefit me? And what if I want it a little richer than that, say 12.8:1? Won't the OBDII fight the piggyback and just adjust back to 14.7 in closed loop? I suppose I can assume that I am only going to get my peak numbers (hp and torque) at WOT, but still, wouldn't I want a nice flat AFR across the board?

I have been reading and learning about this for some time now, as I have not done this type of tuning before, so any and all input would be of assistance.

Thanks, I appreciate all the help I have got so far. Here is a glass of Christmas Beer (or Cheer) from Wisconsin...

Bratcop

[SilverXJ]

From what I understand is that taking the fuel trims to 0 allows the PCM to make as little adjustments as possible. Giving you better economy at least because the PCM won't have to swing from + to - to accommodate the correct AFR. Since teh O2 sensors aren't wideband they don't feed the PCM a lot of info, so the PCM only knows if it is rich or lean with very few points in between.

Basically it will run fine with out adjusting the fuel trim, but will give you at least better economy with the fuel trims close to 0. As for more power, maybe a little bit, but the PCM is still trying to achieve 14.7:1 afr.

You have a very good handle on how a piggy back module works.

[bratcop]

Please correct me if I am wrong...
The PCM does try to achieve 14.7:1 all the time...in open loop it uses a set table in the pcm, i.e. x value from MAP with y value from rpm=14.7:1. My understanding that it isn't using the O2's at this point, just the table in the pcm(speed density system), so if we tweak the MAP signal, we can alter the AFR as the computer wouldn't know it was running rich because it is going off of the MAP value provided to it via the Piggyback and applying that to the table, not the actual MAP value. So at WOT and idle, we have quite a bit of tweakability, right? Not so much under normal driving conditions, which I guess is acceptable. I have read that running it at 12.8:1 as opposed to 14.7:1 at WOT could get me as much as 15-20 hp. I assume the fuel tables in ECM try to achieve 14.7:1 as well...Does anyone know the triggers that kick an 01 ECU from open loop to closed loop and vice versa? Which rpm? Which TPS value?

[SilverXJ]

The PCM does not try to achieve a 14.7 AFR. Only in closed loop does it want the 14.7AFR. I.e. as cruise, and below WOT (~<70% throttle). In open loop the PCM runs a set fuel curve. It ignores most sensors aside from MAP and AIT. This is why a piggy back is needed. The PCM will actually allow the engine to run very rich or very lean under the right conditions, smaller or larger engine, etc. Most of your adjustments will be in open loop, when it is running off the predetermined map.

[bratcop]

Thanks for the info, I appreciate it. I was just assuming that the fuel table in the ECU uses the variable of IAT and MAP and RPM to dump a predetermined amount of fuel, via it's table, into the engine. Since manufacturers seem to be concerned with low emissions (just a little concerned ), I assumed that this table would also be attempting to equate to a 14.7:1 AFR. I understand that changing injectors and displacement, porting manifold and head, etc. will throw this off, because the tables are based on stock parameters. Don't you think that the tables in the ecu are designed to optomize the burn to 14.7:1 as well?

Bratcop

[SilverXJ]

The the PCM is using the tables, open loop, it is looking for different AFRs around different RPMS. However, it will be around 12.5.

[Mgardiner1]

Other then hooking up to an OBD computer with a scanner, is there any way to test for for open loop mode? Or is a scanner the only way to really know? Other then assuming 70%ish throttle would default it to open.

[gradon]

I haven't fooled with my psc in a while, but will get back to it right after christmas. I use the lm-2 to tell me when I'm in CL or OL. Zero the trims first and then watch the pt where it goes to open loop. You will want to blend that 14.7 to 12.5 WOT. Hopefully the point right before OL the fuel trims aren't positive(adding fuel) or else it'll be running lean once in OL and the pcm is no longer looked at to keep the 14.7(it could get 16+ quick if the ft was say +20%). Oh and another thing is that at idle-1K, don't zero the trims--you will want the ft to be a little negative(pulling fuel out) because those cells will be looked at at the cold OL startup, which you want to be rich(13-13.5AFR), not 14.7. It does kinda suck being confined by the pcm, but it works. I've been tempted to give Nick @ Kolak a call to get the pcm reflashed, but since he's out in AZ(not on the east coast), it would make it difficult to make changes on the spot/have it dynoed while tuning. Once you pay for a reflash, Nick will tweak it as many times until you're satisfied(you can send it back).

[bratcop]

and here is where i get a little lost...zeroing the trims is done by adjusting map values via the psc, correct? is that what we do in closed loop, simply adjust map value so the ecu doesn't need to adjust? Then in open loop, we shoot for like 12.8 ish with the psc? and we want a blend from 14.7 to 12.8 at the point of transition from CL to OL? is that for smoothness only or does that transition affect performance?

[gradon]

You've downloaded the u-tune pdf on the split sec site, right? From that doc, they give the impression that you use one cell to split the diff between 14.7 CL to 12.0 OL(that is magic # for the boosted yotas). They also give you a heads up that some of those cells will be CL at times and OL at others. You're shooting for 12.8 based on what Hesco tuned the uni? I know when I tuned the psc at WOT, I had to use the 25 value(25.5 is max) to get the 12.5, so those LT1s are barely cutting it in my app.

[bratcop]

12.8 is from some online research stating that is best for a n/a engines' best performance. I did download and scan that PDF. i have not hooked psc up yet...want to fully understand it's use first so as to minimize Laredo's down time. Still gotta get my zeitronix orded...u using LT1 injectors? flow rate?

[gradon]

the lt1s flow 25.5# on the 49psi obd2 rail.

[bratcop]

So I think I got it now. reading that pdf again helped. First tune via the psc in closed loop trying to zero the fuel trims. After getting the short term and long term trims to average as close to zero as possible, then on to WOT tuning. Do we know at idle and after warm up if a an 01 Jeep ECU will operate in closed loop? or does it only go to closed loop at certain RPM? Either way, after tuning closed loop, then on to Open loop.

For closed loop, just use an OBDII software interface to monitor fuel trims and open/closed loop status. For Open Loop, install a WideBand O2 and tweak the fuel via the psc to about 12.5:1 or similiar. Then it talks about the transitional areas (blending) from open to closed and closed to open. I am still a little foggy on what causes these transitions (aside from mashing the throttle to the floor) I realize deacceleration will bring it from open back to closed, and that is where we should smooth out the map to keep the AFR smooth so there isn't a big jump in values in a few cells? And do the same at the point the jeep goes from idle (open ) to normal operation (closed), as soon as I figure out what causes that.

How does that sound? I hope I cleared some of this up for someone other than myself...

Merry Christmas,
Bratcop

[SilverXJ]

When the engine is at idle and warmed up it is in open loop. I don't know what exact point the PCM goes into closed, but it uses the TPS and MAP sensors as its main inputs to determine when, and the coolant temp sensor to determine when it is warmed up. The best was it to use a scan tool of sorts on the OBD II port to see what the fuel systems status is.

[seanyb505]

Please forgive my ignorance, but what exactly are the differences between open and closed loops, and when exactly do they happen?