I have three questions:

Does the PCM continue to learn, establishing BLMs, when there is a trouble code?

Does spraying the MAF with nitrous make any difference to the PE fuel curve in WOT open loop?

Does spraying the MAF with nitrous just before WOT influence the PE fuel curve?

Thanks. And, please direct me to any links containing pcm philosophy. Thanks again.

 

1. Depends what the trouble code is for. Obviously if its for an O2 sensor, its going to disable learning/closed loop. I honestly do not know the details of what sensors would cause the PCM to disable learning.

2. Spraying N2O on the MAF sensor results in an incorrect flow measurement. Spraying directly into the elements, impinging liquid N2O on the wires will screw it up completely. Place the N2O nozzle further ahead of the MAF sensor, and you can eliminate direct impingement. Now you have two things in play. The nitrous/air stream is colder, but the MAF sensor can pick that up, maintaining a constant delta-T across the wires. But, the calculations the MAF sensor makes are based on the assumption that the wires are heating air, and not a mix of nitrous/air. The calculations are based on the specific heat of the gas that is being heated, and the specific heat of the nitrous/air mix is not exactly the same as the specific heat of the air alone. Exactly how much that affects the MAF sensor would need to be calculated. In the LS1 cars, it seems to work. In the LT1 cars, it seems to be hit-or-miss. Read the posts in the "wet vs. dry" sticky thread for excruciating details. If the MAF sensor is no longer accurate, the fueling based on a target A/F ratio will not be accurate. And that's what you are doing in PE mode.

3. First, note that PE mode is not exactly WOT. Its based on a table of throttle positon vs. RPM. It goes into PE mode before the throttle is fully open. Spraying nitrous before it enters PE mode will affect the A/F ratio - first based on the error induced in the MAF (as discussed in 2., above), and then based on the O2 sensor feedback. The stoichiometry of a nitrous/air mixture + fuel is not the same as the stoichiometry of an air + fuel mixture. How that affects the O2 sensors is not clear to me. A wide-band measures lambda, which it the ratio of the A/F mixture to stoichiometry. Is isn't clear to me if the stoichiometry measured with a wide-band on a nitrous/air blend is referencing to the new stoichiometry, or if its still assuming its looking at an air + fuel combustion process. It would seem to me that the wide-band needs to be calibrated to the correct stoichiometry. But that's just a guess. Using the stock narrow-band sensors, it becomes even less clear whether they are feeding back the correct voltage to the PCM, which would affect the accuracy of the fuel addition, and hence alter the BLM's in Cell 15 as you approach PE mode at high RPM/high MAP.

Bottom line is..... I don't know. You might want to try Rich Krause and see if he ever followed up on this with his buddies at Kennedy. I believe he had assumed that the reaction of the wide-band O2 sensor was not affected by how that chemical combination was arrived at, but after we batted my thoughts around for a while, he was going to check further.

 

Thanks Injuneer.

Seems smart that any sensor failure related to correct engine operation would disable BLMs.

I've sprayed everything from 30 to 150 hp through the MAF, all back at the air box. I'm always looking at O2s. I've always only seen a continual reduction in O2 voltage with the addition of nitrous, the more you spray the lower the O2 voltages. This is a casual observation, not scientific. It's so reliable, I take it for granted at this point.

It almost sounds like I'm incorrectly calling PE mode WOT open loop. I was under the impression they were the same. That is one uses the PE table to control the amount of fuel spray at WOT. I thought all the sensors were pretty much along for the ride at WOT? The sensors are measuring, but the only active input seems to be from the knock sensor. I do understand how the MAF is not able to accurately "measure" the incomming air due to the addition of nitrous. Is PE mode a step along the way to WOT open loop?

I'll try to read the "wet vs. dry" sticky thread for the excruciating details.

 

Once the O2 sensors warm up and it goes into closed loop, GM always calls it "closed loop enabled". When you go to PE mode, the PCM changes the target A/F ratio using the PE tables, to produce the richer mixture that will produce peak power/torque. The operation of the engine remains the same, and all the sensors continue to be used, except the O2 sensors. The engine still needs all the basic info to operate correctly, so it doesn't ignore anything. The only thing that is changed is the richer target A/F ratio, and the elimination of the A/F ratio correction that would normally be provided by the short term fuel corrections. Those corrections are determined using the O2 sensors, and the O2 sensors can not be used. In effect, the feedback "loop" has been "opened", but GM doesn't call it open loop - they call it power enrichment mode. And its not limited to WOT (100% TPP). With stock tuning, it will enter PE mode at only 66% TPP below 2,800 RPM, and at 4,000RPM will only require the throttle to be 38% open. In effect, PE mode has nothing to do with true WOT.

The stock narrow-band O2 sensors aren't accurate at the richer target A/F ratios used in PE mode. So using them for closed loop control would not make any sense.

 

It took us a while to get there, my fault, but at the end of the day the o2 sensors are the primary driver to correct AFR. So whatever effect the remainder of the engine sensors have in correcting to a target A/F ratio once in PE mode, even with air temperatures droping from 45 degrees to -5 degrees celcius (typical of what my IAT sensor reports), again spraying nitrous in through the air box, the sensors are largely unable to compensate for changing engine operting conditions? Which is exactly what I've seen in data logs on car after car.

Although, you said some LT1s are hit or miss, and LS1s (OBDII) perform in this manner? Can you elaborate?

Thanks.

 

I've seen people "say" that have successfully sprayed through the MAF sensor on an LT1. NOS does not recommend it, that's why they came up with the increased fuel pressure on the LT1 dry kit. Given the fact that not many people have done it on the LT1, and NOS claimed it wouldn't work, I'm guessing its "hit or miss". I have never tried spraying through the MAF, so I have no hands-on knowledge to back up my opinion.

But many people have successfully sprayed the LS1 through the MAF. NOS claimed they used spraying through the MAF in their LS1 dry kit because the LS1 PCM is a much faster processor than the LT1 PCM. I've seen it work, and worked with people using the LS1 system, and I have complete confidence in it.... as long as you keep the N2O impingement off the sensor elements.

I don't think any other sensors attempt to alter the A/F ratio based on feedback. But other sensors are used in setting the target A/F ratio for open loop, and for PE mode.

My knowledge of the LT1 PCM is a case of "arrested development". I got rid of (most of the functions in) my stock PCM in 1999, and never spent a lot of time doing anything other than scanning it, never tuning it.

 

Sounds like one more great reason to step up to the LS1. Although, I'm having so much fun with the LT1, it'll be years. LT1s are so cheap and robust it's just hard to imagine.

Speaking of processing speed any idea what the LT1 computer speed may be in terms collecting all the operational data and making an alteration? I gather data at 20 hits per second. Often I see repeat entries. I'm not sure if it's computer, communication, etc.