Just hypothesizing here, and sorry it'* a bit wordy, but I hope you get through it all:

To those that have moved the PCM out of the stock airbox, and have left it open in the engine bay, I have a question for you.

The question is this: Is there a way to tell how much lag heat causes the PCM from crunching numbers versus colder air? It would be interesting to see if you could shave even just 1/10 of a second off the 1/4 mile stretch by helping the PCM to run as efficiently as possible.

This close-looped computer, meaning it is gathering information from dozens of sensors from coolant temperature to the amount of oxygen that'* in the exhaust. With this data, it performs millions of calculations each second, including looking up values in tables, learns from it, then changes its own values in the calculations, then tells what the various components to do, such as telling the injector when to close.

The processor in our PCM is likely only a 32-bit, 40 MHz unit. Granted, the code it'* running is very efficient, and usually takes less than 1MB of memory. The controller area networking (CAN) speed is 500kbps. Since the CAN is the actual BUS speed, we can assume a transfer rate from the processor of no more than 500kbps.

Given that the design of the embedded PCM is so that it will provide good processing within a small and thermally constrained environment, and can run without the need for active cooling or a cool environment, that'* not to say that it won't benefit from such. In operation, the temperature levels of the processor will rise until the temperature gradient between the processor and its environment is such that the heat flow matches the input, and the temperature of the processor reaches equilibrium. For reliable operation, the equilibrium temperature must be sufficiently low for the structure of the computer'* circuits to survive. The only logical explanation for GM moving the PCM to the engine bay is that it is place in the air box, away from any ambient heat. For those that take it out, it is uncertain how much they are actually pushing that line.

Now, from a performance standpoint, as the processor nears the brink of equilibrium, it will begin to "lag" when processing information. The maximum operating temperatures I can find on low-power embedded processors are when the case temperature is in the 85*C (185*F) range (the case, not the ambient air). Other indications are less, to around 55*C (131*F) when no airflow is present. If one could sufficiently keep the case cool enough so that no lag is present, or even cool to the point that the conductivity actually improves over "room" temperature (as it does in winter, we all notice it, although don't attribute some of that to the PCM'* reaction time), we could see improvements in overall engine performance to possibly add even a tenth of a second to the quarter mile.

Now, we know active cooling is pretty much out of the question, as the environment is too dust-ridden to maintain efficient conductivity on the circuitboard in general. But what about a cooler environment? We are restricted by the length of the wires leading to and from the PCM. Cold air box? Mount the tray in the fender well?

Here, I'll open it up to discussion. If anyone wants the actual mathemetic formula for determining the maximum operating temperature of the PCM, I'll post it. It is quite lengthy and involved, and I can tell you the answer you'll get is between 55*C and 85*C no matter what environmental variable you plug into it.

For starters, the PCM harness on the 96 and newer Bvilles isn't long enough to reach the fenderwell.

I suppose you could do a couple track passes with an iced-down PCM (don't drip on the track) to see, but my bet would be that a good performance PCM would be hard to improve on with cooling.

It is more logical to believe that GM put the PCM in the airbox because it saved them 0.00001 cents per car.

I meant versus out in the open in the engine bay. It'* becoming common knowledge that cooler environments mean better running electronics. Especially with the phenomenon of superconductivity being published.

It'* not so much that a cooler PCM case will provide some gains, but rather it could prevent loss of performance. I've watched several runs of vehicles with the exposed PCM, and it seems there'* quite a pattern that the more runs one makes, a good percentage of the time the runs get worse. Not by much, but up to a few tenths.

It seems a logical explanation that one reason is that the PCM was quite warm and thus "lagged" a bit when closing the injector, or caused some spark retard. It can be logical to imagine that the warmer the PCM, the slower the processor will run the math, and could also very well have a few "lag spikes".

It is also worthy to note that the warmer the PCM case, the lower the output voltage to the various components will be.

Any further thoughts? Anyone else that would like to hypothesize here? It would be nice to see if someone wouldn't mind running some numbers of the temperature of the case after various times and loads on the engine. Mine'* still in the airbox, and I don't have one of those point and shoot thermometers, either. :P

Anyone game?

Granted conductivity is directly related to temperature I can’t see the pcm becoming that hot in which it would hinder performance. I would think if the pcm was having trouble communicating due to the extra resistance caused by the heat you would suffer more then a slower car; you would likely experience some sort of odd engine behavior.


Ed

Somewhat off topic, But that area of my '93 hits 140*+ in traffic, which I would assume to be pretty close to track conditions Your theory does sound like it has merit..

Of course there'* always the fact that thermal expansion causes SMD components to lose connection after repeated cycling.

If I'm thinking correctly, PCM in the airbox requires MORE wire since the wires jump to the body up high then loop around, verus just jumping to the body and popping through.

Not concerned with increased resistance in the wire here, but the processor. As it nears even 75% of it'* maximum running temperature, it will start to slow down in processing speed. Near 90%, and it will likely start to "lag", maybe even to the point of dropping the timing to default for about a half-second or more, then picking it back up (for instance).

That could cost up to 0.2 seconds off the quarter.

(Notedly, the above is pure conjecture.)

I'm not talking about just wire I'm speaking of the resistance in the semiconductors contained in the pcm itself. Although I think your theory has substance I just don't think it'* very likely to be seen in real life, other wise I’m sure it would have become any issue in the 3800 community by now and dealt with accordingly.

Ed

Well, it'* no secret that performance suffers on hot days. Sometimes, I even see stock vehicles with the PCM still in the airbox start to perform evenly with modded cars, or darn near.

To account for at least some of it, I have to wonder if perhaps that PCM is just getting too hot...

PCM trays laying open in the engine bay can easily exceed 120*. Seeing as how the max operating temp is not much above that, one can begin to surmise that the processor might start having difficulties.

One more thing here: Those that have modded have, at times, seen issues with performance when associated with heat and have come close to chasing their tails when trying to nail down what'* causing intermittent perfomance loss when the ambient temperature is high, and have not thought to try to cool the PCM to see if that resolves the issue.

In order to deal with an issue with the PCM accordingly, one has to first recognize the issue.