I agree with your comments on the effects of cold air on carb'd engines..... unfortunately I am old enough to have wrestled with a Carter AFB on an early 60's Chevy Impala, a 3x2bbl Rochester setup on a 66 GTO, etc...........

The speed-density system MUST have an IAT sensor. The only way a speed-density system can know what the density of the incoming air is, is to calculate the density using the perfect gas law, which requires the absolute air temperature (degF + 460) and the absolute air pressure (MAP), at the point it enters the intake.

In a mass-air system, the MAF sensor is "self-contained". It measures the temperature of the incoming air, then calculates how much energy it takes to keep the thermistor on the wire at roughly 100degF above the incoming air temp, then knowing the specific heat of air, it can calculate the pounds of air flowing through the sensor. It does not need the data from the IAT sensor on the intake elbow.

However, as wile2k pointed out, on failure of the MAF sensor, the system will default to speed-density, and then it needs the inlet air temp. I believe there is a suspicion that the IAT sensor is also used for some minor ignition timing offsets...... maybe the elusive 3deg advance that seems to show up from time to time. I know with my MoTeC we trim ignition timing based on inlet air temp and on coolant temp.

The only time the PCM changes A/F ratio at away from the stoichiometric 14.7:1 is on startup in open loop, based on coolant temperature..... sort of like the old days when you had a choke on the carb to make it run richer on cold startup. At WOT, the "target" A/F ratio is calculated by a formula, using values that are a function of coolant temp (PE vs. coolant temp) and rpm (PE vs RPM). So it doesn't seem to depend on IAT.

I'm a little limited on knowledge of the stock PCM, because I pretty much eliminated it about 3 years ago, before LT1_Edit and TunerCat were available to poke around inside......

 

Injuneer, theres nothing wrong with a Carter AFB. I got one sitting on my chevy truck right now. Too easy to work on not to have one vs a Q-jet. At least I don't have to worry about trashing a PCM when re-jetting it! Haha. Thanks for the explination. And looking back at my scan data before losing the PCM, I was also getting KR at high MAP and high RPM at WOT. Not during shifts though. MAP values were running in the 23 range, RPM's were in the 5500 to 5800 range and it was running with between 41 to 42 deg of advance. What do you make of that? Should I be looking at losing a little timing to get rid of it or make changes elsewhere to try and eliminate it? Just curious. Also, where would I make changes in Tunercat to get my AF mixture down if I want to cut my fuel pressure back closer to stock? It's high now to keep my AF mixture low, but I don't think I want to run it like that. If I drop my pressure closer to stock, i'll kick up my AF mixture and lean it out. Thanks in advance.

Ken R 95Z