Exactly how does it differ from MAF? It doesnt rely on 02's as much does it? cuz my car is having all kinds of problems 02 related.

 

speed-density vs. mass-air has nothing to do with how the PCM uses the O2 sensors. It determines only how the computer measures the actual air flow into the engine.

Mass-air is very simple... the MAF sensor measures the mass air flow (#/min or grams/sec). Its a direct measurement. The mass air sensor produces signal that varies frequency with air flow, and the PCM has a table that tells it what each frequncy means in terms of mass air flow. Then it adds the fuel required to produce the target A/F ratio.

Speed-density is a little more complicated. First the PCM multiplies engine speed times displacement. It looks up the volumetric efficiency in a table stored in the computer, as a function of rpm and engine load (MAP). Now it knows the volume of air flow into the engine... cfm. But it needs to know the "mass" (pounds) of air flow into the engine. So it measures the inlet air temp, and the inllet air pressure (manifold absolute pressure, or MAP) and uses the perfect gas law to calculate the density of the air (#/cf). Then it multiplies volume flow rate by air density to get the mass-air flow rate in grams/sec or pound/min. Then it calculates the required fuel to meet the target A/F ratio, just like the other system.

Same end result..... EXCEPT:

The mass-air system is self-sufficient, and only needs to measure the actual air flow to accomodate modifications to the engine (within reasonable limits). Speed-density is dependent on an accurate table of volumetric efficiency vs. rpm & MAP. It needs a new table programmed into it to adapt to significant changes in the engine, particularly improved breathing.

 

The only SD ECUs that have a intake air temp sensor are the aftermarket ones, arent they? I have looked over a SD LT1 and my friend has a SD 5.0 and I didnt see one on the LT1 and I know that my buddies doesnt have one on his 5.0

 

The 93 LT1 runs S-D from the factory, and uses the IAT sensor on the inlet bellows. I run S-D on my MoTeC M48Pro, and use a stock IAT sensor in my Ram Air box. There is absolutely no way to run S-D without knowing both the inlet air temp (some systems call it "MAT" rather than "IAT") and MAP.

A 94-97 LT1 uses the MAF, but if the MAF fails, it defaults to S-D, using the MAP and IAT readings. Not sure why you are saying there is no "IAT"?????

 

Both of my 93's have IAT's.

 

Then why do you have to tune in similar outside condidtions?

 

Depends how you are tuning.... if you've got it on a dyno, the dyno program will correct whatever conditions you are running at to SAE standard. Differing condition shouldn't affect anything.

If you are making passes at the track, and using MPH to gauge HP, you need to correct the MPH for atmospheric conditions so you can accurately evaluate the effects of the changes. Condtions can vary, but you have to make corrections to the data before you use it.

Where you get into problems are extreme changes where it might bring into play parts of the program you aren't "tuning"... if there are large timing offsets for IAT, or other things programmed into your system, that could mask changes made in other areas.