However I feel SD is more suited for the track, than it is for the road. This for someone that just wants to do a few mods and tune it, and leave it for a long time like it is, maybe some minor fiddles, the MAF tune would be better. If you want the most outa your car, race etc and are always changing things, retuning etc then SD is more suited.
Why, one point only really will spend some time and try to explain better than I usually do, think a bit, it hurts however. The purpose of any EFI system is to inject an amount of fuel into the engine. To do this, the PCM must know how much air is going into the motor at ANY given point.
What you should know is how much oxygen is in the combustion chamber at the time of ignition. The only way to tell how much fuel to inject in to mix with the available oxygen ( counting the molecules is hard) So you need to calculate the mass of air in the engine.
The amount of mass of air/gas in a given volume (the cylinder) depends on the temperature and pressure of the air. So SD setups measure the pressure in the intake manifold, and the current engine speed (which gives volume), from calculating these the pcm knows the mass of air in the combustion chamber. Speed density.SD is very fast, pretty accurate, and because it measures the intake charge in the intake manifold (close to the combustion chamber) it gives very good results.
But they have one problem, unless I’ve missing something else. The temperature and pressure of the intake charge in the cylinders wouldn’t be really the same as it is in the intake manifold. The air passes through the cylinder heads and past the valves first, and other factors can reduce or increase the amount of air that actually makes it into the cylinder. To allow for these differences, a speed/density PCM needs more info, the correction table called volumetric efficiency.
With volumetric efficiency (VE), you run the engine at each load point on a dyno, and manually adjusting the table until you get the correct air/fuel ratio as measured by an exhaust gas oxygen sensor. Then, after its is all perfectly tuned and producing nice power anytime after this you do anything else to the engine that changes breathing, you must recalculate all the VE points again.
This means as the engine ages, accumulates carbon deposits, cat converters clog up, etc etc etc, the VE of the engine is constantly changing, the perfect tune is constantly deteriorating. Emissions are also increasing overtime. It also means that if you do something to make a big change in VE - like change catbacks, extractors, intake systems, for example - that perfect SD engine tune will suffer. You need to go and reset the tune again
So holden like GM have used a different method of determining the mass air charge. Instead of the speed density systems, they use a sensor that measures air mass directly, a maf.
From the point of view of the factory, a maf is a great. As air mass is measured straight off the sensor, the maf compensates for wear, or ppl modifing the car to improve it (why you may need to reset the pcm to relearn). Since it has been mapped a given air mass vs RPM in the computer's tables, you can do what you like to the engine's VE and the computer can adjust for it. So you can make small changes and not stuff up your maf tune to much.
So it all depends on what you want to do with the car which tune to get. A maf tune is more forgiving, so could be said to be safer due to the point about VE and SD tunes.
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