| I do have a question for you though regarding your EMS setup. You mention that you are going to swap out the MAS for a speed-density system and I wanted to pick your brain about it a bit. I understand the fundamentals as to how these two air metering systems function and will state up front that I am highly biased towards the MAS-based configuration. I have tuned Z's primarily using the OE MAS-based configuration but I have had a few opportunities to tune a few Z's using the VPC setup. (for those who dont know what this is exactly, it is a system developed by HKS some time ago which converts the air-metering system of the Z from a MAS-based config to a speed-density based system). I know you are fully versed in these two systems - my only question is why you are choosing to go to a MAP (speed-density) based setup. In my experiences with the VPC, I found there to be inconsistencies in how the MAP system performed as the engine experienced heatsoak. When I made several pulls back to back on a VPC-based Z, the fuel mixture would shift a good bit even though I changed none of the fuel map parameters. My suspicion was that the inlet air temperature sensor which was mounted into the intake manifold back near the passenger side of the balance tube was experiencing a sort of "heatsoak" by way of the intake manifold temperature itself - the temp of the aluminum manifold was sinking into the sensor and causing the VPC to inaccurately provide information to the ECU - causing the shift in mixture. If you dont mind, I'd like to take a moment to divulge here into the nuts and bolts of a speed-density system... A "MAP" based system, or speed-density system, uses a pressure sensor and a temperature sensor to measure the condition of the air within the intake manifold. From these two points of information it is able to determine the density of the air itself - i.e., how many oxygen molecules exist within a given volume of air in the manifold. This system also links into the engine RPM as well. In order for the ECU to make a proper determination on how much fuel to deliver to each cylinder, it must first know precisely how much air is entering the cylinder. With a MAP based system, you do not directly measure the quantity of air entering the engine - rather, the system depends first on the determination of the density of the air and then based on a volumetric efficiency table it looks up what percentage of that air will actually make it into the cylinders. This system is dependent on the tuner providing accurate volumetric efficiency values for all engine RPMs over the range the engine can operate within. When changes are made to the engine system that affect how the engine breathes at any RPM, the volumetric efficiency of the engine has changed in some way or another across its operating RPM range. What this leads to is the necessity to re-tune the ECU each and every time a change is made to the system. With the Z32's OE configuration using a MAS-based setup, when a change is made to the system that affects how the engine breathes, it will see the change in airflow through the engine and account for it initially through the load calculation. This calculation, in the simplest of terms, takes the air mass value and divides it by the engine RPM - the result of this calculation reflects the exact quantity of air consumed per intake stroke - it knows exactly how much air has entered each cylinder by direct measurement rather than by extrapolation of massflow from air density times volumetric efficiency. The measurement of air density with a MAP based system is quite accurate - temperature and pressure is all you need to determine that - however, in order to determine how much of that air actually enters during the intake stroke requires a value representing the volumetric efficiency of the engine at all RPM and air densities. Perhaps I am misunderstood on some aspects of speed density systems and I've obviously opened myself here to being "set straight." :) In the fuel maps for MAP systems, it is obviously correlated by engine RPM and what I presume to be either manifold pressure or air density. If it is arranged by manifold pressure and map variables represent injector pulsewidth, in the tuning process of adjusting fuel map parameters to achieve ideal fuel mixtures, the injector pulsewidth variables actually would have the volumetric efficiency built into them. In and of itself this isn't any different than tuning a MAS based system - you are just manipulating variables to achieve desired results. However, if any changes are made to the system that affect the VE of the engine, the previous tune will no longer accurately apply. Ex: At 18psi of boost with an IAT of 275F, the density of the gases will be "X". If you upgraded some parts on the car which cause the engine to perform better, VE-wise, at say, 4500RPM, the boost will still be 18psi and the IAT will still be 275F but the actual quantity of that air entering the engine at 4500RPM is actually going to be higher and require additional injector pulsewidth to maintain proper fuel mixture. The system will not actually see that more air is moving through the engine since the temperature and pressure of the air is still the same as before the mod was made. Granted, a MAP based system can be precisely tuned and perform just as well as a MAS based system, but a MAP based system lacks the ability to self-adjust when anything changes. I have heard the argument that a MAP based system is superior because it eliminates a "restrictive" MAS sensor. While I do agree with this in certain circumstances, such as the OE 240SX MAS and others which aren't sized for big power, the fact is that the filters themselves are the biggest point of restriction - the MAS outflows even the best flowing K&N filter. I bring all of this up as I really think it is a topic of significant importance and I have a lot of confidence that you can bring a lot to the table in this discussion. :)
[ ashspecz.com ]
[ agpowers@bellsouth.net ] Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the tailor.
Albert Einstein
|
