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Okay I know that is a really noobie question but I have never understood the difference here between the MAF and Speed Density. Can somebody explain? When I bought my truck the PO had put on a K&N air filter, and the air filter connects to one pipe that splits into 2 before it enters the manifold. Also, the filter has a little nubbin hanging off the back with wires coming out, temp sensor? Which setup do I have??
Oh and by the way, I tried searching this forum for MAF but it told me that no results were returned. This is why I hate that search function. I even tried using the google search trick by searching: MAF site:https://www.ford-trucks.com/forums/forum28 and still did not really get what I was looking for. So yeah, just wanted to add that before someone yelled at me, because I know there has to be threads about this but I just can't find them...
-Chris
Last edited by Skandocious; Apr 26, 2007 at 03:45 AM.
Mass air flow is difference from SD because It measures the amount of air and its temp coming in through your mass air meter (the device on your intake tube with the wires). Once the air is measured, the computer knows what kind of fuel mixture is necessary in order to run properly. This gives you quicker readings resulting in better throttle response and more accurate readings leading to better fuel efficiency. Speed density measures how much air is being used by calculating manifold vaccuum and throttle position. SD uses pre-set "equations" to deliver the correct fuel mixture. This takes longer, is less efficient, and limits performance mods by not allowing you to greatly change manifold vacuum. MAF uses active readings and makes its own "equations", which allows you to greatly modify the engine and change the manifold vacuum without upsetting the comp. Few! now my fingers hurt.
The speed density system is fine with a stock camshaft. It doesn't like when you change the cam. SD compares the barometric pressure to the manifold pressure to determine how much air is moving through the motor. If you take into effect the temperature of the air it can be fairly accurate. I know that many Hondas use the speed density system. It has the benefit of not needing to place a MAF in a forced induction setup.
As others have said, the mass air meter gives the computer a direct input for the mass flowrate of air entering the engine. This in itself does not improve throttle response or the quality of idle or anything like that. The tuning is done the exact same as it is with speed density in that for a certain input and rpm, a desired injector pulse width is set in a table (both MAF and speed density have "pre-set equations"...MAF doesn't just tune itself). This is true for both systems.
The speed density refers to two things: the speed of the engine (RPM) and the manifold pressure (MAP). Imagine a 3-D table with two axes and set values in the middle. The difference between MAF and speed density is one table has MAP vs. RPM (or MAP/BAP vs. RPM), the other has MAF vs. RPM. Obviously the table values themselves will not be the same. The computer (MAF or speed density) uses the O2 sensor to monitor the result of its calculated fuel values and can adjust the fueling as necessary. Both MAF and speed density use air temperature sensors, but play a greater role in a speed density system.
There's no difference in the time it takes the computer to make a calculation between MAF or speed density (because the calculations are the same - the computer just uses a different input). On the exact same engine, between fuel calculated by MAF or fuel calculated by MAP, there should be zero performance difference when tuned properly. The improvement seen on these trucks due to a mass air swap is mostly due to the computer being tuned for a sports car (Mustang). The improved throttle response has a lot to do with the common MAF swap computers using sequential injection, and again it partially goes back to the computer being from a Mustang.
The reason MAP tuning cannot adapt is that for a given manifold vacuum, if the flow rate changes, the fuel required is also changed. If this is accounted for with higher table values, it will run perfectly fine. MAF can adjust to changes in air mass flow because it just reads a value higher in the MAF axis in the table. Even MAF computers should be tuned to maximize performance because the spark timing tables are often based on MAF and rpm, so a higher mass flow at the same engine load would use the incorrect spark advance. MAF can adjust for some decent changes, but it certainly isn't a magical solution to aftermarket upgrades. MAP based fueling can be just as accurate as MAF based fueling, plus you don't have the delay from throttle application to mass air flow rate change at the air meter.
Outstanding explanation, couldn't have done a better job!
Those ford powertrain engineers are one crazy smart group. I'd love to hang out with 'em and someday hope to work with there after this EE degree is done.
Thanks all for the great replies! It's a MAF truck.... it IS actually up and running, save for the one mid-throttle hesitation that we were seeing... as a result, I think the best bet is to just put some resistors in the pins that were expecting to "see" signal from the trans and move on. I was wondering if a manual computer would have been an otherwise easy solution.
