MAF Conversions

[Edward Hernandez R]

"But you can calculate Mass Airflow from manifold temp, air density and engine speed.  Why is this so inferior?"

Because you cannot calcualte it. You must reference a table, established by hours of dyno work(for OEM) or hours of on road calibration for the DIYer. Thankfully, most aftermarket SD systems conme 'precalibrated' so you can at least start the car and drive around to see how to improve it.  

VARIOUS SNIPPINGS ON FUEL ENRICHMENT DURING TRANSIENTS:

"Throttle openings and closings can cause transients in the manifold/plenum that the MAF sensor won't see, right?...Isn't the throttle based enrichment algorithm a simple way of approximating this transient condition?"

NO, the MAF will see the transients as Anthony pointed out. Change in airflow is what causes transients in the first place. Remember, carburetors simply have an accelerator pump that worked very well for decades before the advent of ECUs. They are based on throttle transients, mechanically compensating for throttle position at the time of the transient. Fuel enrichment based on throttle position transients work fine whether you have a carburetor or MAF or SD, but...

"An additional benefit that I want to point out for the use of MAP sensors in MAF-based systems is for ignition advance calculations. This would need to be modified for engine loading as well, which a MAF alone could not."

MAF systems can and do calulate and engine loading parameter by(oversimplifying here) comparing throttle position, engine speed and mass airflow. For SD systems, yes dn, you can have the same MAP under varying engine loads because it varies with engine speed and throttle position. You simply cannot measure something called 'engine load'(realistically meaning engine output) with one sensor, whether it's MAF(airflow) or MAP(manifold pressure) or TPS(throttle position). Of these, only MAF tells you exactly how much air the engine demands by explicitly measuring it. Assuming one is interesting in measuring engine load to determine it's fuel requirement, MAF wins because you ought to be measuring air anyway. For ignition requirements, which are based on load and speed, you need a table or function using at least two inputs whether you have SD or MAF. BTW, I thought there was a lot of good input from the list on this subject. I had a lot more to say about fuel enrichment(really, you ask?), but much has already bee

n answered. Besides, think KISS. That accel pump worked for decades...

EGR can and does represent a large volume of air but it doesn't directly affect manifold pressure to a great extent. What it does do is cause wider throttle settings at a given engine speed(to achieve the same net engine output), which does change manifold pressure. I've left a hook here, two of them, actually...

"Maybe I'm confused or jumping to conclusions here, but is this implying that the quality control from the Blue Oval factory is so bad that the maps for a SD system would have to be individually tailored to EACH car coming from the factory to keep A/F correct?"

Let me put it this way: GM went SD when Ford went MAF. For 1996 Northstar, GM went back to MAF. One of the prime reasons? Emissions. You guys have to worry about mods that change your VE curve, throwing off SD look-up tables that a DIYer can fix with some effort. OEMs worry about degradation of the existing systems throwing off the same look-up tables that no one can fix. Customer doesn't necessarily notice any performance feel, but will gets pretty upset when their vehicle suddenly doesn't pass the sniffer test and they can't get plates. We get even more upset when a state makes us recall cars that don't pass, such as the catalytic convertor recall we had a couple years back. MAF makes it much easier to maintain emissions performance overtime, and even with MAF we need adaptive control, and even with that, if a component fails or degrades beyond the ECU's adaptive capability, it needs to get replaced.

There are several sources of air leaks in an engine, even an engine you blueprint by hand. The throttle body bearings are one of the biggest sources. Then imagine each and every joint and gasket between the MAF and the combustion chamber(or for SD, between the throttle body and the combustion chamber). Then imagine that each component between the gaskets can leak. Our problem is trying to build  half a million engines a year with exactly the same leak(not possible) that don't change this leak over time(also not possible). So we count on the ECU to adapt itself. But that's OEM stuff. Like I said, the DIY can adjust and fix his/her own system when needed.

I can't respond as often now since I have taken advantage of the Family Leave Act to care for a newborn, so I have to post long ones. Working four hours a day has its pros and cons, but I'd thought I'd explain the epistle and reasons why I haven't posted lately. dn, thanks for letting me know about your Eudora(and asking for my input:). I saved a lot of time not placing carriage returns on this one, but let me know if you or anyone has problems reading it.

PS: I don't have access to the Web, but I would like to know the list's experience and recommendations on aftermarket O2 sensors, especially the ones that use factory sensors and claim wide range readings with fancy LEDs. Can't I build my own meter or is there one you 'trust' the most(mistrust the least)?