MAFs & Injectors: Are they Married or Friends w/Benefits

[cgrey8]

I understand MAF systems are very docile when it comes to engine mods compared to Speed Density systems because the computer doesn't need to know the flow characteristics of the engine in order to calculate the correct amount of fuel to inject. The computer can calculate how much fuel to inject based on how much air is being consumed measured by the MAF and the temperature of the incoming air. Thus it comes VERY close to injecting the proper amount of fuel regardless of heads, cam, intake, or any other mechanical mod. The O2 feedback simply fine tunes the inaccuracies and gets it even closer for future injection calculations.

Just thinking about this a bit, the comptuer needs to know how much fuel a pulse on the injector is going to release when it sends a timed ON pulse to the injector. On a heavily built engine, the air intake consumption will exceed the flow capabilities of the injectors. The logical fix for this problem is higher flow injectors. Although, it doesn't seem logicical that the computer would be expected to use the O2 sensor to figure out that it's fuel delivery calculations are way rich and to keep compensating until the O2 is reporting good. With programming like that, what happens when the O2 goes bad? This would not be good control practice.

Bottom line, the computer needs to know there is extra fuel coming from those higher-flow injectors somehow in order to calculate and control the injection correctly. I see Summit Racing sells higher flow injectors and matched MAF sensors so you can get around this by simply installing a different MAF which basically lies to the computer reporting a lower air flow than actual to compensate for the higher fuel flow. With this unique union, the computer the calculates the injection pulse dwells and durations correctly.

Maybe in the automotive industry this is just the way you do things, the MAF sensor is married to the Injector flow rate and that's fine if it is. However, working in the HVAC computer controls industry, we don't change the input sensor to give the desired output, we change the computer's program or the parameters the program uses to give the proper output with the sensors and controllers the computer has been installed with.

It just seems to me there would be a way to adjust a parameter in the computer so the computer realizes it has higher flowing injectors thus avoiding spending another couple hundred dollars on a new MAF. Such a parameter would likely be done via an interface programming tool (although I don't know of any that specifically do this). If I'm going to spend a few hundred dollars, I'd rather it be on a TOOL that I can use again and again, rather than replacing a perfectly good component in order to spoof the computer into doing what I want.

Anybody know/recommend a device that can make such modifications to the factory computer (specifically a '97 V8 Explorer computer)?

Are there any flaws in my understanding of how this all works?

Anybody have injector flow rate vs. HP charts that can be used to proper size injectors to an engine's expected performance? I assume that even if money wasn't an option, buying the highest flowing injector will have "resolution-of-control" issues. A computer can only send so short of a pulse to an injector. If the shortest possible pulse still runs the engine rich at low RPMs/idle/cranking, then that's no good for an everyday driving vehicle.

I want to hear peoples thoughts and expereinces with upgrading MAFs, Injectors, and Computer Parameters.

Chris

[MalcolmV8]

it doesn't seem logicical that the computer would be expected to use the O2 sensor to figure out that it's fuel delivery calculations are way rich and to keep compensating until the O2 is reporting good. With programming like that, what happens when the O2 goes bad? This would not be good control practice.

Actually in the Ford EFI system the O2 sensors are very important and always need to be working. Even in a factory setup your O2s always need to be in good working order or your vehicle will run horrible and get the worse gas milage you've ever seen.

Maybe in the automotive industry this is just the way you do things, the MAF sensor is married to the Injector flow rate and that's fine if it is. However, working in the HVAC computer controls industry, we don't change the input sensor to give the desired output, we change the computer's program or the parameters the program uses to give the proper output with the sensors and controllers the computer has been installed with.

It just seems to me there would be a way to adjust a parameter in the computer so the computer realizes it has higher flowing injectors thus avoiding spending another couple hundred dollars on a new MAF.

Yes the parameters can be changed in the computer. In fact this would be the prefered and most accurate way of doing this. None of the tools I've seen (such as tweecer, ecc tuner etc.) have very easy to use interfaces and it seems they are usually used in conjunction with a dyno to get the correct tune. Most of us don't have a dyno in the garage or even easily available and an expert tuner to tweak the code in the ECC. So the alternative that's most common is simply match a MAF to some injectors. Or like in my case I have a C&L 77mm MAF which has an air sampling tube that can be changed so I can use either 19lb or 24lb injectors. A new tube costs around $30.

Anybody know/recommend a device that can make such modifications to the factory computer (specifically a '97 V8 Explorer computer)?

Not sure. The units I've seen only change the pre OBDII (started in 96) ECC IV. Best I know of is you might be able to get a custom chip made per your mods and then plug it into the ECC to over ride certain values.

I assume that even if money wasn't an option, buying the highest flowing injector will have "resolution-of-control" issues. A computer can only send so short of a pulse to an injector. If the shortest possible pulse still runs the engine rich at low RPMs/idle/cranking, then that's no good for an everyday driving vehicle.

I've seen a guy put in large injectors into a Mustang and it would idle super rich even with a so called 'matched' MAF. Because as you say there was just to much fuel dumping at idle. However a custom chip which narrowed the pulse width at idle fixed the idle. As I recall these were something like 45lb injectors. The guy was bolting on a Kenne Bell.

Just as an interesting side note. It's amazing how well the computer adapts to your engine based on the sensors and feed back etc. After having the battery disconnected for a while to blank all info it has stored I hook it back up start my truck. My motor is far from stock and not what the computer is expecting with its default factory programming. It won't hardly run. It just keeps dying and I have to feather the gas. The idle surges and it runs bad for the first few minutes. After a short while it 'learns' what makes my engine run right and remembers. From here on out I start up my truck it idles and runs great. That is until next time I disconnect my battery.

Later
Malcolm

[cgrey8]

Malcolm, thanks for the detailed feedback. I was aware that failing O2 sensors could make an engine run terribly, however I thought that once the computer discovered that the O2 sensor was misbehaving (either stuck rich or lean), it would start operating in "Open Loop" mode which would be the same as if you unplugged the O2 sensor completely.

However if the O2 is failing but still feeding back a legal signal, just the wrong signal for the condition, the computer would have no way to know this right off so it would respond and cause problems as you indicated.


As for the ECC modification tools, I'd like to check out anything that's available. Are those that you mentioned Internet downloadable or are these programs that you have to pay a decent amount of money on in order to even view it? If the reprogramming or parameter resetting is not user intuitive, then I see now why the recalibrated MAF when installing higher-flow Injectors is preferred.

However would the computer eventually figure out that it's over-juicing the engine based on the O2 feedback or does the O2 sensor only have a small margin of influence on the fuel mix? If I do move forward with the mods I've mentioned before, I just want to be prepared for what kind of pocket-book hit it will make.

BTW, I reserved the engine today. They'll yank it out this week and I'll pick it up Friday when I get back into town. I've taken a vacation to Fla to visit my relatives down here in Tallahassee. I checked the mileage, it had 128kmiles when it wrecked. It's not terribly high, but not a low mileage engine either. Whether I add aluminum heads or not, I think I am going to rebuild it...

I'll let you know more/ask more questions as things develop. However with the purchase of the engine, the project has officially started.


Chris