QuickSilver2002

Most of us running FI will blow our motors at WOT if the MAF fails.

Mine did not, but I just realized this is the case and figured most people would not know it either.

It's actually pretty scary when you think about it; those little tiny wires are a potential catastrophic point of failure point for the entire motor.

I had an incident a few weeks back where I accidentally bent one of the pins when I plugged my MAF back in. I did not notice this when it happened and took the car out for a few WOT sessions. It gave me a code when I started it, but I pretty much ignored it since I had just flashed the PCM and wrote it off the to typical misc codes that get set after a flash. I cleared the code and went out for my drive. I had my logging software up and running and it immediately alarmed me that my AF was way out of whack. No harm was done, but I was a little confused about why this happened. Especially since I got no consistent check engine light...Just a random code when I first started the car. I found the bent pin and knew it was at the root of the problem, but was a little surprised that the car ran the way it did (pretty good before WOT), but way way lean at WOT. I just decided to write it off as a fluke and move on.

After doing some more research and trying to resolve some other problems with my part throttle tune, I realized almost exactly what happened to my car that day. It was running in speed density mode (basically with no MAF). This is normally not a big deal on an NA motor. The PCM has a VE table of sorts (not a true VE table) that defines how much mass flow there should be at a given RPM and map value. Mass flow is what the PCM uses to calculate all fueling. It pretty much says I have this much air how much fuel do in need to hit 14.7 a/f and then it puts on the various PE/eq modifiers. Well, guess what happens when you are running boost. That stock VE table is so wrong it is not even funny. It only goes up to 105KPA, so boost is something that it cannot understand. And the values that are generally in it (without tuner intervention) just represent what a stock car would flow at the given MAP.

As an example, my car flows about 420g/s of air at 4400 RPMs at WOT (that is probably only about 4-5 psi of boost). The stock VE thinks the car should flow about 250g/s at that same point. And trust me, 99% of the tuners out there do not mess with this table for WOT conditions since the MAF pretty much puts a blanket over most of it.

Anyway, I don't want to freak people out or anything, but you should know that your tune is 100% reliant on the MAF working correctly. Even if you run on a maxed MAF, your entire tune is referenced off that max frequency signal that it can send. If it does not send a signal in that range you are in deep dodo.

If you ever get a MAF related code, make 100% sure the maf is working correctly before going WOT.

I think there is a safety net that you can tune into the VE table to prevent this, but it may have some other ill effects that are not worth dealing with. I'm going to try it here in a few days and see if it causes any other part throttle issues. If not, it may be worth throwing in just in case the MAF decides to hiccup.

SleepieAce

I had never thought of that. Definitely something to keep in mind.

Shinobi'sZ

I am not sure about this. How does this fit in with the Aussie's ability to tune with LS1edit and no MAF. They fine tune the VE all the way through the rpm range at part throttle and WOT. I guess it is because when tuning with a MAF, the VE is more of a failsafe when the MAF is maxed out..so whatever the default is in the VE is not good enough to save the motor. I bent one of the little diode sensor things in the MAF..didn't break it..but now you have me concerned. Don't know whether to thank you yet or be stressed out about it.

This is also why I am looking at tuning without the MAF. I am working with another TT guy (CR) who made 1000+rwhp using the AEM. I will use an on board wideband to monitor AFR.

Av8ter

My MAF failed and all it did was bog down when I tried to floor it. It did have hard starts but once it was running it wouldnt idle very well and was surging. When driving down the road it was ok, but like i said before, when i tried to floor it, it bogged down big time and surged back and forth. The MAF was replaced under warranty and then everything went back to normal.

Hool97

I modified all my MAP 95 100 105 values above 3000 rpm to reflect the increased air flow compared to stock with a little extra. This way i am way rich if the maf fails. In addition i changed my part throttle values as well to improve transitional KR.

QuickSilver2002

It bogged because it was so lean that it could not make any power. The car can bog for two reasons (too rich or too lean). I guess a lean bog is what would happen on most cars, but still not a good thing to rely on.


A MAF less tune is a speed density tune. In this mode the car is 100% reliant on the VE table and the tuner does almost all of his work here. In normal MAF mode, it only uses the VE table for throttle transitions and other areas where the MAF signal is unstable. Once you go above 4000 RPMs, the VE table is completely ignored (this info is always subject to change, but I think it is now generally accepted that this is how it works). This is why most people say the MAF puts a blanket over the VE table and pretty much hides most of it. It’s also why most tuners who tune your car on the dyno don’t mess with the VE table much. It has just been within the last year or so that people finally started agreeing that that VE tables were used at all when the MAF was working.

It seems impossible to get a good mafless tune with the stock PCM and boost. You only have MAP to 105kpa to work with. So you basically can't read any boost at all. You could still get the car to make a nice pull on the dyno, but your part throttle would be horrible. With a MAF, you can actually get the computer to read up to 512g/s of air (which is well into boost even at higher RPMs). A maffless car would have to fuel the same at 5000 RPMS with 0 psi as it did at 5000 rpms at 10psi. That would stink.

Hool97

I agree with you Quicksilver it would stink but then you would not be really using the VE unless the MAF failed. The only other way to fix this is to use a 2 bar MAP adjusted to give 5 volts at 2 bar. Take the table and reduce the values to that of 50% less MAP. Also as far as the stock PCM max. of 512gm/s air flow this also can be made up for by electronically having the MAF read 50% of the flow and modify the injector flow rate by 50% with it also adjusted for MAP/FP etc.

I don't know if that would work but it could. Also i have thought of splitting the air flow between two MAFs and using a small processor to average the signal and then change the MAF by 50%( because the two MAFs would double the air flow yet the Freq would not MAX out)

99C5Vert

I haven't looked at the MAP sensor setup on the C5, but is it possible to get a 2 bar or 3bar sensor and recalibrate the speed-density tables?

QuickSilver2002

I've been wondering the same thing. But I really don't think it would work unless you could get at the MAP vs voltage scaling tables (if those exist).

You could hook up a 2 bar sensor and manually apply different known pressure and see what the scanner thought those KPA values were, but I'm guess that high vacuum conditions would be off the scale and you would have major trouble getting the car to run at idle and part throttle.

The PCM is such an unknown in reality; you never really know what will happen until you try it.

The next issue would be the loss of granularity. You would cut your cells in half for part throttle tuning and I think this would cause some drivability issues.

It does seem worth investigation.

I think the more proven approach is using a MAF that is calibrated with the PCMs frequency range to support big air. You still loose granularity on timing... but that is not really a big deal since the maf signal is really in full control.

Raedon

funny..

I would think the PCM would set the trim rich as hell if the MAF went out as it's getting info from the VASS and if the two don't add up I would assume the PCM would set everything rich as possible to save the engine.

Or are you saying that the MAF doesn't stop sending data but it starts sending false data?

Man, you FI guys have *****, so many things to worry about.