What Happens When the MAF Limit is Exceeded?

[BlackGS]

I have a new turbo setup running twin-charged. I'm exceeding the 11500hz limit on my LQ4 MAF, usually around 5500rpms. Each time I hit 5200+ rpms and the MAF hz is above 11500, fuel is pulled and my afr goes to around 14. It is showing a big drop in calculated cylinder air, which leads me to believe that it is no longer using the MAF table to calculate fueling.

The MAF fail is set to 14000hz, and I bumped up all the VE cells in the higher rpm/kpa ranges (set to 90). Neither of these changes made any difference.

So, I am trying to find out just what happens when the MAF limit is exceeded. Does it fail over to VE? What is causing the calculated air to fall so drastically? Does anyone know?

[Perforator]

I don't know the answer to your question but I would love to see some pics.

[Bill@HPTuners]

calculated cylinder air is a CALCULATION based upon MAF airflow & rpm, obviously if maf airflow plateau's & rpm doesn't the cyl air will go wherever it plateaued at. You need to adjust your maf & pe tables to get your fueling dialed in over the point where the maf maxes out.

[BlackGS]

I don't know the answer to your question but I would love to see some pics.

You got it!

[BlackGS]

calculated cylinder air is a CALCULATION based upon MAF airflow & rpm, obviously if maf airflow plateau's & rpm doesn't the cyl air will go wherever it plateaued at. You need to adjust your maf & pe tables to get your fueling dialed in over the point where the maf maxes out.

What I don't get is that the calculated air and afr fall off very quickly. As soon as I hit above 5000 rpms the calc cyl air drops from like 1.25 to .95 and afr spikes to 14.0 when it was 11.5! If I am running around 4500 rpms, the MAF can hit 11500-11600, and no problems. I just don't understand why the drop off is so dramatic. Is there something else going on?

I seem to be dialed in pretty good at the 11000-11500 range.

[bilemke]

What I don't get is that the calculated air and afr fall off very quickly. As soon as I hit above 5000 rpms the calc cyl air drops from like 1.25 to .95 and afr spikes to 14.0 when it was 11.5! If I am running around 4500 rpms, the MAF can hit 11500-11600, and no problems. I just don't understand why the drop off is so dramatic. Is there something else going on?

I seem to be dialed in pretty good at the 11000-11500 range.

Dont take this as 100%, but I will step in here with some basic math.

At a point in the calculation for cylinder air mass, the value is divided by RPM. Thus, assuming all other values stay constant, higher RPM values will decrease the end calculated cylinder air mass.

Regarding the peak MAF being seen at a RPM less then peak RPM, this is volumetric efficiency. Your engine doesnt flow as well at the very high RPM ranges. It actually peaks in flow in a slightly lower engine speed range..

On a stock (or near stock) L67, peak seems to be around 4000-4400 RPM.

[Perforator]

That looks really sweet Bob. I think it's time you find an AFC or get a bigger MAF.
Did you have to put the stock exhaust manifolds back on?

[EaglePerformance]

Sometimes when the MAF is pegged, the load will shift to a lower zone, not sure why this happens, but it does. Short of changing the MAF, if you can figure out the zone it shifts to, change the A/F, timing etc., to your needs up top.

[BlackGS]

That looks really sweet Bob. I think it's time you find an AFC or get a bigger MAF.
Did you have to put the stock exhaust manifolds back on?

I am looking around for a used AFC. I can't find any on the market right now.

The manifolds are custom powerlogs from zzp.

I still don't understand why the afr goes instantly from 11.5 to 14 or higher. I attached a scan from the track a while back where I exceeded the MAF with just the supercharger. The first time it goes over 11500, the afr jumps to over 16! When it gets back under 11500, it goes right back down to low 11's. The strange thing is that the commanded does not change. I really think there is something else going on here, but I can't figure out what it is.

[Perforator]

Bob, have you done a open loop ve tune yet? The reason I ask is I had something similiar happen to me in as far as the IDC dropping for no apparent reason. I believe what was happening was the pcm was switching to VE for fueling instead of MAF at WOT. I started to do a open loop ve tune and with the MAF sensor unplugged the upper columns of my VE table's a/f ratio equaled the wierd IDC's I was seeing. I increased the VE table in the upper columns to get my a/f ration matched and it seemed to fix it. Can you post what your VE table looks like?

[BlackGS]

Bob, have you done a open loop ve tune yet? The reason I ask is I had something similiar happen to me in as far as the IDC dropping for no apparent reason. I believe what was happening was the pcm was switching to VE for fueling instead of MAF at WOT. I started to do a open loop ve tune and with the MAF sensor unplugged the upper columns of my VE table's a/f ratio equaled the wierd IDC's I was seeing. I increased the VE table in the upper columns to get my a/f ration matched and it seemed to fix it. Can you post what your VE table looks like?

I have raised those cells after the above scan, but the changes did not seem to have an affect. My VE table in the upper cells was 80-82. I increased those cells to over 90 last night, and I'm going to try to get a WOT run that will hit those cells. If I see an improvement in fueling, I'll try running with the MAF failed. How high did you set your upper cells?

[5_Liter_Eater]

I cannot speak to the V6 PCM's but if it's the same as the LSx PCM's if you're running in a MAF/VE tune it will solely reference the MAF in WOT. It's not just going to revert to VE once you are off the MAF scale. You simply are not metering the air after that, thus you go lean. You have to go pure SD/VE (assuming you're within the bounds of that table) to maintain airflow calculations. Either that or you will need to increase the PE from 5000 on to add fuel as RPM and boost increase.

[Perforator]

The last row from 4400rpm on are 88 but I'm not getting close to maxing my MAF.

[BlackGS]

I adjusted PE vs RPM vs Time and added a ton of fuel over 4800 rpms. This seems to have done the trick. I now have safe fueling in the extended MAF ranges. Commanded AFR goes to low 9's, but actual is right around 12. I probably want to tweak a little more to get it around 11.6-11.8, but this keeps me less stressed about blowing something up.

Hopefully, someone will sell me an AFC soon and I won't have to hack it like this.

[danthurs]

I had read somewhere, sorry, can't find it now. But when you exceed the MAF flow rate it will cut the fuel flow in half causing a lean condition. This can cause a major backfire threw the exhaust, scaring the heck out of you, the engine then stalls out. This happens so fast the PCM never records the event. I will keep looking for the article.

I have to ask, what kind of boost are you getting with that setup? I'm pushing about 14psi with just the supercharger.

Edit.

Ok, found it.
In our 3800 cars the MAF (Mass Air Flow sensor) Max's out at 37.93 lb/min or 11500Hz. when this happens the PCM thinks the MAF has failed and cuts the injectors PW (Pulse width) in half.

This is usually the case with FI cars. Remember PSI is not what matters but CFM or lb/min of air flow. Some Turbo cars are maxing there MAF at 15+ psi with smaller turbo's and my car maxed the MAF with only 7 PSI (larger turbo)

When the PCM cuts the injectors PW in half it causes a lean condition but more so the car cuts out and backfire's loudly, it sounds like a bomb goes off and you will think you just blew your motor.... it is not nice like a rev-limiter it's sudden and fast. You will let off your gas and usually the car will stall or stumble. Also the car will not usually throw a code, it was not an extended time for the PCM to run its checks and the car stalling don't help.

NOTE: FWD 3800 cars do not have this problem as most people upgrade to the LS1 TB. There are no kits that fit the Camaro/Firebird
They also use the MAFT that dial's down the MAF HZ to the PCM and then they tune from there, We can also use this method. how eve it is still a "band aid"

Below is the HP Tuners 2.0 screen for the MAF fail Fq. Simply adjust it to the max you can. For 1997 cars the max is 16000HZ, 1998 and later cars have a higher number.

[BlackGS]

I had read somewhere, sorry, can't find it now. But when you exceed the MAF flow rate it will cut the fuel flow in half causing a lean condition. This can cause a major backfire threw the exhaust, scaring the heck out of you, the engine then stalls out. This happens so fast the PCM never records the event. I will keep looking for the article.

I have to ask, what kind of boost are you getting with that setup? I'm pushing about 14psi with just the supercharger.

Right now, I have the boost controller limiting boost to 12psi. Even at that low boost, with the turbo I'm pushing 12500hz on the MAF. I just picked up a mini-AFC 2.2, and I will be installing that tomorrow. That should take care of any MAF problems. This turbo will go easily to 25psi, but I won't see that until I upgrade the tranny.

[danthurs]

Ya, you'll leave that tranny all over the road. There's a few members at the car club I'm in that has regals.