Autothority Mass Air Flow (MAF) and detonation?

[jc]

Hi,

Has anyone here had a problem with the Autothority Mass Air Flow sensor and detonation?

The PO had installed the MAF on my '88 Carrera and the engine has always detonated above 5500rpm with the MAF. Disconcerting when taking out another car around a corner turnout on Highway 9. When ambient temps are high it would ping at even lower rpms. I've recently solved the problem by replacing the trimpot on the Authority wiring harness with a temperature sensor. Now I can drive it hard with no detonation, the car runs like a champ and it idles well to boot.

If you're interested , drop me an email.

Cheers,
jc

[RickM]

I recall when receiving Autothority's MAF that they were very clear to check and adjust air-fuel mixture. Has this been done?

[jc]

Hi,

The A/F was fine with the stock unit in. The problem with the Autothority setup is that the trimpot was put in to simulate the temp sensor in the stock unit. But the trimpot sets a fixed temperature, and we know air intake temps are going to vary. Adjust it to simulate very hot conditions and it will no longer detonate, but the car won't idle well at cooler temps, and vise versa. Not a good design IMO, that's why I got rid of the trimpot.

Regards,
jc

[RickM]

Hopefully SteveW will chime in. He's the expert on this subject. You may want to PM him.

[Steve W]

Hi jc, the factory air flow meter has an air temp sensor built in to provide the dme with ambient air temperature information allowing the dme to trim the ignition timing accordingly. However, due to the Autothority MAF sensor's lack of a temp sensor, the MAF system is programmed to operate under the fixed assumption that the ambient temperature is ~70 degrees. The hot wire does not compensate this ignition adjustment because it's only purpose is to adjust the fuel mixtures based on air density. Your solution is very interesting as there is another Pelican member - Randal Jacoby trying to combat this very problem you are having as he lives in Phoenix, Arizona and regularly drives in 120 degree heat. I'll email you for further details.

[RandalJ]

jc, I have been wrestling with this issue all summer long. Living in Phoenix, with daytime temps that have reached as high as 117 this summer, I think that I've probably put the Autothority MAF to its limits.

(one of the previous owners had installed mine too)

I have not only an engine knock problem, but also an intermittent off-throttle stalling problem in the hotter weather. The knocking does not occur until the engine is hot - above the first hash mark on the temp gauge - but the stalling would happen during warm-up more often than when hot.

I have had the car about a year now, and I remember experiencing this for a short time before it cooled off last October. Both problems disappeared from October to late May (The car ran superbly, and I even did a DE in November with no knocking), and I had put them out of my mind, but by June both issues were back.

I have pretty much tamed the off-throttle stalling problem this summer with some advice from Steve Wong on adjusting the trim pot to a leaner idle mixture. I think it was at 5.2k, and now is at around 3k (I'd have to double check). Now, my understanding at the time was that this control was for idle air/fuel mix - and maybe indirectly it is - but I did not realize that this was the ambient air temp sensor circuit, which would have relevance under all throttle conditions.

In any case, the "plunging tach" stalling problem was tamed a bit, but the detonation remained. I would get the detonation when the engine was hot (above the first hash mark), A/C on, and when accelerating wide open beginning at about 2700-3000 RPMs. The A/C is a big factor because the condensor on the lid is pre-heating the air into intake. I can get knock even with milder temps (80's) inder hard throttle if the AC is on.

I ran a 50/50 101/91 octane mix (approx 96 octane), and even higher, and still had the knocking at the higher RPMs on really hot days (above 107 or so), so I knew the problem was pretty severe.

I tried a chip from Steve, with the timing retarded a bit from the Autothority map, but still above stock. It helped quite a bit, and now I have the knocking above about 4000 on the hottest days. He and I are corresponing to try to fine tune, but he mentioned your posting to me.

I am curious as to what you used as a temp sensor to replace the trim pot. Do you have the specs on it? I'd like to set my pot to what your sensor would read at, say, 100 or 105F, just to see if I can duplicate your good results, with the current chip from Steve, and with the Autothority chip back in. I'd be content to just to seasonally adjust the pot with summer setting, and a fall/winter/spring setting.

Thanks

[jc]

Randal,

Disconnect the Autothority harness and adjust the trimpot to around 900 Ohms to simulate hot conditions. You must disconnect the harness because the low EMU input impedance will otherwise affect the reading. Connect it back up and try it out.

Happy Motoring,
jc

[jtratza]

I have about 85,000 miles on my mass flow from autothority. There was some detonation during Houston's hot summers at first so I sent the chip back and they reprogrammed it. Seemed to cure the knock except when given near to full throttle after the engine heated up in traffic. If I let the engine cool down a little it seems ok.

The plunging tach problem sounds like a problem I had a couple of years ago. After months of replacing all sorts of things we traced it to a bad potentiameter in the wiring harness from the sensor. I replaced it for $90 and the problem went away.

[RandalJ]

Thanks for the reply, jc.

I will give it a try. I won't be able to put it to the real test until it gets hot again (it's overcast and in the 90's today).

After I posted I realized that the temp in the engine compartment would actually be a lot higher than an ambient 100 or 105, expecially if the A/C is on, so my idea of setting the pot to match your sensor specs at those temps would probably have been way off. It's probably 130 or more in there.

Probably best to go with a sensor like yours. Do you have a manufacturer and part number? Where you bought it?

Thanks again...

[jc]

Tom,

Remapping the chip is one way to go, but it will mean that when running under cooler conditions you will be losing hp, ie you could be running with a more aggressive mapping when ambient temps are cool.

Regards,
jc

[RandalJ]

jc, what did you use for your temp sensor, and where did you purchase it?

Thanks in advance.

[jc]

Hi Randal,

I had to fab one up. I'm gauging interest and will be in touch.

Cheers,
jeff

[AND2008]

Hi
I have a Porsche Carrera 1987 with an Autothority Mass Flow.I need to replace the hot wire sensor module (the wires are broken) but I don't have the part or serial #.Anybody know this inf. I think that is an "Ford" sensor...
Thanks

[Lorenfb]

Insights might be gained from this thread:

http://forums.pelicanparts.com/showthread.php?t=391950

[Fast951]

The DME should pull timing based on intake air temp. If the DME is reading a constant value, it will not be able to compensate properly. So adding a IAT will take care of this. This is the good part! The bad part is that the DME will also lean out the mixture when it's hot. However with True Air Mass (MAF), this should not be done. So when you add a IAT sensor to the APE MAF, ignition will be reduced (no problem), but at the same time the AFR will be leaner (problem). The MAF compensates for changes in temps, having the DME compensate and adjust fuel is not needed with a MAF.

There are various types of Intake Air Temp (IAT) sensors. The correct one must be used! You can pull the IAT out of the stock AFM, but you will destroy the AFM. We use a aftermarket IAT similar to the factory unit with some of the MAF sensors that we use.

During warm weather, assuming the pinging is caused by advanced timing (and not incorrect AFR), it's best to use the Fuel Quality Switch (FQS) to retard the timing. Usually Position 4,5,6 and 7 will do this. FQS is a 8 position rotary switch on the side of the DME. This might fix the pinging.

The correct fix it to use the proper software with the IAT & MAF.

I'm not here often. Feel free to emal me directly if you need help.

[Lorenfb]

"The DME should pull timing based on intake air temp."

VERY limited and in MOST cases won't compensate for the excessive timing
used in performance chips. That's why Porsche/Bosch used twin plugs on
the 964/993 engines with knock sensors. A real joke to assume that the
IAT is a real solution under all conditions.

Bottom line: Poor & unreliable insurance!

[Fast951]

What is "Very Limited"? The amount of timing the DME pulls based on hot IAT?

The factory chip pulls timing based on IAT... Nothing new here.

I can program the DME to pull as much timing as I want based on Intake Air Temp.

As I said, the proper software must be used!

[Lorenfb]

"I can program the DME to pull as much timing as I want based on Intake Air Temp."

So! The problem is that the IAT is NOT a good variable to use for major timing
control, e.g. given it's range of variability. Again, that's why knock sensors are
used for a timing retard of up to 9 degrees. Using temp as a variable is basically
a GUESSING GAME which NEVER achieves either optimum timing nor proper
timing retard AND obviously doesn't consider load.

[Fast951]

Bosch uses the IAT input to alter timing, this functionality exists in the factory chip! With the factory AFM, the IAT is used to alter ignition and fuel! It is not a guessing game.

The claim that the amount of timing that can be pulled based on IAT is limited, is a false statement. I differ by saying that the amount of timing can be set to whatever the tuner wants to set it to. So it is not limited. (Of course you have to know the code to do this).

Cars with a knock sensors (951, 944S2, 944S, 964), have a additional variable to work with "Knock Sensor". However even engines (964...) with knock sensors, still use the IAT to alter timing.. The above cars I listed also use the IAT to alter fuel as they use AFM.

Cylinder Heat Temp is another input that is used by some of the DME (Air cooled engines)....
Same as IAT and CHT other sensors are used...

Proper tuning takes into consideration MANY variables and inputs from various sensors. ONE of which is IAT!

Timing control based on IAT is not limited. It's a programmable variable!

Back to the main issue. Adding a IAT to the APE MAF will address a problem (timing), but will introduce another one (leaner AFR). At the same time, depending on the timing used on the chips, the values being pulled based on IAT may or may not be enough... A proper Air Mass conversion will use a IAT and the code must compensate correctly.

[Lorenfb]

"Timing control based on IAT is not limited. It's a programmable variable!"

Again, so what! The key variable is ENGINE temp and NOT air temp which is
the MAJOR factor for detonation, e.g. the air temp maybe moderate, but
because of driving conditions (traffic/load) the engine maybe very hot.

"Cylinder Heat Temp is another input that is used by some of the DME (Air cooled engines).."

It's not useful on the air cooled 911 because once the engine reaches operating
temp, the head temp sensor resistance delta changes little.

"A proper Air Mass conversion will use a IAT and the code must compensate correctly."

Hardly, as it will ALWAYS be too little or too much not knowing when detonation
really occurs (a guessing game), i.e. the need for knock sensors and is the reason
all modern engines use them.

Bottom line: Relying totally on temp to control detonation is NOT a good approach!!!!!!!!!!!