RE: Ported Cobra Mass Air Meter

AOD86GT · Nov 18, 2004

My car used to have no idle problems until I ported out the air divertor in my Cobra mass air meter. Now I cannot even hold an idle unless I have my foot on the gas pedal. Now it surges and dies on stoplights. Idle screw/tps adjustment doesn't help. Thought I would get more air flow porting it, now I think it is out of calibration. Thanks.

stang2841 · Nov 18, 2004

thanks for experimenting with me on this...... Now I know not to do that

Mustang5L5 · Nov 18, 2004

You answered your own question here.

Yes, you now screwed up the calibration of the meter.

The way the meters work is they flow a certain set amount of air due to the diamter. The small flow tube is a certain calibrated size....say 1/16 of the air that flows through the meter flows through the sampling tube.

Now they computer only sees the air flowing through the sample tube but knows it's 1/16th of the actual airflow, so it bases it fuel curves on the Y time 16 ratio.

You cannot enlarge the bore of the meter unless you adjust the bore of the sample tube as well. This is how aftermarket mass air meter calibrate for injectors. They "trick" the computer by changing the ratio of the mass air meter.

When you bore out the meter, you let more air in but the computer still sees the same airflow in ther sample tube so it doesn't add fuel. The bigger you bore it, the leaner and leaner your car runs.

Another thing i see is that you have a Cobra meter and 24# injectors. WHat computer are you running? If your Cobra meter is a factory '93 Cobra meter, it's calibrated for 19# injectors. If you run 24's with the meter only and no chip or computer change, you will run rich.

autoXr1 · Nov 19, 2004

Sorry you had to learn the hard way.

Kids, I know there have been a few individuals who have ported their meters and then have claimed improvements or that "it worked great". Chances are they likely had other mods which made the stock calibration less than optimal and when they ported out their MAF meter, they lucked out and perhaps it did run better. That said, unless they have the proper measurement and calibration equipment and was able to transfer that new information into a chip on their computer, their tune is off, despite their "improvement" claims. Be careful of what is on the internet.

Pro-M not only has OEM meters you can purchase with the correct calibrations for your application, but can also recalibrate your existing meter for changes in injector sizing. Switching to an OEM 70mm MAF meter is a great way to upgrade your intake system, but make sure that the calibration is correct for your application or else you can have all sorts of drivability issues and certainly, the engine will not be running optimally.

Bottom line is that in order for the engine to run properly, the MAF meter calibration should match the injector size AND the computer should be re-calibrated for, at the least, the new MAF meter voltage table. Too many folks stop at the new MAF meter and end up with a crappy tune...even when they may be making more power.

AOD86GT · Nov 19, 2004

Mustang5L5: Right now, I am using a stock 1990 computer from an aod. Is that okay? Can you clarify more. Does that mean I have to use the Cobra computer with it. Yes, it is a factory Cobra meter that was sold as a kit with 24lb inj.(using it) and Cobra ecm.

AutoXr1: Thanks, for elaborating. So, I guess I can sent it to Pro M and they can re-calibrate it for me. Thought it was going to be another paper wt.

stang2841: That's what I love about this hobby-tweaking (NOT). Thanks to the internet we can all share this wealth of information.

Michael Yount · Nov 19, 2004

The sad part is that with your mods, the 70 mm maf was flowing likely capable of MORE than enough air to meet the engine's needs. IN short, it wasn't a bottleneck to flow - it's doubtful that porting it would offer any significant performance benefit.

AOD86GT · Nov 19, 2004

Michael Yount said:
The sad part is that with your mods, the 70 mm maf was flowing likely capable of MORE than enough air to meet the engine's needs. IN short, it wasn't a bottleneck to flow - it's doubtful that porting it would offer any significant performance benefit.

Wish I knew that before. Had too much time in my hands with a new Dremel I had bought. But that's kewl because people can now learn from my mistake.

Prochrgd1 · Nov 19, 2004

I did the same thing on my 94 Cobra & had the same problems. But, I found the fix & it worked great afterward. I'm assuming you cut/grinded the post up to the sample tube & took the thin metal plate off the back of the post. I did the same & the car would'nt stay running. Put the thin metal plate back on the sample tube so it looks like you're actually plugging air flow. Grind the metal backing up flush with the sample tube. plug it in & go. Give it a try, it worked for me.

AOD86GT · Nov 19, 2004

Prochrgd1 said:
I did the same thing on my 94 Cobra & had the same problems. But, I found the fix & it worked great afterward. I'm assuming you cut/grinded the post up to the sample tube & took the thin metal plate off the back of the post. I did the same & the car would'nt stay running. Put the thin metal plate back on the sample tube so it looks like you're actually plugging air flow. Grind the metal backing up flush with the sample tube. plug it in & go. Give it a try, it worked for me.

Yes, I did all that. You are exactly right. My car does not want to idle at all or stay running anymore. It keeps dying out on me. I have to keep one foot on the gas pedal all the time and the other on the brake pedal to keep it from stalling. Was a "biotch" trying to stage on the line and keep her running. Wasn't like that before--porting the Cobra meter. It used to idle nice and smooth with no surging and stalling.

Mustang5L5 · Nov 19, 2004

AOD86GT said:
Mustang5L5: Right now, I am using a stock 1990 computer from an aod. Is that okay? Can you clarify more. Does that mean I have to use the Cobra computer with it. Yes, it is a factory Cobra meter that was sold as a kit with 24lb inj.(using it) and Cobra ecm.

The meter is not calibrated for the injectors, the Cobra ecm is. If you run 24# injectors and a stock '90 AOD computer, that cobra mass air was only calibrated for 19's. You were probably running slightly rich with this setup.

I am under the assumption you will be shopping for a new MAF now. Make sure you get one calibrated for 24's. If you get another OEM cobra MAF, it will be calibrated for 19's and you'll either need to use the Cobra ECM or get a custom chip.

AOD86GT · Nov 19, 2004

Mustang5L5 said:
The meter is not calibrated for the injectors, the Cobra ecm is. If you run 24# injectors and a stock '90 AOD computer, that cobra mass air was only calibrated for 19's. You were probably running slightly rich with this setup.

I am under the assumption you will be shopping for a new MAF now. Make sure you get one calibrated for 24's. If you get another OEM cobra MAF, it will be calibrated for 19's and you'll either need to use the Cobra ECM or get a custom chip.

Now I understand. Thanks for the tip.

Michael Yount · Nov 19, 2004

Sorry I missed it - I just caught this same issue in another thread. Good catch 5L5.

Before you send the Cobra meter back in, try either using the Cobra computer, or pulling the 24's and putting your 19's back in -- see how it runs then. As 5L5 stated - either use 19's with the Cobra maf, or use 24's with the Cobra maf and ecu; or use a chip to select 24# injectors with your HO computer. But the way it is, I'm surprised it's running as well as it is. You have a mismatch between maf and injector size/computer.

AOD86GT · Nov 19, 2004

Michael Yount said:
Sorry I missed it - I just caught this same issue in another thread. Good catch 5L5.

Before you send the Cobra meter back in, try either using the Cobra computer, or pulling the 24's and putting your 19's back in -- see how it runs then. As 5L5 stated - either use 19's with the Cobra maf, or use 24's with the Cobra maf and ecu; or use a chip to select 24# injectors with your HO computer. But the way it is, I'm surprised it's running as well as it is. You have a mismatch between maf and injector size/computer.

Thanks for your input Mike. I am borrowing a friend's Bullet meter (24lb) with a conical this weekend and see if that clears all my horrible state of tune. Will post results.

AOD86GT · Nov 22, 2004

Life is so sweet again. Running Bullet meter from Pro M with a conical calibrated for 24 lb. injectors. She idles steady at stop lights, no more bad surging and fluctuating rpms. And also does not cut-out or die on me anymore. She runs like a million bucks, now. Thanks everyone for all your help! Too bad, drag season is over in NJ.

Michael Yount · Nov 22, 2004

"Too bad, drag season is over in NJ. " Heck, hop on I-95 and bring it south! Glad to hear it's running good again.

sn9550 · Nov 22, 2004

I did that to the meter on my 95 and had all kinds of problems. That thing would ping constantly under the slightest load. Now with my 88 I don't have to run one , yet. Glad you got it straight.

crunchie12268 · Nov 22, 2004

Thats funny

RE: Ported Cobra Mass Air Meter

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