Fox Mass Air Flow 42lb injectors

89wreck · Aug 18, 2025

Greetings, I own an 89 GT and recently did a 331 stroker rebuild on the block. I have an Edelbrock RPM II intake manifold with a 70mm BBK throttle body, and 42lb bosch injectors, naturally aspirated. I bought a Megasquirt 2 computer for the car. Seeking input on the right Mass Air Flow Sensor for the 42 lb injectors. I ordered a Pro M 92 MM meter & filter but they have been back ordered for close to 3 months now with no idea when it will be back in stock. Looking for good alternatives, any feedback is appreciated.

Monkeybutt2000 · Aug 18, 2025

Don't run a mass air with the MegaSquirt,not required. I run the MSPNP 2 with no issues.

AeroCoupe · Aug 18, 2025

If you were to run mass air then I would tell you to get an 80mm MAF off of a 2000 GT. That is what I run with my 331 and it works great.

Scrapla347 · Aug 19, 2025

On my 347 I ran a Lightning Mass air and Lightning green top 42# injectors.

89wreck · Aug 20, 2025

Monkeybutt2000 said:
Don't run a mass air with the MegaSquirt,not required. I run the MSPNP 2 with no issues.

Thanks for the feedback. First attempt at tuning did not go well and cost me quite a bit. I thought maybe a sensor to match the injectors might help. I appreciate hearing what has worked for others!

Noobz347 · Aug 20, 2025

89wreck said:
Thanks for the feedback. First attempt at tuning did not go well and cost me quite a bit. I thought maybe a sensor to match the injectors might help. I appreciate hearing what has worked for others!

There is really no such thing when you're talking about an aftermarket ECU or chip-tuning.

A MAF has a transfer function that is used to tell the ECU what mass of air is flowing at a given voltage so that it makes the correct calculation.

A "calibrated" MAF is one that tries to skew the transfer function for an ECU that only knows one transfer function. It's a band-aid and not a very good one when you're talking about injectors over 30 lbs.

With your Megasquirt, the only thing you're interested in is the accuracy of the transfer function. Somebody familiar with tuning can 'reverse engineer' an accurate transfer function for any quality MAF (meaning one that is consistent).

AeroCoupe · Aug 21, 2025

I would add that the mass transfer function for just about every Ford MAF is readily available. I found the one for the 2000 GT 80mm MAF I used in a matter of minutes.

Scrapla347 · Aug 21, 2025

89wreck said:
Thanks for the feedback. First attempt at tuning did not go well and cost me quite a bit. I thought maybe a sensor to match the injectors might help. I appreciate hearing what has worked for others!

I had a mass air metered for my 42# injectors and it would never run right until I got it tuned. This was back around 07. The local shop put my car on a dyno and actually burned a custom 4 bank chip for my ECU. It ran perfect after that. From my understanding anything over 24# injectors and the ECU can't really handle it well. It's also good to get a tune so you can make sure your AF ratio is correct. I got my tune mainly to make sure it was safe to daily drive not to squeeze every ounce of power.

AeroCoupe · Aug 21, 2025

Anything over 19 lb/hr will cause issues without a tune or a "calibrated" MAF unless you use a 1993 Cobra ECU which is setup for 24 lb/hr injectors. Most of us here have had good luck with "calibrated" MAF's with up to 30 lb/hr injectors. I am sure there are folks that have used ones with larger injectors and they were okay but I would bet money they would have been better with a tune.

Noobz347 · Aug 21, 2025

I will add this and explain later if necessary:

Take any Mass Air Meter and "calibrate" it to run a larger injector on a given ECU.

How do we do this? We reduce the size of the air sample flowing over the sensor with a sample tube (the larger the injector, the smaller the orifice in the sample tube) or adding resistance to the sensor circuit. By doing this we just all over the sample size and accuracy of this MAF and degraded the resolution of the meter.

When you are tuning, it is better to [not] have a "calibrated" meter. You are much better off with one that makes use of its full resolution.

And finally, these changes in sampling only account for the commanded duty cycle of the injector and not for the other differences in the physical characteristics between the two injectors. For instance, when a larger injector 'ramps up' from one commanded duty cycle to another, it is physically capable of pumping more fuel, faster during those transitions. That can only be accounted for with tuning as it is determined by the ECU strategy and not the MAF.

89wreck · Aug 25, 2025

Scrapla347 said:
I had a mass air metered for my 42# injectors and it would never run right until I got it tuned. This was back around 07. The local shop put my car on a dyno and actually burned a custom 4 bank chip for my ECU. It ran perfect after that. From my understanding anything over 24# injectors and the ECU can't really handle it well. It's also good to get a tune so you can make sure your AF ratio is correct. I got my tune mainly to make sure it was safe to daily drive not to squeeze every ounce of power.

Yes, I fully intend to get the car to a shop with a dyno. Hopefully that will get things running more smoothly. The guy I paid to come out to me was not able to get it running very good.

Noobz347 · Aug 25, 2025

89wreck said:
Yes, I fully intend to get the car to a shop with a dyno. Hopefully that will get things running more smoothly. The guy I paid to come out to me was not able to get it running very good.

If you pull a data-log and send it to the dyno-shop, they can probably get your tune good enough that you can drive it in, if it's relatively close.

Monkeybutt2000 · Aug 26, 2025

89wreck said:
Yes, I fully intend to get the car to a shop with a dyno. Hopefully that will get things running more smoothly. The guy I paid to come out to me was not able to get it running very good.

I have an amazing Megasquirt tuner if you need a contact.

Jrogers17 · Sep 5, 2025

Scrapla347 said:
On my 347 I ran a Lightning Mass air and Lightning green top 42# injectors.

Anything you had to do to run that Mass air and Lightning injectors. Looking at adding 42 lb injectors.

Noobz347 · Sep 5, 2025

Jrogers17 said:
Anything you had to do to run that Mass air and Lightning injectors. Looking at adding 42 lb injectors.

Yes. You need a tune.

Scrapla347 · Sep 5, 2025

Jrogers17 said:
Anything you had to do to run that Mass air and Lightning injectors. Looking at adding 42 lb injectors.

Yes I had to have it tuned. I took it to a local shop and had a custom chip burned for my A9L. Even though I had the matching Mass Air calibrated for the injectors the car ran like total crap. The stock computers can only do so much.

Fox Mass Air Flow 42lb injectors

89wreck

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Monkeybutt2000

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AeroCoupe

lube between the nut and the face. I know my lubes

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89wreck

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Noobz347

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AeroCoupe

lube between the nut and the face. I know my lubes

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lube between the nut and the face. I know my lubes

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Fox Mass Air Flow 42lb injectors

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