Question about mass air meters and injectors

NotA4.6 said:

Say for instance I have a Pro M met
er that's calibrated for 24# injectors and I want to use 42# injectors. Can a tuner make that combo work via SCT chip?

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Yes.

NotA4.6 said:

I finally talked to my tuner today and he confirmed that it can be done. I asked him for a temp tune and he told me sure, but told me that if I drive it easy and leave the MAF unplugged that it will make it just fine, he said it will run in "speed density" mode. I trust him so I'm going to do it. It's a good ways away so I hope I can at least drive 50 or so MPH or it'll take me a year to get there.

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This is not true. In order to run in speed density mode, you would require a speed density computer and your BAP would need to be plumbed into the manifold to function as a MAP sensor. In reality, your car is running in, "limp" mode with a predetermined fuel and spark tables that ignore all input from your remaining sensors. The EEC's adaptive strategy is also disabled.

NotA4.6 said:

I wanted to update this thread. While getting tuned it turns out that since there was such a huge margin between the calibration and the actual size of the injectors, the meter ended up pegging at about 8-9 psi of boost. I ended up having to install a MAFia right there on the dyno in order to tune the car. The cost of the MAFia was about the same as a new meter would have been and the tuner said it serve me well with future upgrades ie more boost.

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This is not a function of how different the calibration of your meter is from the injectors. It is a function of the mass air meter's ability to convert the flow of air into a variably resistant output that the EEC can read (voltage is sent to the meter by the EEC and the meter element varies that voltage so the EEc can determine the mass of air being ingested). In other words, you could have a meter that is capable of reading 5000 CFM (for instance) and still be calibrated for an A9L computer and 19 lb injectors.

A word of caution about units like the MAFia: What these things do is rescale the resistance properties of the MAF by varying the input voltage to the meter. While it may function very well for your current application, you need to know that the more you scale this thing, the more resolution you loose, the less accurate the the meter will be at each end (the minimum and maximum CFM ranges for your combo). If scaled enough, it will effect driveability at low RPM/CFM. Something to keep in mind if you decide to boost your combo later an stretch the scale even further.

NotA4.6 said:

I do have an issue when I go from load to part throttle where the car feels like it bucks ever so slightly. Doesn't do it all the time and it's not severe. It's less noticeable if I'm at low RPM in 4th or 5th gear. Could this be caused by the MAFia?

I'd hate to think I spent 175 bucks on this MAFia and it be useless. I could have easily bought a lightning meter for the price I paid for the MAFia.

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That to me sounds like you're using too high of a gear at too low of an RPM. If you're at say 1500 RPM or lower and in higher gear requiring more torque then I would expect the car to not be happy with that. You CAN tune for that to a point but there is a limit.

Low resolution symptoms would be more like, bucking at cruise when leveled off the gas in an appropriate gear. Engine stumbling at when transitioning from a cruise output into or out of the power band. Inability to level off the gas pedal at speed. AFR bouncing between rich and/or lean at low speeds or cruise. Exhaust popping when decelerating. Things of that nature.

The hairy part is that many of those symptoms can also be caused by other malfunctions as well.

NotA4.6 said:

That's exactly what it's doing. I said it was "less" noticeable at low rpm in a higher gear. Say I'm driving at 65mph on the highway. When I level off the pedal to cruise I can feel the car jerking slightly. And I do get popping out the exhaust during deceleration. I told my tuner of this and he said he'll fix it next time I'm in the neighborhood.

With my last supercharged car (95 GT) I used an 80mm Pro M that was cal'd for the 42's and it drove perfect! I did some searching around and it seems like Don95Vert likes the MAFia and says it can be tuned to drive like stock in the right hands.

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To a limit... and yes... your tuner should be able to get that straightened out and should also be able to ensure that it is the tune that needs tweaking and not a sensor malfunction.


By the way... ProM is VERY good at what they do. I run a ProM that is not calibrated to ANY specific injector. I use the 30 point (I think it's 30) mass air transfer function that they provide and dial in the injector size, the EEC, displacement, etc. so that the air and fuel match. Your air fuel ratio is a function of the EEC and not the function of either the injectors or the MAF or any other sensor for that matter. All of those things are just inputs/outputs that the EEC and tune use to come up with the correct answer/mixture.