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Originally Posted by eschaider
Irrespective of whichever model, TBI or MP-EFI, you choose, the fueling strategy you use will determine how sensitive the system is to atmospheric, temperature and baro (altitude) changes that the tune has to accommodate. You have two models to choose from, speed density or mass air flow.
Mass Air Flow (MAF) based systems are correct everywhere. It is what Detroit uses to insure emissions compliance no matter where their cars are driven. Speed density systems will fail emissions compliance testing when you get altitude, temperature, barometric or humidity changes in the ambient air.
Most of us do not need to worry about emissions compliance so, the compliance part is gone. The remaining headache for us however is component changes in the engine and ambient air conditions. Different heads, air cleaner, cam advance, cam shaft, compression, headers etc will all require a retune. When your altitude changes by more than a a few hundred feet so does your tune — ecxcept speed density can not accommodate the change in ambient conditions without, you guessed it — a retune.
Keep the car in one location and you'll be fine until you modify anything on the engine to improve performance or go up a mountain or down into a valley and then you will need a new tune.
MAF based systems comfortably side step these issues because the MAF directly measures the mass air flow into the engine — hence their name. The MAF based ECU can take the mass air data data right from the MAF sensor and directly calculate injector pulse widths to provide the commanded AFR or lambda you coded into the tune irrespective of ambient air conditions.
Changes in ambient air conditions or air pressure from alititude changes have no effect on the tune whatsoever, because the MAF is making a direct measurement of the air being consumed by the engine. The direct air measurement allows the ECU to do an immediate injector pulse width calculation based upon your commanded AFR or lambda for the engine operating condition at that instant. Speed density fueling models can not do this.
Ed
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I wish to elaborate on Ed's post. A few excerpts:
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"Different heads, air cleaner, cam advance, cam shaft, compression, headers etc will all require a retune. When your altitude changes by more than a a few hundred feet so does your tune — ecxcept speed density can not accommodate the change in ambient conditions without, you guessed it — a retune."
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Quote:
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Keep the car in one location and you'll be fine until you modify anything on the engine to improve performance or go up a mountain or down into a valley and then you will need a new tune.
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These examples are enough. Do the conditions [change anything] and correction ["retune"] sound familiar? Does it sound like what happens, with, say, a carburetor?
That's because it is.
A "speed density" EFI system is essentially a very expensive carb.
If you're using an EFI system and NOT using the MAF configuration, you've essentially got an electronic carb.
Why bother? If you go through any of the changes to the engine or alititue and want to keep peak performance, you need a "retune". But here's the catch: A EFI retune costs 10x more than a couple of carb jets.
Use MAF or save your money. Unless you stay at one altitude and don't change anything.
You are forgiven if your altitude change is transient. Going from sea level to the Eisenhower tunnel will cause you to run rich until you get back. But so will a carb. You'll survive... When I was a kid we had a Suburban. We took it from Iowa to Rocky Mountain Park and drove Trail Ridge alot. After that first time my dad had the carb rejetted before we left.