Quote:
Originally Posted by twobjshelbys
A carb should have no trouble with that altitude change as you well know. Plus remember any altitude compensation is not dynamic under operation. The maf only infers altitude at key turn without engine running so if you drive up and back and don't stop for lunch you made no adjustment whatsoever.
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What Tony said about MAFs and how they sense air mass entering the engine is correct but not entirely representative of the process today.
A modern MAF does not sense altitude, although it can be inferred. The way that the MAF informs the ECU of what is happening air-mass-wise is by sensing the temperature of a small length of a special wire that is electrically heated to a specific temperature. The wire’s temperature is monitored, and the ECU continuously supplies current to maintain that temperature with a closed loop circuit.
As engine speed increases, the air flow over this ‘hot wire’ also increases. The increasing airflow has a chilling effect on the hot wire. When the MAF is calibrated, the change in wire temperature attributable to increased air volume passing over the hot wire is measured and recorded. The amount of current used to maintain wire temperature is directly convertible into air mass.
When we go to higher altitudes, the air is thinner and has reduced mass to cool the hot wire. At lower altitudes, the air is denser and has increased mass to cool the hot wire. Modern Mass Air Flow EFI systems will frequently use two inlet air temperature sensors (IAT’s). The first may be identified as IAT1 and the second as IAT2. The reason for two sensors is accuracy in the air mass calculation.
The IAT1 sensor provides information about ambient conditions. The IAT2 sensor, typically located in the intake manifold and before the intake valve, provides information about the intake charge temp. Intake charge temp is a density metric but also a contributor to detonation. If the ECU had detonation detection/prevention logic, it pays attention to this data for optimal ignition timing and fueling before the occurrence of detonation.
Temperature data is foundational to determining how much air mass is in the intake system for use by the engine’s cylinders. The gas laws (remember those from High School?) use pressure, volume, and temperature as metrics to determine the mass of a gas at a given temperature and pressure.
More than a hundred years ago, several scientists, in independent efforts, began to characterize the behavior of a gas when one of its metrics, Pressure (P), Volume (V), or Temperature (T), was held constant while the other two were manipulated. This “team” effort resulted in what we today call the Ideal Gas Law, which can be represented as,
PV=nRT
where;
- P is the absolute pressure of the gas
- V is the volume the gas occupies
- n is the amount of the gas
- R is the universal gas constant, and
- T is the absolute temperature of the gas
Using the Ideal Gas Law and knowing the ambient air temperature and pressure, the mass of the air ingested by the engine can be accurately calculated. The actual calculations are done in real time by the ECU and are used to calculate the injector pulse widths necessary to match the ingested air with the correct amount of fuel to meet the tuner’s commanded Air Fuel Ratio (AFR) or lambda. Lambda is simply 1/AFR, which makes the tuning job easier.
Modern ECUs have enough sensor data and computing power available that this calculation can be made (with the right ECU) on a cylinder-by-cylinder basis in real time. Today's CPU silicon technology allows these calculations to be accomplished, at elevated engine speeds at or above 10,000 rpm, even for ten and twelve cylinder engines.
With all this enhanced capability and computing power reserve we have on hand today, it is possible to precisely change individual cylinder fuel charge requirements in real-time while driving as you move from low to high altitude or vice versa. Very early EFI systems, like the Accel DFI systems Roush used early on, were challenged (read unable to perform) at this level.
The Haltech, Megasquirt MS3Pro, Pro-M Racing Universal EFI system, and some others are all capable of this level of performance today. There are yet others that are not — and also cost more! Some can cost much more ($10K+) but still do the same things.
As Tony said, your carburetor is a good solution unless you are moving between significantly different altitudes. If you do move between significantly different altitudes, you owe it to yourself to explore the Haltech, Megasquirt, and Pro-M Racing Universal EFI systems. Any one of them will do an excellent job for you.
You will spend the most on the Haltech and the least on the Megasquirt, with the Pro-M Universal System coming in a whisker above the Megasquirt pricewise. I have used Haltech and Megasquirt and looked extensively at the Pro-M system offering.
Today, I use a Megasquirt MS3Pro Plug and Play (P-n-P) system for its capabilities, its ability to work with any sensors, ease of tuning (programming), and the ability to use an off-the-shelf Ford OEM engine and under-dash wiring harness rather than building my own harness.
The move to an EFI system is a big step in fuel management tech, no matter who you are. It is advantageous to invest the time to learn and compare before buying. It is stunningly easy to spend $7K to $10K and get it wrong! Even when you do it right with the lower price point MS3Pro, it will still cost you about $3,500, possibly more, depending on what you already have that is suitable for reuse in the EFI system.
If you are supercharged, it is impossible not to use EFI. From a piston-burning perspective, you are playing Russian Roulette with more than one bullet in the pistol when you try to avoid using EFI.
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