This is great info. If I'm running a Windsor, what are the options? The only aftermarket solutions I've found (Holley Sniper, FAST EZ-EFI, Edelbrock Pro Flo 4 XT) are all speed density.

I think MegaSquirt 3 Pro does MAF, but...where do you get the MAF? I have a '95 Mustang intake lying around (last year of the 302), and I suppose I could cobble together the components, but I was kind of hoping for a kit.

I guess maybe the question I should ask is what I would do to make fuel changes if I had a modified '95 Mustang...

With regard to fuel, I've read that vapor lock is much less likely with a return line, and I live in a pretty hot climate...

All the MS3Pro P-n-P units support Mass Air fuel delivery and will use a Ford OEM wiring harness. So do the Haltech units. Megasquirt units are thousands of dollars less expensive and come with a lifetime warranty.


Pro-M Racing (https://www.promracing.com/pro-m-80.html) has the MAF meters and the meter you saw in the YouTube video. They also offer an excellent Universal EFI system that is essentially the Ford Factory Race EFI system built by the same people and on the same production line as the Ford Factory Racing EFI system. It fits pricewise between the Megasquirt and the Haltech systems but a closer to the Megasquirt price point.


You would tune the engine.


A fuel return line has nothing to do with it at all!

Vapor lock is fuel that is boiling or has boiled and turned into vapor. Just like you raise the boiling point for your car’s radiator by using a higher-pressure radiator cap, you also raise the boiling point of gasoline by raising its pressure.

Instead of a 5-7 psi fuel pressure in the feedline to the carburetor(s) EFI runs at a 40 psi or higher (in some systems) fuel pressure in the feed lines to the injectors. Just like the higher-pressure radiator cap, the 40+ psi fuel line pressure to the injectors raises the boiling point of the gas and takes the vapor lock problem off the table. The fuel return line has nothing to do with it at all.

Interesting. Then why bother with a return line at all?

I'm starting form essentially scratch. No factory wires, but some factory intake stuff that I expected to toss...but might be able to repurpose?

What is your motivation for an EFI system in an old technology engine? It gains you virtually nothing over a properly adjusted carb provided you don't have adverse usage conditions:

1. Track/autocross
2. Huge changes in elevation, ie, sea level to 10,000+ feet for extended periods of time, ie, Phoenix in the winter, Aspen in the summer coupled with aggressive driving (ie, having fun on twisty high mountain roads)
3. You have or want a faux weber setup. I wouldn't wish a set of real mechanically connected Webers to anyone. Really. Maybe there are a few that are talented enough to keep them properly adjusted but unless you're one and want the Weber look, EFI is the only way to go.

In the case of #2 the only way you gain an advantage is IF you use a MAF system since without it the EFI system will not do altitude compensation. Please note the MAF does not dynamically adjust for altitude. There can be no flow over the MAF during the period altitude is sensed since it uses static air pressure. (It's not really measuring altitude but air pressure/density from which it infers an oxygen content for determining AFR.)

If you do not have a MAF then you end up with a static tune for the EFI at your altitude and you have the same issue as with rejetting a carb if you change altitude. Both a carb and a static EFI tune will operate just fine if you change altitude operating conditions for short periods - ie, a drive across I70 tunnel or if you visit Denver for a week.

What is the difference between a EFI system and a well adjusted carb? About $5000.

EFI systems in a Cobra are an even more compromise than putting one in say a Mustang. The reason is that there is no IDEAL place to put the O2 sensor. My setup actually had two locations (not sensors - only one at a time). One in the header right off the cylinder exhaust and the other in the passenger exhaust just after it went from the 4 tubes collected to 1. It is not possible to put the O2 sensor in the ideal upright position and the near horizontal is not ideal, and the muffler location is prone to false readings at low idle speeds due to air getting back into the exhaust. The single cylinder location overcomes that but the downstream at least gives you averaging of 4 cylinders. I don't know of anyone that is running two O2 sensors.

Then after it's all done, you've got to seek out a shop with a dyno and a tuner that knows your system. The "self learning" systems are OK only approximate. If you're OK with that then you'd be OK with a carb out of the box.

Just (consistently) my 2c worth.

PS, I had EFI on my engine and very nearly had Roush switch it back to the carb version. I admit it was a system they didn't understand and have since abandoned due to its complexity. I needed EFI because my typical driving DID involve 5000' to 12000' and the engine would backfire when going high because it ran rich. In all it cost me nearly 20K to get it running correctly and I got to learn alot about EFI systems which is what led me to the conclusion that if you are in a static environment, a carb will perform as well for a lot less money.

It's not uncommon for me to go from 70°F at sea level to 40°F a mile up in the same day...Indeed, most of my fun driving involves driving a mile up a mountain and back.

My goal with EFI is to just turn the key and have it start and run optimally in all conditions...and never have to clean and rebuild it. I have two carbureted cars, and they start and run reliably, but there's always a bit of choke and throttle finesse at one time or another.

A clean, well-adjusted carburetor can be great, but I'm not sure that your position that they're as good and trouble-free as a well setup EFI system is universally accepted.

...$5k is nothing to sneeze at, though...I'm interested in fueling configurations mainly to have the option down the road without having to take the car apart again. Regardless of the intake system, return lines are nice to have, and a lot easier to figure out before the car is together!

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.

Putting in return line is good planning.

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,


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.