One thing that worries me about your hardware is that you have a relatively large dead-end fuel rail. That means you're going to be vulnerable to heat soak effects. if you haven't looked into this you might be surprised how much the viscosity of petrol changes with temperature. If you have any chance, I'd suggest you replace the center feed fuel rail with an end feed one - supply from one end, go past the injector (or injectors) and put your pressure regulator at the other end. This means the rail never stagnates so you don't the temperature swings in the fuel. I actually have a bakelite (sp?) spacer that goes beneath the TBI to insulate it from heat. There's actually very little chance that any fuel would "stagnate" in this rail, as it flows from center to the injectors, which are at the termini. Literally the end is where the injector lies. I am slightly confused by your suggestion on the positioning of the regulator... The fuel needs to go through the regulator in order for it to regulate the pressure. Are you by chance talking about the pressure relief valve?You will also need to measure the injector supply voltage very carefully and adjust your pulse length to compensate for voltage changes. The higher the supply voltage the faster the injector opens. The closing speed of course is determined by the characteristics of the injector itself and relatively fixed. If your injector is sized correctly, at low load / low speed you will be running with pulse lengths similar to the opening time (I believe erogation is the term EFI people use) and you will need to control the pulse duration very precisely.The charging system on most cars is fairly stable to my knowledge... I can understand it making a difference with a couple volts of variation, but the regulator (again, to my knowledge) pretty well hovers right at 14V... Please correct me if I'm wrong. If it does vary much, I can simply add in volt/amp to the program.With TBI you also have to pay particular attention to the temporal distribution of the fuel that goes into an engine cycle. With multi-point injection each cylinder's fuel is dropped into the port and more or less has to go into the right cylinder. But when you are injecting a sequence of pulses into a column of air that is being divided into multiple cylinders, not always smoothly and continuously, you need to design your injection system to mix the fuel/air together rather than have lumps of rich/lean mixture chasing each other down the manifold. Of course you don't have this problem with carbs because the fuel is mixed evenly with the air as it passes through the carb.Much consideration was given to the proper "atomization" of fuel while I was building the unit... The fuel has to pass the air coming in from the throttle body in a cross-pattern, then due to the tubes having open sides the air has to cross in a figure-8. The whole system is open enough for good airflow, yet turbulent enough (hopefully) to mix the fuel well. That's also part of the reason why I want to have the injectors fire alternately, so that it's a more consistent fuel distribution vs. the effect of both injectors firing at once. And actually, even with carbs (depending on quality and the type of manifold) "pooling" of fuel or just lousy mixture is still an issue. Only time will tell whether this manifold will mix it well.It's doable (I've done it) but it's not easy and there is a lot of factors to deal with to get this working right and I'd guess you have several man-years of work ahead of you to develop from scratch something that works as well as a simple carb. It's a fun project, but it's a lot of work.I'd love to hear more about your project! Was it also with an Arduino, or another custom circuit? That's actually half the purpose of this project. I'm pretty confident that it'll work (and get better MPG than the current carb, @25mpg ), but a main point is to improve my programming skills and also to better understand EFI in general.
You CANNOT get by with as few sensors as you think. How are you going to know when to trigger the injectors? As magnethead was saying, you need to perform a timing map on the pulses. They need to happen just before each intake valve opens.How do you do this? With a crank position sensor… You can't just put one of these on easily so you need to come up with another way to find position and from revolution to revolution this timing map and pulse cycle will probably be changing. You can maybe do this without a crank position sensor, but you need to know when and how long each of the 4 intake valves are open and time your pulses on the injector accordingly. If you try to just randomly inject fuel without this, you run a risk of blowing a lot of things. You could possibly use a sensor on the tachometer for the pulses, but again, you have to tie your pulses back to the opening of each cylinder.
note the lawnmower can get away without a map sensor because it's a governed engine designed to run at a single RPM