Quote:
Originally Posted by FUNFER2
Can a cam be too big, that you have little vacuum, meaning do we need vacuum to run the system properly ?
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With Carbs (Webers) - Yes. With Throttle Body injection (TBI) - Yes.
With Multi-port injection (MPI) - It depends on the computer.
Speed density MPI (Also called "open loop") systems use manifold absolute pressure (MAP) as the primary air measuring device for calculating the injector pulse needed to achieve the correct A/F ratio for any particular throttle position; and cams with a lot of overlap will cause a lot of instability in the MAP, the same way they wreak havoc with vacuum. This makes the MAP sensor send erratic signals to the computer, making the computer think it has to make hundreds of adjustments per second. So tuning the idle and part throttle settings in a speed density controller can take lots of time, and lots of trial and error on the dyno, when you throw a big cam into the mix.
BTW- all the current crop of 8-stack style EFI systems out there today are speed density MPI designs, and so they will require this more sensitive tuning, (and probably more time on the dyno to accomplish it)
The other style of MPI is MAF/MAP "combination" (also called "dual circuit", "sequential" or "closed loop") systems. These systems use MAP for calibrating the WOT A/F ratio, and they use Mass Air Flow (MAF) to calibrate the idle and cruise throttle positions.
MAF systems are, hands down, the BEST EFI design for running with a large cam (especially in a street driven car); because the cam would have to be REALLY extreme to influence the air flow in front of the throttle valves.
The reason none of the 8-stack EFI systems use a closed loop controller is that there is no practical/feasible way to incorporate a mass airflow sensor into each air horn (yet), nor is there a controller out there that can aggregate 8 MAF Sensor signals into a single value that the computer can use. (This is what Olddog and I were talking about in the other thread you opened up about the Ebay manifold you had found)
Quote:
Originally Posted by FUNFER2
Are there any other specs of the engine that will screw up the working of this type of injection, thus the engine will not run well ?
Any special engine build needs for this injection ?
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Nah- the cam profile (overlap most specifically) is really about it. Tight lobe separation angles (below 112 degrees), and mega long durations (above about 230 at .050) are where you are going to start running into potential part-throttle hassles with a MAP sensor system (These values are just yardsticks, not hard values)...