Holley systems have also been around longer. i remember when the used to use the same gm style coils that came on 8 cyl engines. the ones that were mounted on top if a module the size if a tablet.So Speed Density in aftermarket form is different? Would a speed density system like Holley be able to adjust to the changes or can the Holley system automatically compensate for the changes (without dyno time or some other calibration method). I'm not really that familiar with the Holley, I just know people like it because it's cheaper and doesn't "require" a mass air sensor (at least those are the most common reasons that Holley enthusiasts give when they say Holley is "better" than Pro-M EFI)..