How can any fuel additive increase power?
Really only one sure way, but one that isn't applicable in every case as Chris142 proved (depending on which additive he used). The sure way is a cetane improver but how much you gain is directly proportional to the particular fuel. Increasing the cetane beyond a certain point has absolutely no benefit. The closer the fuel is to what the engine "likes" the less you gain... and vice versa. Unfortunately, diesel fuel standards for cetane ratings are not Federalized beyond a minimum of CN 40 (Cetane Number, this test uses a test engine to measure ignition delay) and there are no Federal mandates to post the actual CN on the pump.
Don't confuse "Cetane Number" with "Cetane Index" CN 40 is about CI 35. You can google CI for more info but it's the "quick-n-dirty" method that doesn't require a test engine. CI is what you used to got with the old specific gravity tester but that test standard is way obsolete now because of the additives used in modern diesel fuel. Furthermore, this method does not account for cetane index improver additives, both aftermarket and those added by the fuel blender.
Various engines "like" various cetane ratings. It can even come down to a new engine versus a tired one, but the bottom line is that the engine designers tune to get the most from the available fuel in the market for which the engine is intended. That's about CN 40 here.
What do the IDI engines "like?" That's a good question to which I can only find general information on paper (and I have a LOT of paper now), such as meeting the D975 ASTM spec for 2D fuel. Back in the day, that was also a minimum of CN 40. I have been told by two engineers who were involved in the development of the 6.9L, Rowells and Hankins, that the optimal point is CN 45. Assuming they are correct, and I have no reason to doubt these guys, after CN 45, there is no, or very little, benefit to increasing the cetane number past that. That fit pretty well with some testing in the '90s on a GM 6.2L (which has the same combustion chamber design, compression ratio and the same type of injection system). There was about a 9 hp increase (8 percent) from 39 cetane to about 45 cetane, after which further gains were not measurable.
What is the current average CN for diesel fuel? Again, good question. Some states have standards in this regard. Because a higher CN results in less smoke and fewer emission, California has mandated CN 53. I have read on Wikipedia that Texas is CN 48 minimum for the same reasons. Other sources state the average across the US is CN 42-45... but again the minimum is CN 40 and that is the minimum for any fuel supplier outside of the areas with other requirements. It's reasonably safe to assume that's what you are getting.
There is also more to making power than the cetane number. The more highly refined ULSD has less specific heat (BTUs) in it than previous fuels. It isn't by a huge amount but it's there. The noticeable losses translates more into fuel economy than power but there is a slight loss in power, though it's probably not measurable in any easy-to-us way (like a chassis dyno or the butt dyno). Also, ULSD is lighter (lower viscosity) than the fuel our engines were designed for and that viscosity change slightly alters the way the injection system works. I've not seen any tests to show the exact differences but I'll bet Mel could tell us about how changes in the viscosity of the test fluid on the pump calibration machine alters pump performance. If I understand correctly, the effects are mostly in the area of timing advance. I'd have to dig around to find the viscosity changes between then and now but in theory, if you increased it, you could probably gain a little. My guess is that it would be more trouble than it's worth to do so.
Adding a cetane improver is an expensive proposition that adds a lot to cost of ownership without a lot of gain day to day. Figuring the worst fuel, the biggest gain you could expect is about 8 percent, all other conditions being optimal. If you have good fuel in your area, your gains could be nil so you are just blowing money at that point. Plus, consider how often are we operating at maximum power and how often do we need it? A practical method would be to use an additive in those times when you do need every lbs-ft. You're going to haul an 8K load over the Rockies... assume your fuel is only CN 40 and put in enough of a cetane improver to bring you to about CN 45. That 8 percent might keep you 5 mph faster at the crest of Eisenhower. Commuting empty to work with light throttle at 45 mph, what good does adding the improver do? None, really.
Assuming Chris142 used the Howes with a cetane improver, the reason he felt no difference is that CA is mandated to CN 53. He's already past optimal on the CN so further increasing it is useless.
