[v8volvo]

The water or water/alcohol injection question is an interesting one.

The goal of doing that (as you undoubtedly know) is to help cool the intake air mass, enabling you to get more oxygen into the engine via a denser intake air charge. However, that is mainly beneficial in the scenario of a turbocharged engine. As I understand it the main reason for that is: the turbo and the pressurization of the intake air create a great deal of heat, so that creates a lot of room for potential benefit from charge cooling techniques such as intercooling and/or water-alcohol injection.

In the scenario of a naturally aspirated engine however, the reasons for doing it would be less clear. Since the intake charge is less hot and more dense already to begin with on a NA engine sucking cool atmospheric air in, there is 1) less heat to vaporize the water or methanol to create a cooling effect from phase change and 2) less benefit to be gained from reducing the temperature of air that's not very hot to begin with.

It sounds like you have already been making great progress with the car but ultimately you may run into a limit as to what can be accomplished without increasing the airflow to the engine. There is a reason turbos are commonly seen on diesels.... they are the best method to achieve that. Porting and other techniques and injection pump tuning maybe (?) can do some good but only up to a point.

If you're at 16sec now with heavy smoke and want to get to 15sec with light smoke, you would need to add a lot more air to the engine. The smoke means your limiting factor already is oxygen, not fuel. A turbo would be the obvious choice but maybe a supercharger would be of interest also?