WHAT is it that dtermian how fast or how qiuck your nitro motor gets on to the pipe?

Many things. Pipe design, Fuel, glow plug, prop type and setup all can have an affect on how quick you get on to pipe. You need to play with one thing at a time or you will confuse yourself. Start at the pipe length (if you are running a tune pipe). If it is a muffled (canister) motor that would not be one of the things you look at. Next would be prop type and prop height in the water. Higher prop would get revs quicker so the motor gets to tuned revs quiker. smaller prop would give the same affect. It takes time to play with your setup to get it right. Their is no short cut tricks.

Thanx bud,I will play around

There are short-cut tricks, One is what engine do you have? Many people "here" generally know timing information on the engine you might have. If you could state what model, make or displacement of engine then maybe someone could help you. What pipe you are using would also be a big help.

I see many pipes and header configurations on here without the important consideration of actually being the right tuned length or not.
A guide for for "tuned pipe length" is contained in the engine your using. Basic engine dimensions, measurements and port timing numbers will determine an area of tuned pipe length. Once you get the supercharging effect from the pipe the key is to optimize the the prop and it's orientation between the boat and water. The idea is to run the pipe as short as possible to attain the highest rpm for the setup you have,, thats really what we all want, isn't it?
( I saw on IW your motor is a K&B 7.5,, O/B?,, still don't know which pipe and header. Picture is worth a lot.)
JW