There IS ALLWAYS 1.
I am not claiming to be right, nor accepting the statement the ip can comp 7 deg's of timing.
WE also had a member swear he could and would build 400 hp dd idi that got great mpg's.
About 10 years back...
Still crickets.
Actually, I believe that being possible. I've seen Justin build a 450HP IDI(and rode in it, no less!), and the thing about a turbo IDI is that it's still the same motor underneath it all(unless you are messing with compression). Of course, you need better connecting rods to handle that 450HP reliably...
So, there's no reason to think that DDing such an engine *wouldn't* produce the same mileage as a mildly turbo'd IDI, because it's doing the same work, and thus requiring the same fuel - the turbo, when not spooled up, isn't adding more air either.
No dought it is possible to ***** the timing and have it run good.
I have seen it, but the timing was way off, it ran like a ***** ape, but,
Why not do it right the first time.
It has more to do with getting the /right/ fuel timing out of a worn out pump. Yes, you can just throw it away and put a brand new one in there, but as the timing bits wear before the rest of the IP, it's entirely possible to 'work around' the retarded timing and keep using the old pump for another 20K or something.
Enough time to save up for a new, high quality IP.
Also, remember that timing isn't just a number. On a DB2 pump, it's a 4D graph, with throttle position, RPM, load, and advance being the four axes.
Factors that affect your timing /curve/ include:
1. Inlet IP pressure.
2. Transfer pressure inside the pump
3. Throttle position, as it pushes against the light load cam(and adjusting the light load cam position will affect this)
4. Spring pressure in the light load advance
5. Spring pressure inside the advance piston
6. Advance piston bore wear
7. Fuel cetane number
8. Injector pop pressure.
With all of these factors affecting your timing, and not in a linear fashion either... it's entirely possible to end up with a setup which is way off at the testing spot(2,000 RPM, unloaded), but perfectly timed under load, resulting in a good-running vehicle.
Or, conversely, it's easy to have it timed 'right', but still off once you put a load on it due to how all these factors affect it.