Yes, I know where 70mpg will take me: 1700 miles club.
I usually do 1 mod a year so I can see if it makes a difference or not.
If you mod your car all the way in a short period of time and something goes wrong, what is it?
Hard to diagnose.
It's my mod philosophy.
What do you mean by non SI diesels?
To answer your question about the chip, it simply controls the engine.
It can advance the timing a little bit or retard it depending on the software.
It can give more fuel at different predetermined moments or not.
It can give more boost (up to a certain level) or less, or cut it to 4-5 pounds in order to protect the engine (called limp mode).
Some chips are smoother than others.
Upsolute for example will give you a big "kick in the back" right at 1900 rpm (peak torque).
It's nice but undesirable.
This "kick" is hard on the engine, tranny, but mostly the clutch.
Older TDI's like mine have stronger clutches, but the new TDI's have clutches with max torque ratings of only 200 lb-ft.
Only a chip will bring the torque at around 210++lb-ft of torque.
So it means a VR6 or custom (my preference) clutch.
Mine should be good up to 250 lb-ft.
There is now new chip tuner that have "finer", more advance chip softwares.
It makes the cars drive like a stock vehicule with a stronger motor.
Everything is as smooth as stock.
1 chip tuner makes complete ECU convertion too.
He gets the most torque out of these engines: 1 member dyno'ed his TDI at 129 hp and 298 lb-ft (he also had the same injectors as I do).
I was there when he dyno'ed it.
2 mods and almost 300 lb-ft of torque! :o