wha exactly is done to a ecu when it is programed??

 

advance the timing ?
remapping the air to fuel ratios ?

or what is physically done ?

 

I could be wrong, but I believe there is a problem with aftermarket ECUs working with 99s. You may want to look into it if you're thinking about getting one.

 

If I got a JWT ECU how would my fuel economy be affected if I was just cruising on the highway at say 70 mph? What about city driving if I didn't put my foot into it?

 

i wanted to the the performance that is done to them,there a place in bk that says they reprogram the 99s ecu so iam just tryin to figure out wha is goin to be done.

so its the air/fuel
timing.

any thing else that it does?
this way i know what i want them to repromgram it for.

 

I believe you only tell them if you are N/A, turbo, or SC and maybe what your mods are. As far as I know the 1999 ECUs remain uncracked. The best you can do is try to rig up a 96 ECU.

 

Does your mpg go down with upgraded ECU?