I experiment a lot with my truck. One thing that I'm at a loss to figure out is this...............When I'm cruising on the highway at 65mph without the AC on and the windows closed and the fan on only, my boost is running 4-5psi and EGT's are about 600*F. Get this though.........same conditions/speed.......I turn the AC on to recirulate mode...............boost drops to a steady 4psi and EGT's drop to about 550 or slightly less. Let me say this, both of these scenarios are running level ground with little or no wind. Does turning the AC on "advance" the timing that much more???? I've done the "tone ring mod" and run the Smarty on default settings. If I advance the timing on the Smarty, I get "timing rattle". I don't see how this scenario works out!!!!???? :confused: :confused:

It might? But that is seriously odd though... :confused: I figured with the additional load of the A/C compressor it would increase the EGT and boost not drop it...

I would lean more to change in air flow, as in dirty air at the back edge of the door. Quick way to check is to tape a tab of plastic to cover the gap at the trail edge of the door.keydl

Modern A/C doesn't have near the drag of older A/C's. I can tell you that. My old gasser (C30 454, built for towing) got 11mpg... loaded, towing 9000# or empty. The only thing that mattered was turning on the A/C cost 1 mpg. I don't know how they did it but my vehicles the A/C doesn't seem to affect mpg. I read tests where better mpg with windows up & A/C on than windows down & it off.I know this is not exactly what you asked... maybe something compensates in the programming. Odd fer sure.Russell

I see two changes - recirc - stops the air flow through the cab and out the rear door gap.and - A/C comp runs as called by the thermostatThe recirk stops the drain from the base of the windshield of the high pressure air. the bubble shape would change in front of the windshield and behind the cab.Try A/C and A/C recircI have an '84 GMC w/ 454, 400, 4.56 gears. 11mpg @ 55, 8mpg@65, drops about .2mpg with a trailer/tractor - 12,200#. I moved a 12x68 mobile home and it dropped to 6.6 for 47k gcw. Better than the old International but not as good as the Ford with a Cat.keydl