Reading the Painless Wiring, Wire harness Installation Instructions, on line at www.SummitRacing.com on page 16 in the instruction book, page 20 on the Adobe viewer. para B. at the bottom the warning, Important: The MAP sensor needs to be mounted with the hose nipple pointed down.
Anyone know why?

 

My guess would be that the the way the inside is constructed, so its easier for the vacumm to pull on the plunger. Not really sure though. I do know that you should use the, shortest possible pastic tubing, (not rubber). This wont allow it to collapse.

 

Although I understand the theory behind having a really short, and dedicated hard tube vaccum source for the MAP sensor, I have experimented with this somewhat. I have tried varying lengths, and T'ing off other vaccum lines with the MAP line. I never really noticed a difference at idle, or while driving. Vaccuum reading at idle was still in the low 50 kpa, and responded just as quickly when you give it some throttle (used a laptop to verify this).

Not trying to say it doesn't make a difference, just that I haven't noticed one.