Vacuum/boost reference hose that doesn't expand
 

The MAP says 9.5 PSI but my boost gauge says 8.5 PSI. I suspect the vacuum/boost reference hose is expanding. Does anyone make hose that doesn't expand or has minimal expansion?

Thanks!

If volume increases then pressure decreases, not the other way around. In reality though, the volume of a vacuum hose is so small compared to the volume of the intake that it is not going to change pressure.

MAP can be any number scale so long as it is consistent. It can be in camel farts per acre and you can still tune with it. You might be able to put a custom transfer function into the engine software to calibrate the MAP if it really bothers you.

Also remember that MAP is absolute pressure so it's roughly one atmosphere off from gauge pressure.

Ho Lee Shit

Right, squishy hose that expands with pressure will result in a lower pressure reading on a gauge at the end of the line.
As long as I see the same max every time it is ok. Actually it gets to 8.5 on the gauge, not sure what RPM, and stays there until I lift.

squishy hose won't give you a 1lb difference. Likely just a calibration/voltage/ground issue w transducers. The push lock nylon hose works well though and won't expand if you want to prove it to yourself. ;)

I like to use small dia 1/4” EFI fuel hose.
Its stiff, and much harder to melt if it gets too close to exhaust parts.

Last time I was worried about potential hose melt, I used leftover hard brake line for vac/boost hose. Just have to make sure not to kink it when you make a bend.