If you want inches of mercury (vacuum), use this formula:
in Hg(vac) = 29.92 - (kPa reading) x 0.2953
So your 91 kPa would be =
29.92 - 91 x 0.2953 = 29.92 - 26.87 = 3.05"Hg vacuum
That's only 3" of vacuum.
This is assuming you are at sea level, where the atmospheric pressure is 29.92"Hg. If you are at a higher elevation you need to change the 29.92 to whatever the local atm pressure is.
In El Paso for example, you might use 26 instead of 29.92, and you would actually have a slight positive manifold pressure with 91 kPa rather than a vacuum.
Just one of the things to remember when talking gauge pressure instead of absolute.
John Estill