I purchased a 3/4" x 5" solid walnut floor which I'll be (DIY) installing over a 4' (dry) crawl space to 3/4" plywood. The manufacturer (Perverco) indicates a 15lb felt underlay and 2" nails. Some "installers" have indicated I should, omit the felt and glue (PL 400) the floor down to minimize any cupping which I don't mind doing but can anyone help me out with the pros an cons of gluing the floor without felt. I live in Vancouver, BC so humidity isn't much of a concern.

My previous house had 3/4" x 5" solid maple that was professional installed over a 4' crawl space without felt and only nailed which I didn't have any problems with.

15 lb felt is widely used in some areas of the US and also in Quebec, which is where Preverco originates. Other geographic regions use other types of underlay. In Ontario we generally just use a wax coated paper, which does nothing more than allow the boards to slide up easily during installation.
Having said all that, if you have a ceiling underneath the space you are installing in, you COULD omit the underlay and use glue. I am not a fan of this but many installers have done it without detrimental results.
We recently have switched our process to include "Vapour -Lock" under all main floor installations, and continue to use the wax underlay in upper levels.

The use of adhesive is NOT to prevent cupping, but to assist the floor in resisting normal, seasonal movement due to changes in the interior environment. And to aid the floor in preventing squeaking. The areas below the floor MUST be dry and remain that way. In any case, 15 # asphalt felt is not a moisture vapor barrier anyway. Typically, a 6 mil polyethylene plastic vapor barrier is recommended to be installed on the earth in a crawl space installation. Many flooring distributors are beginning to recommend the use of adhesive, along with the proper fasteners, for the installation of wider plank flooring. A 5" wide plank is not considered WIDE in the most traditional sense. And should be able to be successfully nailed down without adhesive, especially if it's North American Black Walnut, which is pretty stable. The install will be even more successful if you properly acclimate the flooring and keep the interior of your home within the recommended temp and humidity levels. That being said, you would most likely gain a little more solid of an install by using a urethane adhesive. But it would be much more time consuming and expensive.

Thanks for the info. I'm still undecided on the installation type I'm going to use for a couple of reasons. I'm transitioning to a tile that is a little more than 3/4" and the 15 # felt will make it close to even. This is the manufactures recommended install. On the other hand I don't want squeaks and would like to minimize seasonal movement. I keep flip-flopping between the 2 installation type and have the glue and adhesive on hand waiting for my final decision.

The manufacturer is recommending 3/4" for expansion which seems like too much any comments? http://en.preverco.com/MEDIA/PDF/INSTAL ... NAL.EN.pdf

Few of us use a 3/4" expansion gap on residential installs. I find 1/2" adequate. And most baseboards will cover a 1/2" gap. You can always undercut the sheetrock to allow for more expansion. But I rarely do, because IF a floor ever moved enough to fill that expansion space, it will have suffered a major flooding and would most likely need replacing anyway.