any info on this would be greatly appreciated. learning learning learning..never ends does it!
This reply is long, but hopefully covers important info on dust collection in general, and you're specific need.
TL;DR- miter saw dust collection is hard, and inconsistent at best with traditional shop vacs or DCs.
Confusion about dust collection is nothing new…and everyone is still learning. The biggest issue is this: large impeller-type dust collectors (like you may think of as a classic DC) are designed to move large volumes of air, but at low pressure. This is great for clearing out dust in a larger area, like say under a cabinet saw. They excel at pulling dust out of the air, but may fall short of grabbing heavier debris like chips, especially if not properly sized or set up. ShopVac style units are designed for high pressure, but low volume. Their suction is most significant right near the end of the hose, and they do great at picking up chips….but since they don't move much volume, they're not so great at removing dust from the air in a larger space. Both machines have their place, but they need to be set up, ducted and deployed the way the manufacturer designed them to be used.
Where most dust collection falls short is exactly there: the system(s) aren't being used the way the manufacturer designed them, so they don't produce what you're expecting. Large dust collectors need to be able to move that large volume of air without impedance in order to keep the dust and heavier debris airborne. This mean smooth, straight runs of properly sized ducting with minimal disruptions. That's the only way to take advantage of the high volume and defeat the low pressure. So, using corrugated pipe or hose, sharp angles, reducing (or increasing!) duct diameter beyond what's recommended, too long a run of duct, clogged filters, etc will all render those big DCs fairly useless. When you reduce the required duct size too much, the DC simply can't move enough air to generate any pressure at all…so very little gets picked up: ie - necking 4" ducting from a traditional DC down to 1" for a palm sander.
On the other hand, shopvacs, with their much higher pressure, do a great job…at picking up what's directly in front of them. But, because of their low volume, if the focused vacuum pressure (end of the hose) moves even a couple inches away, the pressure reduction is so massive that the system fails to perform. For these systems to effectively collect dust, some sort of confinement has to be built to help focus the airflow and direct the light dust towards the hose. This could be the internal construction of the shop tool itself (as is seen in some benchtop tools), or through sealing up leaky tool casings or cabinets and allowing fresh air to enter through a small opening at a point that puts the dust production between that opening and the end of the hose…which isn't always an easy feat!
The easiest way to negate these pitfalls is to use the right sized ducting on the collector, and use it as it's intended. As a basic rule of thumb, any tool with dust ports 2" or less needs a shop vac-style collector, and anything with dust ports 4" or greater needs a larger volume DC. The manufacturer (probably) designed the interior of the tool to be in alignment with the dust port size, and the proper type of collection device takes advantage of that design. There are a couple collectors that bridge that dust port sizing gap, the Oneida Supercell being one of them. The Supercell essentially combines three shop vac motors together to generate really, really high vacuum pressure, and also move a relatively large volume of air. It's a somewhat niche machine aimed at the small shop with a single woodworker, using machines with dust ports 4" or smaller, but it does what it does incredibly well. Truly awesome when used correctly.
So, all this info leads us to your quandary about collecting for a miter saw. The issue is: large chips AND dust being thrown over a wider, unconfined area. Soooo…high pressure doesn't work great because the collection area is too wide, and high volume doesn't work great because the chips are heavy and the existing dust ports on the saw necks the opening down too much for the DC to be effective. Funnels like the Big Gulp try to address these by bypassing the saws dust port, allowing a 4" connection to function behind the saw at the end of the funnel which helps collect and consolidate the chips, and hopefully the high volume grabs the dust. But, it takes up a lot of room and requires a large volume of air to be moved…which isn't going to work with a shop vac. Another option is to add on a small homemade funnel (a lot of people just use duct tape to add some sidewalls and widen the funnel to the saw's existing dust port. This allows the blade to fling the chips into the duct tape walls, consolidating them down into the existing dust port where a high pressure (shop vac) style collector can effectively move them, while hopefully also directing the dust closer to the end of the hose where the system might grab that too.
Both models are incomplete for their own reasons, but both work better than the stock adaptation. IMO, the best option is using a high pressure, moderate volume system like the Supercell. This provides super high pressure to pull in the chips, but enough volume to gather dust as well. Absent that option, most of your options are going to either require some sort of an adaptor, or a different collection method. Building off what you already have may save money, but may (or may not) fix your problem. Whatever you decide, try to keep in mind the strengths (and weaknesses) of your system and design your solution around that.
I'd encourage you to look at your shop dust collection and mitigation as a whole. Look at what size and type of collector your machines were designed for and make decisions from there. Often, a small shop requires multiple, different dust collectors to be efficient which in itself is pretty inefficient. Dust collection is an often misunderstood and overlooked part of a woodshop, but the health, safety and efficacy of a properly laid out and used dust collecting system cannot be overstated. All that said, the miter saw is a challenge for many shops!
Hope this helps at least a little.