I tend to to agree with you now after I went through this. Dc pumps are not worth the money honestly.
Thay being said, my RD80 is pushing steady 1300gph on the same plumbing, so I am ol with the loc lines.
I would say no harm with loc line as long as you get a pump than can handle the pressure. AC pumps dp fantastic job there, and if you want DC oump pay attention to the real flow given they are all expensive any way.
I though I was saving 200$ by going vectra not RD I now realized I just wanted 500$ lol.
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Man, I just read through this whole thread and one thing is abundantly clear; You just don't get it.
Your comparisons of the RD3 and the Vectra are basically equal to you complaining that you bought a ford F-150 pickup with a gasoline engine but it has a hard time towing 5,000 lbs, so you went out and bought a Chevy pickup with a Durmax Diesel and Allison transmission and now you can tow 10,000lbs - so obviously the Chevy is a better truck.
It doesn't work that way.
A pump can be designed for pressure, for flow or for both. You could put 20 different pumps all rated for 3100gph in your system and get 20 different flow rates. That alone doesn't mean that any one of those pumps is any better than another.
When setting up your return, you have to do one of two things - pick a pump that matches the plumbing that you already have or will have
OR design your plumbing to match the characteristics of the pump you want to get.
You can't complain about a pump not performing as you expected it to when you put a high flow, low pressure pump in an undersized, highly restricted return setup.
You say that your plumbing is simple and indicative of the average? I beg to differ. Barb fittings are TERRIBLE. The inner diameter of a 1" barb is close to the inner diameter 3/4" hose. That restriction adds huge head losses; but it's not just the restriction - you also have to think about the turbulence that those hard corners add to the flow in the pipe.
You want the least restrictive flow possible? Get the BSPT to 1.5" Schedule 40 adapter for the Vectra, run spaflex up to a 1.5" bulkhead and use a 120 degree wye in the tank - it has a 1.5" spigot that will slip right into a 1.5" bulkhead and two 1" slips that you could then glue a 1" slip to 3/4" thread bushing and screw in your loclines.
Lastly, remember that you're not just paying for hardware. For many people the ability to sync up their Vectra, Vortech and Radion from a single app is worth the price of the pump over a $150 Syncra.
The Vectra didn't work for you? That's too bad. The RD3 works better? Great! But don't knock a pump that plenty of people are having no issues with because you put it in a situation that brings out it's worst attributes.
Remember; if you judge a fish by it's abilty to climb a tree.....