I have one simple question. I saw many dump valves out there and I am looking to buy one. Many of them are adjustable. What do you actually adjust on the adjustable ones ?
Spend your money on a good dump valve, adjustable ones aren't worth bothering with.
All your adjusting is the preload on the spring so once its set at the right preload you wont touch it again. A Forge or Bailey valve will last for years and give good results straight from the box.
Cars with MAF sensor should not use atmospheric valves IMO, as you are letting measured air out of the system. Perhaps not enough to do any major harm, but I'll stick with my recirc valve. If you want an atmospheric valve, atleast get a twin piston one. On MAP equiped cars it doesn't really matter.
If you want a adjustable one then go for the HKS, I have one and my mate. Not intstalled yet but my mate has. I can tell you there is no mutch difference when you adjust it. It is not cheap this one. -------------------------