OK. You have convinced me and confirmed what I already suspected. 2" isn't going to work it. I've come up with a different solution that isn't as good due to physical constraints but I believe I can make it work. First, please note that I will use a fully enclosed hood with a 4" exhaust so no reduction from 6" to 4" and so no back pressure there.

Consider this: I could enlarge my external exit to accommodate a 4" exhaust and then Y join my 2" vacuum PVC exhaust into the 4" duct. It shouldn't impact my vacuum. So that would give me a 4" exhaust all the way through the system. Will that work for the setup that I'm considering? 3'6" x 3'6" x 7' with 400w HPS.

bigtopsfinn, I have not bought my light yet and I could be sold on the LEDs. I need to do more research. I did read a lot of the LED discussion under the lights thread. It is long, rambling at at times off topic so I'm sure I missed some good info that is tucked away there. A tutorial on this subject is sorely needed. If I go this direction then I'll work on one from a noob perspective. Without violating TOU here, can you point me to some resources? Can you give me some cost estimates to setup a *good* 300w LED system and what components go into a "good" LED setup? I need to know the start up costs. I recognize that there are long term costs savings but the immediate bottom line is important to me. That said, I come from the "do it right the first time" school of thought and would rather pay a bit more up front to have it done right. Any help, pointers, resources you can offer in this regard appreciated.

redtails, I have seen 4" inline at 178 cfm although I'd prefer to keep it a bit smaller for noise issues. Does the fact that I am not reducing under my new 4" straight through exhaust scenario change the equation? Can anyone comment further on whistling / fan noise issues?

I guess I need to go back and do more research on these fan issues. I'll report anything I find here.

Thanks for the input.