Well, since I have a 71mm arc, a 135mm lens, a 330mm and 550mm fresnels, I can say this for sure. The Arc length makes a difference. First, the white ends light up enough to add to the amount of light that gets into the lens, that is if infact they fit. In my case, the ends are outside the lens area, thus I am losing brightness. Also, going with the 71mm arc, I am barely inside the edges of the 135mm. So having a the right arc is very important. Just thought I would give a fyi, for people thinking about the 1000w bulbs.

Sorry, didn't clarify what I have. I have this 875w metal halide. http://www.venturelighting.com/LampsDataSh...alide/33759.pdf
I am going to get this one http://www.hydrofarm.com/content/PDF/hortilux.pdf because of the arc length. I posted this, because some people think that the 1000w with a 80mm arc will make their projector work a lot better. I just believed you needed to know some facts before going that direction. The 80mm or larger arc will miss light getting into your lens. Even my 71mm arc does. So that is why I am going the 55mm direction.. Another thing the larger arc is doing, is that the whites are kind of blurred, I guess because the light is hitting the fresnels at a different angle than the middle of the arc. The smaller the arc the better, which most of you already know. But here are some reasons they are better. peace.

Which one of those 1000w bulbs has a 55mm arc? Where did you find specs?
Thanks

Sorry for the off-topic but I cant resist. Does the title of this thread sound like an Indy-Rock band name to anyone else?

Yup! And the sub-title sounds like the song they sing!

The hortilux bulb does. It is listed in my post. look down to arc length on the specs.