Hey Guys... i just ordered a
RX-3900-1 because i was a little worried about space, but noow that i look at it i can go to any size. I have it on order but i think i can call them tomorrow (if they are open) to change my order.

Will it change my airflow enough to worry about?

I was kinda rushed while ordering and i just now realized that its reverse tapered, but is that better becase it can let more air in at the front? I kinda feel like an idiot...

I am making my own custom pipe wiht EMT so that is not an issue, ill have pictures up as soon as i can get cardomain to work for me

I know willwren tested this one and i think allmachtige did to?

You can get all the info here:
http://www.knfilters.com/universal/X-stream.htm

Thats the one Willwren and allmachtige have, kinda oval from the pics.

I've got it, it'* a cone, that'* stretched on the "triangle" part.

Hrmm, but we dont know if having the backwards cone will affect anything? (aka the one i ordered is BIGGER in the front and SMALLER at the back where it attaches to the pipe...)

Actual surface area is much larger on the 4750. Other than that, I don't know what to tell you. If it'* in a CAI or heatshield, I don't suppose the reverse taper will do any harm. One thing that concerns me is that the 3900 only has a 7/8" long flange for mounting. The 4750 is 1 7/8".

They'll all outflow your engine, but that 7/8" mounting flange is tiny... I picked the 4750 because of the mounting ID, and then it'* size.

The reverse taper will make shielding much easier though :?

The 475 0 also has the advantage of the large base for mounting a CAI or heatshield from the base. Not sure if the 3900 has an IAT hole, but the 4750 does.

I think all-in-all, the 4750 has the edge for all the features. That little 7/8" flange may be an issue, especially if you attach your CAI or shielding to the filter.