12"

If you want, you can chop it off 1/2" at a time, but if a 12" header will fit, that will give you the best midrange. Call Dave and talk to him if you want to verify, but the shorter you keep making the header, the more top end RPM you will get at the expense of your midrange. I think they have run them as short as 10.5" but then there becomes a little surging (about 100 rpm) in the midrange.

Hope this helps

John