Long tube headers vs short??? Power loss??

I'll take opinions, charts , 1st hand info, or Doug what his name for the gospel. How much Hp do you, lose( if any) going from long to short tube headers. We are talking bout a 400 plus or minus Hp Small blk. They're arguing out in the shop at work, and I'm tired of hearing it and told em so. So they told me to get some answers. They didn't tell me where. SOOOOOO! here I am. Lets have em. Oh, and I'm not so dumb I look, cause I ain't sayin which side of the track I'm on. >>>>>>>>>>>>>> YET!
Thanks Guys.