Tony-B5

Hi guy & gals wondering if anyone out there can help me after much pondering i decide to see what all the fuss was about and fitted the N75J Valve to my 1.8 T.Q i first fitted it as it was in the engine bay road tested & was amazed at the result. But on second glance at the old valve it seemed as if i had fitted it incorrectly & guess what ? i had on this engine you have to fit it upside down on doing so POWER was back to standard why ?
p.s. is there any harm to the engine if i fit it back incorrectly & drive it like that or do you have to fit it correctly any help would be much appreciated.
Thanks.