It never hit me until I got older how clever Tyrell is - he knows he's close to perfection with Rachel and the V-K test is his chance.

"I want to see it work. I want to see a negative before I provide it with a positive."

Afterwards when he's debriefing with Deckard on how hard he had to work to figure out that Rachel's a replicant, he's working really hard to contain his excitement.