[Wisha Haddan H3]

Haven't used a spacer, but I'm very skeptical that they do any good on Vortec engines.

Vortec heads are already designed to swirl the air right before it hits the injectors. But if the spacer promotes any "swirling" at all, it's too far away to do any good. I'm more inclined to think the extra turbulence is counter-productive, since the engineers designed the heads without them. You do get more sound after you install a spacer, but I think it's the sucking sound of your money going down the drain.

imho, you're better off putting that money towards a cold-air intake and high performance exhaust.