why dose a holley need it's pv referenced on a roots blower setup

 

It doesn't, strictly speaking. The theory is that the lower intake will be into boost while there is still vacuum above the blower's lobes under heavy part throttle conditions. So you could be slightly into boost in the engine but the carb is still running at a lean cruising A/F ratio. Possible engine damage could result.

Referencing the PV to a source BELOW the blower's lobes means that any time you get very low vacuum down there, or go into boost the carb will go into power enrichment (PV will open) and you will have less chance of damaging the motor, and possibly better throttle response.

I have found that this is overkill and not strictly necessary in my experience. The relationship between the vacuum above the lobes and the vacuum/boost below them is pretty consistent. A typical relationship I have witnessed is that you will just start to cross into boost in the lower intake when you still have about 5" vacuum above the blower. Using a typical 6.5" power valve in the carb will ensure that you will be into power enrichment before the engine ever actually goes into boost. In other words, pretty safe. Most Holley carbs come stock with a 6.5" power valve.

If you're the nervous type then switch up to an 8.5" power valve and you will definitely be on the safe side. Don't go to far with it, however, since you don't want to be going into power enrichment at idle or at light throttle cruising.

If you're the nervous type then rig your carb up to be boost referenced and then you won't have to worry. i just haven't found it necessary to spend the time or effort to do this when I can get roughly the same resuts from playign with the PV opening point.

 

thanks