Throughout this redesign that was another whole set of hurdles that we had to consider. And the way we manufacture these units actually gave us the idea of going through the bottom. If the carburetor needs any adjusting during the flow stand testing (one of the final quality control steps) the techs could adjust it right on the flow stand with this idle bypass circuit.

The idle bypass circuit is a really cool addition that you won't find on an old HP. "This was really designed for that guy who is running a big camshaft with a lot of overlap. He has no idle at vacuum so the first thing he does is crank his idle up," says Jay McFarland. "You know the drill, it gets out of the transfer slots, gets to the top and it starts pulling fuel into the main circuits and ends up getting a rich idle. He can't adjust his idle and then his carburetor is a piece of junk and he throws it out, even though it's not his carburetor's fault. The idle bypass circuit allows him to fine tune it so he can get his primary throttle plates back down into the proper spot on the slot so that he has idle control. It's more of a fine tuning tool."

Metering Block
Both McFarland and Shehan are excited about the fact that Holley will be machining the billet baseplate and metering blocks in house. "One of the wonderful things about machining in house is that it allows us to make changes easier," says Shehan.

In the past, Holley technicians had to drill and press in plugs since this carb has an external accelerator pump. In the new HP however, Holley is machining in the gasket locating pins as opposed to machining the holes and pressing the pins in place.

"This alleviates the condition where the pins are put in crooked, fall out or are omitted all together as in some billet designs," explains McFarland. "We have also changed the machining of the accelerator pump channel to have the metering block plugged internally rather than the outside to eliminate a potential leak path that exists with external plugging.

"Further, we redesigned the vent baffle such that it is press in place and round, instead of the operation where these are drilled and you put the little plastic baffle in place and then on the line you drill through that plastic and install the pin. It's already there. There's no debris. It's less costly to manufacture the oval slot versus all of the drillings. That's another benefit to doing things in house."

A few other significant changes in the metering block is that they've lowered the power channel restrictors to keep them lower in the fuel and they've hollowed the block out to gain fuel capacity but not so much that it cost twice as much to manufacture. Engineers are also working on designing a recess that racers can easily use to pry the block from the main body when working on the carb. Another benefit is that since the metering blocks are billet Holley will be able to manufacture them in different colors. Want a pink block? Go for it!

Inside the fuel bowls, Holley designed internal baffling all the way around, something the previous fuel bowls did not have (see picture 5 for a comparison) along with a larger diameter needle and seat boss.

Through testing and research, the team discovered that fuel entering from the needle and seat goes down and hits the float tab. That contact sprays the fuel onto the flat bottom, causing a lot of aeration and foaming. With the additional baffling and the center shelf design however, when the fuel enters it hits the shelf and has a tendency to dribble down the side, almost like a slow waterfall. The net result is less aeration and less foaming of the fuel. Some foaming does still occur but it does so up on the shelf and not down in the main fuel level.