Another great improvement can be found on the screw for the squirter. That screw has always been either a slotted screw or a Phillips head. Granted, this is a bigger improvement on a street carburetor where you have to fish down deep into the choke plate, but since they've gone to a rounded allen head instead of a big flat head you've again achieved better airflow than the previous design. While McFarland insists that particular improvement is nothing major, perhaps a 0.5 percent gain, it shows the depth that Holley engineers have thought through the overall design of the new HP carb.

Development Work
The HP design has been around since 1993 while the original 4150 was brought out in 1958. This is the first time any design changes have been undertaken on the HP in almost 20 years. A lot of these changes are subtle but when taken together, it's a carb for the 21st century.

The team took a very methodical approach to designing the new carb. At every step it balanced the new designs it was considering with how they would impact the manufacturing process together. In addition, the team staggered the parts coming in so it could do the development work on the main bodies before it started dealing with fuel bowls and metering blocks.

The testing protocols were equally as methodical. The team documented the flow curves on the current 4150. It replaced just the main body with the new design, leaving everything else (metering blocks, boosters, and so on) exactly the same. What did the team find out? The results were very consistent on the main bodies with the new main body 3 percent richer than the current main body.

When it started working on the boosters, the team again got very consistent results, only this time the flow curves laid over top one another perfectly. At that point, it knew it could put this booster in the main body and not have any unknowns and continue development work. The engineering team took that same approach with the throttle bodies and the metering blocks. While the throttle bodies had similar flow curves from new to old, the new metering blocks ran richer as well. "I attributed that to better fuel flowing passages versus the old design, the changes that we made allowed the carb to pump more fuel," explains Shehan.

What's Next?
Both McFarland and Shehan are itching to get this carb out into the real world and test it. After all, it has been a long road to this point. "Right before Christmas we released the models to the casting foundry," says Shehan. "Something like this HP is not an overnight development. This is an 18-month, almost 2-year project to get where we are at right now."

They estimate that the carbs will be on the track atop test cars within the next several months and they've invited us back toward the end of the year to go play with one in a real world test. You can bet we'll be there! Stay tuned!

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