Recently while on the AMSOIL Great American Circle Track Tour, Bob Bolles and I stopped by Tour sponsor Holley's corporate headquarters in Bowling Green, Kentucky. While there, we got a special treat as we were given unrestricted access to its brand-new all aluminum oval track 4150 HP carb. We got to sit down with the engineers who designed it, Laura Shehan and Jay McFarland, for several hours. They let us take pictures, video, and answered all of our questions. The opportunity was significant because this is the first redesign of the HP carb since its introduction almost 20 years ago. It's so new in fact that you'll see in the pictures that some parts are still just prototype castings. As of press time, certain aluminum components were not even completed. To make things even better, Bolles and I were the very first media guys to see this carb and we've got the complete expose right here for you.

Laura, Jay, and the other engineers at Holley took a clean slate approach to redesigning the HP. They left no stone unturned and in Jay's words, "a lot of the changes are subtle but when you add them all together they are really significant. And many of these changes were ideas that we had developed over time from just working on these carburetors for so many years."

Significant is an understatement. One of the main goals in the redesign is to make the carburetor more efficient, from airflow to fuel consumption. Another is a increase in fuel capacity, the new HP has a 13 percent greater capacity over the old design. That extra capacity will require you to run different fuel lines, but Holley can help with that through its Earl's brand. Finally, and the most obvious one, is the fact that this HP is all aluminum, a design that has shed nearly 4 pounds off the original HP. That's a substantial weight savings and weight that, given its placement on top of the engine, is high above the center of gravity of the car. Let's get right to it and take a look at the new HP part by part.

Main Body
Starting with the main body, the first thing you'll notice is the newly designed aero boosters. This is one of the changes that affect the airflow in a positive way. Gone is the trademark bump on the arm of the booster. It is now one smooth transition from the side of the body to the end of the booster. Check out photos 1 and 2 for a side-by-side comparison.

In addition, the air bleeds have been moved farther out of the main airstream of the carburetor. Photo 3 on the following page shows the side-by-side comparison. Like the booster change, this allows the air to flow into the carburetor uninterrupted.

Also in the main body you'll find the idle bypass circuit-reference photo 4. "We still have some development to do with this system. It's not rocket science machining," said Shehan. "What we've done is provided a needle of sorts coming up through the bottom of that air cleaner stud, it's spring loaded so it holds tension and won't self adjust. You pull out your air cleaner stud and make your adjustment. It's all contained within your unit and you don't have to redrill anything." Shehan says the system works by opening up or closing off the airflow passages that you can see in photo 4.

Development of the idle bypass circuit is a microcosm of the entire redesign of the HP. "We had three different designs (of the idle bypass circuit), says Shehan. "The first one we came up with was one where you never took the air cleaner off, but there was good and bad with that design, such as how do you adjust the stud length for people who run different sized air cleaners? Then we went to the next generation design where the spring-loaded mechanism was on the top, but we had to consider that design's affect on the manufacturing line.