Background

Holley introduced HP carburetors back in 2004, with new features such as removable air bleeds and a contoured main body. Those features were a significant improvement over the previuos style 4150 carburetors that existed, but over the years racers have asked for even more features. While the HP carbs do have some adjustment capability, the idle circuit and the power valve circuit are still controlled with pressed-in jets. With the advent of low cost, wide-band meters, the demand for fully adjustable metering has greatly increased. For that reason, Holley recently introduced a complete redesign of their classic 4150 series carburetors. These new carburetors, known as the aluminum Ultra HP series, are so loaded with new features that they make the previous 4150 models obsolete--not really, but you know what we mean. In the interest of keeping our readers in the know, we put one of the new Ultra HP carburetors under our microscope, and then bolted it on the dyno. Living up to the buzz, this new carburetor performed perfectly.

A True 950

There has been some confusion over the last several years about the 950HP carburetor that Holley sells. Many people assumed that since Holley calls it a 950HP, that it actually flows 950 cfm. More experienced people knew from looking at the specifications, that the 950HP used a 1.38-inch diameter venturi which is the same size venturi as the one used in the 750-cfm carburetors. As a point of reference, 850-cfm carburetors use a 1.56- inch venturi, and 650-cfm carburetors use a venturi with a diameter of 1.25 inches.

While I'm sure that the marketing guys at Holley had a good reason for introducing the 950HP and 1000HP carbs with those names, it did cause a certain amount of confusion in the marketplace. It's still common to hear someone mention that they are going to pull off their 850 carb and "step up to a 950." The ironic thing is that in many cases, the 950HP actually works better than the 850. Not because it is bigger, but because it's smaller. The smaller venturi size provides increased air velocity and a better booster signal for improved part-throttle performance. While the smaller 950HP works well on the street, for all out racing the 1.375-inch venturi size can limit the power output.

Weight reduction

One of the most significant changes is the use of aluminum for the major pieces of the carburetor body. Previous Holley carburetors have been constructed from a zinc alloy, which is substantially heavier than aluminum. By switching to an all-aluminum construction, Holley was able to reduce the weight by 4-1/2 pounds. This is a significant weight savings on a race car due to the fact that the carburetor location is over the front wheels. The metering blocks and the baseplate are also machined from billet aluminum rather than the die castings. The machined aluminum components are lighter than the old zinc castings, and they do not suffer from any porosity issues.

Main Body

The main body has the same smooth, contoured shape as the HP main bodies, but there are some subtle changes such as the location of the air bleeds and the shape of the reinforcing ribs on the sides. One very significant change to the main body is the incorporation of an idle-air bypass valve in the center of the carburetor. The idle bypass valve allows a small amount of air to flow into the intake manifold when the throttle blades are closed. This additional air is usually required to maintain the higher idle speed necessary with long duration camshafts. On previous carburetors, small holes were drilled in the throttle blades to allow in extra idle air. Drilling the throttle blades was a modification that wasn't easily changed if the carburetor needed to be used for a different application. The idle bypass valve is a much more user friendly feature, which allows the carburetor to be quickly re-adjusted for a different engine combination.