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.
Metering Blocks
Holley started using billet metering blocks a few years ago on the Ultra HP carburetors. The billet metering blocks are lightweight and are fully adjustable, with jets in all of the fuel circuits. The emulsion jets are replaceable, as are the power valve restrictions and the idle circuit jets. The replaceable power valve restrictions allow street guys to run leaner main jets for crisp freeway cruising, but still have plenty of fuel flow capability at wide open throttle.
The billet metering blocks are fully machined on all surfaces to provide better gasket sealing, and they have integrated pry points for easier disassembly. The metering blocks are black anodized for improved corrosion resistance, as well as good looks.
Fuel Bowls
The fuel bowls are all new for these aluminum Ultra HP carbs. Not only are the fuel bowls lighter in weight, but they are slightly larger than previous fuel bowls. The extra size provides an additional 20-percent fuel capacity, but the longer length of the fuel bowl means that you might need to modify your fuel lines. The fuel inlet threads have also been changed, to a -8AN o-ring style inlet rather than the long standing 7/8-20 inverted flare fitting.
Holley started to use the clear Pyrex sight windows on some of the HP carburetors recently, but these new aluminum float bowls step up the game by putting sight widows on both sides of the bowl. Another handy feature on these new float bowls is the drain plug located on each bowl. The float bowls also have some internal baffles and troughs to reduce fuel slosh, minimize aeration, and route the fuel to the main jets.
BasePlate
The baseplates are machined from billet aluminum and then anodized. The mounting holes have been elongated so these carburetors will bolt on an intake with a Dominator flange, as well as any intake with a 4150 pattern. Even though the carburetor will physically bolt onto a Dominator flange, the linkage will not always clear the larger mounting flange used on 4500 style intakes. We ran into this problem when trying to install our 950 carburetor on a M1 intake, but the fix was fairly easy.
The throttle linkage has been simplified for racing by removing some of the passenger car brackets, and the idle screws have been fitted with knurled knobs for easier adjustment. The secondary linkage is adjustable and can be converted to 1:1 operation by simply selecting the alternate attachment point.
Dyno Testing
We were fortunate enough to get our new 950 carburetor onto our 514-inch engine that has been on the dyno multiple times. It usually makes about 825 horsepower with an 1150 Dominator carb, and we didn't want to change intake manifolds for this test, so we built a flat plate adapter to center the 4150 throttle bores over the large plenum in the Super Victor intake. The best power came on the third pull when we recorded 800 horsepower at 6,700 rpm. Swapping back to the 1150 Dominator produced a pull of 820 horsepower at 6,700 rpm, so we considered that to be a good A-B-A comparison test. It is possible that our flat plate adapter was costing us a little power when compared to a tapered adapter, so that is another variable that could be explored down the road.
The extra adjustability of Ultra HP design allowed us to quickly make the changes necessary. The idle-air bypass feature was handy for setting the idle speed without disturbing the relationship between the throttle blade and the transition slot, while the bowl drain feature makes jet changes a little faster and cleaner. All in all, we had great success with this new Holley design and look forward to using it on future projects.
The Cash Outlay
PartPart NumberCost Holley 95080805BK$749 9.75 inch fuel inletAT104195ERL$90
