Tech Feature: Holley Ultra HP Carburetors

Holley’s new racer-inspired Ultra HP line of carburetors leverages the latest trends and technologies learned on the track – from reducing weight to redesigning the bowl for increased capacity and fuel control. In fact, Holley added 30 new features not found on the previous generation HP models to help engine builders take advantage of improved cylinder heads and more aggressive camshaft profiles.

“Engines continue to get bigger and make more power and race fuels continue to evolve, leading us to more testing and calibration of our carburetors with the current race fuels. With all of these changes, there was a need to provide the additional air and fuel curves to match,” explains Holley’s Bill Tichenor. 

Custom Carb Features

It has 30 new features over the traditional HP and racers are more savvy than ever on how to use them. – Bill Tichenor

The Ultra HP is not simply an old 4150 HP carburetor with a couple of new features and a new name to market it with. This is a radical, new design of what has been the go-to carburetor for many racers and enthusiasts for years. According to Tichenor, “It has 30 new features over the traditional HP and racers are more savvy than ever on how to use them.”

To start, 97% of the parts in this carburetor are aluminum. It is 38% lighter than it’s predecessor (about 4 1/2 pounds), and while that does not equate to a large amount of weight, every bit is important in drag racing.

Metering Blocks And Fuel Bowls

The Ultra HP utilizes billet metering blocks for improved durability and sealing, These blocks also incorporate an integrated pry port for easier disassembly. The fuel bowls have 20% more capacity than those on the standard HP carburetor, greatly reducing the risk of fuel starvation and stabilizing air fuel ratios.

There’s also internal baffling in the bowls to reduce fuel slosh under hard g-loads — whether they come from starting line launches at the drag strip or hard cornering in a road course car. There is a fuel shelf below the needle and seat to minimize aeration, and a fuel trough has been built into the bottom of the bowl to route fuel directly to the jets to keep them covered — again allowing for more stable air fuel ratios. 

The no-nonsense throttle lever eliminates provisions not needed on hardcore street or race cars, making swaps faster and allowing for tighter space constraints. The linkage mounting point is slightly different than what you would find on a traditional 4150, which required us to make a small adapter bracket.

Base Plate And Throttle Linkage

Outside the carburetor there are several features that racers and hardcore enthusiasts alike will appreciate. The base plate is made from 6061 aluminum to allow for better gasket sealing and a true surface. It also features elongated dual pattern mounting holes, allowing the Ultra HP to be mounted on either a standard square flange intake, or a Dominator flanged intake, further expanding it’s versatility. The throttle lever has been designed as race-only, with all of it’s unnecessary attachment points and tangs removed, further simplifying installation while removing unnecessary weight and clutter. 

Other Features

Fuel inlets can be plumbed from either side and come standard with a -8AN o-ringed inlet size. A -6AN male adapter is also included to further expand fuel line options. The primary and secondary curb idle screws have been knurled to allow for tool-free adjustment by hand. The secondary linkage is stainless steel and can be adjusted for progressive or 1:1 activation.

The integrated idle bypass valve helps maintain idle quality even with very radical camshafts and is perhaps one of the most user friendly innovations on this carburetor. The airbleeds have been moved outwards to allow for smoother transition of airflow. Even the squirter screws have contoured hex heads to help improve airflow as it enters the carburetor. 

Color choices available include black, polished, and a new Hard Core gray. The finish on these carburetors is hard anodized which should allow for years of easy cleaning and corrosion resistance. 

 Swapping and Testing

Top: We're using Holley's Ultra HP 850 in Hardcore Gray for this test. Left: Our BluePrint 427 LS engine baseline dyno with carburetor that came with the crate engine. Right: The Ultra HP 850 out of the box with no tuning versus the 850 it replaced.

For our purposes we tested two new Ultra HP carburetors on our project BluePrint Chevelle. For this comparison we strapped the car to our in-house Dynojet chassis dyno and left it in place for the duration — this will help ensure consistency throughout our testing as we’re not doing this test over the course of several days, but rather, during the course of one afternoon in the shop. With the included out-of-the-box carburetor from BluePrint engines, our Chevelle’s 427 cubic inch LS engine made a best baseline pull of 417 hp and and just shy of 404 ft-lbs of torque at 5,400 rpm on the chassis dyno. 

Easy Idle Adjustment

A feature we found quite impressive on the Ultra HP carburetors is the new Idle Bypass Valve. An idle bypass has been used on fuel injected cars for years to keep airflow when the throttle is closed. With carburetors this has been done previously by two methods the first being holes in the throttle plates and the second by adjusting the curb idle screws.

The idle bypass is already setup from Holley and can be easily adjusted with a screwdriver to compensate for radical cams. Holley recommends trying this adjustment before touching the curb idle screws. We found that a small adjustment on the idle bypass was in the end the only change we had to make to our Ultra HP 850 to get the best performance straight out of the box. 

We unboxed our new Ultra HP 850 carburetor and performed Holley’s recommended setup procedure, along with a couple of modifications for our setup. The conventional fuel line threads are different and a new line using -AN lines, with one coming from the carburetor  down to a T, and a short -AN line coming from the front down to the T, that then runs to the fuel pressure regulator was made. Also fabricating a small sheet metal bracket to riveted to the throttle linkage to connect the transmission kickdown cable was required.

The carburetor was ran with it’s out-of-the-box tune to get a baseline air/fuel ratio number. Tichenor told us, “On the base level, they [racers] can expect a much lighter carburetor and racers always appreciate weight savings, but they can also expect a race-ready carburetor out of the box.” There is a weight difference between the old carburetor and the new aluminum Ultra HP.

The new idle bypass valve made setting up the idle on the carburetor very easy, and far less complicated than a traditional HP. Other than setting this up, checking float level and a few other basics from Carburetor 101 outlined by Holley in the instruction sheet, the Ultra HP 850 was ready for action right out of the box. The secondaries on the test carburetor has been set for 1:1 activation instead of progressive.

What we found on the dyno was an interesting story when comparing the 850 cfm carburetor that was included with our engine to the out of-the-box UHP 850. While the power numbers untuned were initially identical at 5,400 rpm, the Ultra HP allowed the engine to keep going, well past 5,400 rpm — in fact past 6,000 rpm and all the way to 6,500. The original 850 cfm carburetor we’d used at that point had given all it had at just over 5,400 rpm and could go no further than 5,700 before it dropped off in dramatic fashion. Our comparison of the two ends at that RPM.

The initial numbers when compared at only 5,400 rpm may be misleading, as the potential for more power comes with now increased available engine RPM. The new Ultra HP 850 allows us to run our BluePrint 427 to significantly higher RPM. This allowed the engine to continue making more horsepower. That additional operating range widens the engine’s power band by almost 20%, and allows the engine to make more horsepower. In fact we gained 37 more hp on average in that span of 1,100 rpm, the engine was making power right up to the peak of 6,500 rpm.

So whereas some modifications might make more power at the same RPM or increase RPM but make the same total horsepower, we’ve actually seen significant gains in both horsepower and RPM. This gives the engine more useable RPM on the street or track. Also, take a look at the air-fuel data at the bottom of the dyno sheets. The AFR’s are nearly identical at 5,400 rpm, again showing that the out-of-the-box UHP 850 was already a good carburetor with no fine tuning performed. 


On the base level, they [racers] can expect a much lighter carburetor and racers always appreciate weight savings, but they can also expect a race-ready carburetor out of the box. – Bill Tichenor

In an effort to make more power, we began to tune on our carburetor and engine setup. Our primary changes were made with jetting. In addition to the jetting adjustments timing was experimented with as well – adding and subtracting as much as two degrees of timing, though in the end we found it best to leave it where it already was at the onset. 

Changing jets on the Ultra HP carburetors is fairly easy, and made even easier by the features Holley has built into this carburetor. The drain plug on the side of the bowl is easily removed and the pry point on the metering blocks allows everything to be disassembled quickly. In our early trial and error testing, we leaned the carburetor out considerably by backing off four jet sizes, allowing us to create even more power.

We were able to pick up 25 hp and 14.3 ft-lbs by simply leaning out the carburetor from a 11.64:1 air/fuel ratio to 13.16.

For tuning purposes on the Ultra HP 850, we leaned the carburetor out by four jet sizes to achieve a 13:1 air fuel ratio, where it made the most power.

What we ultimately found was that Holley had this carburetor made its best power right at a 13:1 air/fuel ratio. This gave us that aforementioned 1,100 rpm increase in the engine’s available power band as well as the 60 hp peak increase we saw as well.

Is Bigger Better?

With our success in gaining more power and RPM with the Ultra HP 85o we were now tempted by the bigger is better demon. Our thinking was simple: if we can gain one thousand operating RPM and make this kind of power with the Ultra HP 850, then the Ultra HP 950 might really make us grin. 

The Ultra HP 950 is what Holley deems a “True 950” carburetor, with massive flow potential. In the past, although some HP carburetors may have been labeled as 950, their actual flow rate was probably somewhere in the 800’s. In an effort to stand behind their offerings, Holley pulled out all the stops on the new Ultra HP carburetor line and offers the new Ultra HP 950 as a true 950 CFM carburetor. While we couldn’t put it on the flow bench, we can tell you for sure that it way out-flowed the Ultra HP 850, as it proved too much for our LS engine when we attempted to step things up even further by bolting it on. 

If 850 (pictured right) was good for 60HP and nearly 1000 RPM we wondered what would the UHP 950 (pictured left) could give us.

With images swirling in our heads of a screaming 7,000-plus RPM LS engine, making beautiful music and astounding horsepower figures, could we breach well into the 500+ zone naturally aspirated, at the rear tires? It was time to find out.

With the Chevelle still strapped to the dyno we swapped on the UHP 950 and gave it a go. We found that even after swapping jets as we had done with the 850 to go up one size and down one size, that the 950 was simply too much for our 427 LS engine. In spite of our best efforts to tune the carburetor, we had neither the compression, nor cylinder head airflow to support such a beast. Proving again that bigger is not always the better route, we went back to the Ultra HP 850, which is where the engine was making the most power. 

In truth, the Ultra HP 950 is best suited for a much higher output engine. Holley recommends this carburetor for engines making above 750 horsepower. While we’d like to have seen some improved numbers with this carburetor, we’ll have to save it for a higher output setup. 


Above are the power differences between the carburetor that comes with our BluePrint engine versus the tuned 850 Ultra HP. In the end a peak gain of just over 60 horsepower and 18 ft-lbs of torque. This yielded a peak horsepower of 477.8 hp, and peak torque of 421.3 ft-lbs at the rear tires. We also gained roughly 1,100 rpm of useable engine speed.

The Verdict

In the end a peak gain of just over 60 horsepower and 18 ft-lbs of torque. This yielded a peak horsepower of 477.8 hp, and peak torque of 421.3 ft-lbs at the rear tires. We also gained roughly 1,100 rpm of useable engine speed. These gains represent an increase in horsepower of nearly 14% and a gain in RPM power band of nearly 20%, both are substantial. Both gains could translate into quicker times on the track or even open up gearing and tire options for the street or the strip. Also, our air fuel ratios were right on target for our naturally aspirated setup. With the original 850 carburetor we were actually running a little bit too rich at 11.74, but the Ultra HP 850 we had leaned out to 13.16. This is a difference of less than 1.5 on the A/F ratio still well within the safe zone on this engine and obviously worth the extra power. 


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About the author

Don Creason

Don Creason is an automotive journalist with passions that lie from everything classic, all the way to modern muscle. Experienced tech writer, and all around car aficionado, Don's love for both cars and writing makes him the perfect addition to the Power Automedia team of experts.
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