Without a doubt, in recent years the evolution of performance carburetors has been revved up to the redline-with the stick in high gear. Modern high-performance carburetors have been on the market since the Holley modular-style carburetor made its debut in the late 1950s. As the lineup of carbs grew from this basic design, performance enthusiasts had just a few basic styles and sizes of carburetors to choose from. Those basic 4150 and 4160 carbs with mechanical or vacuum-operated secondaries served well for decades, covering the needs of street enthusiasts, racers, and even OEM manufacturers. While most of those original carb configurations from the '60s are still being manufactured today, there has been an explosion in choices available in the performance carburetor market.
What better way to test Holley's...
What better way to test Holley's new Street HP Carbs than running them on the chassis dyno on our Street Sweeper Chevelle's 496? These carbs are designed to be big on performance, while being easy on the budget.
Why all the carbs? As the needs of enthusiasts and racers evolved to demand ever more sophisticated carburetors, custom Holley carburetors created a sub-industry, and this sub-industry in modified carbs played a role in feeding that demand. It wasn't long ago that a carb specialty shop or end user would take a basic double-pumper carb and doll it up with all manner of modifications, with some of these mods proving more successful than others. The market reacted by filling these needs at the manufacturing level to the point that Holley now offers carburetors off-the-shelf with features and materials that make some of the old custom tricks obsolete.
Engine builders looking to maximize airflow no longer need to mill and streamline the airhorn for more flow, when the HP-Series is already cast with maximum flow in mind. The old practice of drilling the precision tuning circuits for calibration changes has been sidelined by replaceable air bleeds and calibration orifices. Carbs can be had with externally adjustable secondary linkages, four-corner idling, billet baseplates, lightweight aluminum bodies, float bowl sight glasses, slabbed-throttle shafts, annular boosters, and the list goes on. Selecting a carb is now far more involved than just picking a double pumper that's sized suitably for the combination. These days, the enthusiast must balance the level of features with the price point.
The Holley Street HP
We had two of the new Holley...
We had two of the new Holley Street HP carbs available to test, PN 0-82851 at 850 cfm on the left, and PN 0-82951 at 950 cfm on the right. We had to wonder whether these lower-cost carbs could produce competitive power to one costing $150 more.
A new series of carbs just released by Holley is the Street HP line. Offered in 650-, 750-, 850-, and 950-cfm versions, these carbs incorporate many of the performance improvements an enthusiast really needs, while maximizing value. The main distinguishing feature is the high-flow HP-style main body, which provides greatly improved airflow and reduced turbulence compared to a standard main body. Up top, you'll find screw-in air bleeds for custom tuning, along with high-flowing, spun-in, downleg boosters. In contrast to the race HP-Series, the Street HP comes with the required vacuum ports to help with a street installation. Vacuum-secondary carbs come with two-corner idle mixture adjustment and a quick change vacuum diaphragm for easy secondary tuning, while double pumpers feature four-corner idle screws. What you won't find are billet and aluminum components as used on some of Holley's higher line carbs, but rather the traditional Holley zinc body in a tumble-polished finish, with conventional bowls and linkage. The carbs come wet-flowed and calibrated for street/strip performance.
When you really look at what these carbs have, you'll realize that most of what makes the HP carbs famous for performance and tuning is there, but you'll also notice while shopping that the Street HP is priced significantly lower than the higher-range carbs.
The 950-cfm carb on the right...
The 950-cfm carb on the right comes through with 1 3/4-inch throttle bores, while the 850 (left) features a 1 11/16-inch size. Both carbs come with conventional Holley linkage arrangements, and are mechanical secondary units with dual 30cc accelerator pumps.
A closer look down the bores...
A closer look down the bores shows the Holley HP-style main body, with replaceable air bleeds and downleg boosters. Four-corner idle circuits are also a part of the package in the double pumper versions.
The Street HP carbs come with...
The Street HP carbs come with traditional Holley components, including the conventional center-hung float bowls, cast zinc bodies and metering blocks, and cast-aluminum baseplates. Shunning the billet reduces the cost of the carbs without a penalty in performance.
On The Dyno
The initial combination on...
The initial combination on our hydraulic roller cam-equipped 496 featured a higher-end 850 carb that had been dyno-tuned in several prior sessions. The big-block delivered 417 hp to the wheels with an ideal-looking fuel curve. We were hoping to at least see the cost-effective Street HP (seen here) approach the baseline performance of our baseline carb. First up for testing was the smaller 850 unit. Out of the box, the carb was close.
There is no questioning that the Street HP looks good on paper, but the real bottom line is just how it performs. To find out first hand, we figured a real-world test was the only way to separate fact from fantasy. Editor Johnny Hunkins volunteered the Street Sweeper Chevelle as our test bed, and we loaded the two bigger Street HP carbs offered by Holley (PN 0-82851, 850 cfm; and PN 0-82951, 950 cfm) and headed to the Westech Performance Group's chassis dyno. The Chevelle's stout 496ci big-block produced 626 hp on the engine dyno, and certainly has a healthy appetite for airflow. The engine was already equipped with a higher-cost, high-end 850 carb, a combination that had the advantage of no less than four previous dyno sessions, and was tuned to a razor's edge. We were curious to see if these new value-minded Holley carbs would measure up.
To open our test session, the Chevelle was run in its original configuration to establish a baseline. When the numbers rolled in, it showed the big Chevy was delivering 417 hp to the rear wheels. Given this combination's previous dyno work, it was no surprise to find the Lambda showing an ideal max-power mixture and a remarkably flat fuel curve. In terms of carburetion, we had to believe the engine was optimized.
Our first swap had us replace the carburetor with the smaller of the two Holley Street HP carbs, the 850. The carb was installed with the calibration as-delivered, and our first pull showed the Holley was within about 10 hp of the base carb, with the mixture showing a moderately lean condition through the top end of the range. Looking over the dyno data, we decided to add some fuel to the fire, and stepped up the secondary jetting four jet sizes. We were expecting to see the gap in performance narrow with the jetting change, but we were surprised to see our budget 850 actually make more midrange torque, while holding on to make exactly the same 417 hp up top as the much more expensive race-style 850. The Street HP matched the output of our seriously tuned baseline with just one jet change.
With an increase in jet sizing...
With an increase in jet sizing at the secondary as the only tuning change, we were impressed to see the 850 match our baseline combination for peak horsepower, and even more in awe of the substantial improvement in midrange torque and power.
Given the excellent results...
Given the excellent results turned in by the 850 Street HP, we didn't expect to find much left on the table with the 950-cfm version. The larger carb did manage a couple of numbers more in terms of peak power, but looking at the overall averages, the two carbs finished in a virtual dead heat.
Since the 950 Street HP was...
Since the 950 Street HP was destined to remain with the Chevelle, we took the opportunity to add Holley's 116-10 jet-extension kit to the secondary. This helps ensure that the jets are in the fuel during hard dragstrip launches, reducing the possibility of fuel starvation under acceleration. The kit comes with the extensions and the required notched float.
With the conclusion of our first series of tests using the 850 Holley Street HP, we could see that these carbs deliver serious power, and we had to wonder if our 496 would benefit from the additional capacity of the 950 Street HP. In a matter of minutes we had the 950 bolted in place and baselined. Not surprisingly, as with the 850 Holley, we found the 496 hungry for just a touch more jet. Once calibrated, we found the power numbers came in to show a virtual dead overlay of the output recorded by the 850 over most of the power curve, and a couple more horsepower up top. Based on the results, it looked like the smaller of our two test Holley carbs had sufficient airflow and metering accuracy to satisfy our Chevy big-block, yet the added capacity of the 950 showed no detrimental effect.
We expected these Street HP carbs to come close to our more expensive baseline carb, but it was astonishing to see both match the top end power while delivering even more midrange-as much as 15 lb-ft of extra torque at 4,900 rpm. Considering these results were turned in with just minor jet tuning, these Street HP carbs pack a lot of punch for the price.
Fueling The Fire
A healthy engine needs a healthy...
A healthy engine needs a healthy fuel supply at peak power. We opted for a Holley 12-454-25 HP-Series pump. This pump needs no external regulator and provides over 170 gph of fuel flow.
As good as a carburetor might be, there's no way that it will keep a serious performance engine happy if it isn't supplied with sufficient fuel. It's a common mistake to build a hot street powerplant while neglecting the fuel system. While a milder street small-block or a stock-style rebuild can get by with an OEM fuel system in good working order, stepping up the power means the fuel system can't be overlooked. It comes with the territory when dealing with ever-larger strokers and higher power outputs that peak fuel flow demands can quickly outstrip the fuel system's capacity. On the engine dyno, our 496 big-block demanded 291 lb/hr of fuel flow at peak power, while a typical high-performance 350 small-block might require less than 200 lb/hr. It doesn't take heavy calculus to see that the bigger, more powerful combination will call for 50 percent more fuel at peak output. For a street machine like the Street Sweeper, we liked the simplicity of a mechanical pump, but a stocker would be borderline at best.
Fortunately, Holley carries a line of high-performance mechanical pumps that are a simple bolt-in replacement, yet are capable of serious fuel flow. The Street Sweeper is equipped with Holley's 12-454-25 HP-Series fuel pump. With a flow capacity of 170-plus gph, it has proven to feed our 496 flawlessly. This mechanical pump is designed with a pre-set pressure of 7.5 psi to operate without a fuel pressure regulator, simplifying the installation. We installed the pump using Earl's fuel line plumbing, and Earl's Ano-Tuff PN AT101285ERL carb line kit.
|SUPERFLOW CHASSIS DYNO
|PHR STREET SWEEPER '68 CHEVELLE
| 496 BBC TESTED AT WESTECH
||417 at 5,800
||417 at 5,800
||419 at 5,800
||421 at 5,000
||432 at 4,900
||432 at 4,900
*Average rear wheel horsepower and torque from 4,800-6,000 rpm
|WHAT IT COSTS
||Holley 850 4150 Street HP carburetor
||Holley 950 4150 Street HP carburetor
||Holley Ultra HP Billet mech. fuel pump
||Holley float kit
||Carburetor fuel line kit
*Summit mail-order pricing at time of publication