The initial combination on our hydraulic roller cam-equipped 496 featured a higher-end 850
On The Dyno
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 at the secondary as the only tuning change, we were impress
Given the excellent results turned in by the 850 Street HP, we didn't expect to find much
Since the 950 Street HP was destined to remain with the Chevelle, we took the opportunity
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.
A healthy engine needs a healthy fuel supply at peak power. We opted for a Holley 12-454-2
Fueling The Fire
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