The Plan
One of the most common performance tricks is upgrading the cam and the carburetor on a musclecar engine. So that is exactly what we decided to do with our 505 motor once we finished the cross-ram testing that was featured in our August '07 issue. We had seen a peak of 705 hp during those cross-ram tests, and we wanted to compare those numbers to a combination that uses a big carb on a single-plane manifold. Since we were going to be twisting the wrenches anyway, we grabbed the opportunity to use Comp's new CamQuest 6 cam selection software to help us pick a cam. Our plan came together like a charm with the new cam-and-carb combo blasting our 505 motor to a new peak horsepower recording.

The Carbs
Holley recently introduced a line of Ultra HP carburetors that have a lot of new features. These super-high-performance carbs have billet base plates, billet metering blocks with full adjustability, notched floats, glass float level sights, and replaceable air bleeds. The metering on these units is dialed in for race-type engines with big camshafts, so they are ready to run right out of the box. We had seen some information on these new carbs at the shows, but hadn't been lucky enough to get our hands on any until this dyno test was scheduled. For this test, we ordered both a 4150-style carb with a 950-cfm flow rating and a big 4500 series Dominator with a 1050-cfm rating, so we could compare them back to back.

These new Ultra HP carbs have replaceable jets in the idle circuit, as well as the emulsion tubes and the air bleeds. What this means is if the original metering isn't correct for your particular application, it can be easily changed. This ease of tuning solves one of the more frustrating issues that carb tuners have had to deal with for a long time. There have been numerous magazine articles written over the years telling people how to stick a small wire into the idle feed jets to restrict the idle fuel flow, and other articles have shown how to drill out the power valve restrictions for more fuel at WOT. Now with the Ultra HP version of the carb, you don't need to drill or fill any of the passages since you can now change the metering of the various circuits with jet changes.

The main circuit in Holley carburetors has always been easy to tune with replaceable jets, and that wasn't changed in the Ultra HP series. Likewise, the acceleration circuits on these new carbs are tunable by changing the pump cam, the shooter nozzle-size, or the size of the accelerator pump just like any other Holley carb. The difference in the Ultra HP series is the addition of jets for the idle circuit, as well as jets in the emulsion wells and in the power valve restrictions.

Sometimes the idle mixture screws just don't have enough adjustment range to get the idle mixture correct when using a large camshaft with a lot of overlap. In the Ultra HP carb you can quickly change an idle jet in the metering block to get the idle mixture screw back into its proper control range. For instance, if the idle mixture screws have to be adjusted almost all the way in for the motor to idle, you know the mixture is too rich. By reducing the size of the idle jets inside the metering block, the mixture screws can be returned to the normal 1.5 turns open.

The traditional Holley power valve circuit is like an on/off switch where the fuel flow is fixed by passages drilled in the metering block. In the Ultra HP carb, the power enrichment circuit also has jets in it so you can adjust the amount of fuel that flows thru the power enrichment circuit without having to drill or epoxy the metering block. This is especially valuable in applications where a tuner would want to set up the carb to be fairly lean while cruising down the freeway, but have the carb be safely rich when operating at wide-open throttle. With the ability to change jets in the power enrichment circuit, tuners now have the capability to easily change the A/F ratio at WOT without disturbing the part-throttle A/F ratio.