Horse Sense: You might think the carburetor on a 1,000hp blow-through system is not much more than a metered waterfall, but as Bob Vrbancic says, "The blow-through [system], has to be rich when you open the throttle. But it has to have a fuel curve." In other words, it still has to work as a carburetor, metering fuel with precision.
Bob Vrbancic makes the final check on The Carb Shop's carburetor atop Holley's 1,000hp demo engine. The dyno is at Vrbancic Brothers Racing, which Bob partners with his brother, George.
Stories on dyno engines are often dry and technical-sort of a death by numbers experience. The trouble is typically in the numbers, so here's a solution: 1,047 hp. Yep, four-digit fun from a 427 stroker Windsor, and it only gets better. For example, try these numbers: 91-as in octane, 19-as in pounds of boost, and 90-as in degrees Fahrenheit dissipated by the intercooler. See, numbers can be fun, just like your eighth-grade algebra teacher used to say.
Generating all this numerical happiness was the demonstration engine from Holley we reported on in our April issue. At that time we followed the engine's assembly, detailing its internal construction. The highlights of that story are that nearly every part in the engine came from a company owned by the giant Holley corporation to arrive at an 8.5:1-compression 427 Windsor wearing an ATI-ProCharger F2 supercharger and blow-through carburetor induction. Parts are premium or heavy-duty throughout (see the sidebar for more engine information). Bob and George Vrbancic and tech Dave Whitehead assembled the engine at Vrbancic Brothers Racing. Furthermore, Bob Vrbancic owns The Carb Shop next door, which supplied the special Holley-based carburetor.
Clearly Holley wanted to demonstrate the wide range of its companies' reach. Furthermore, Holley wanted to show a significant engine such as this could be assembled by a serious enthusiast without resorting to a crate engine. As for the performance goal, that was 1,000 hp on pump gasoline.
The results were spectacular, with the engine churning out its 1,000-plus horsepower in a walk, and in a manner that makes it look extremely driveable for a hobby sort of street car. Make no mistake-that 1,047hp figure is just the beginning for this engine because it was running a conservative tune in deference to the 91-octane pump gas it was drinking.
Blower and Carb
Huge is a quick way to describe ProCharger's F2 supercharger. Measuring 10.5 inches across the volute and capable of up to 38 psi and 2,700 cfm, the F2 is rated to produce 1,600 hp, so it's right in its working range on this engine. With its own oil supply, the self-contained F2 needs no oil plumbed from the host engine. And check out that mount-it's beyond stout.
In our first article on this project we mentioned the major elements of the blow-through supercharger arrangement and promised details in this dyno article. Perhaps the first question is, why is it a blow-through system in the first place? The answer, says Bob Vrbancic, is the ability to produce tremendous power via high boost pressures while retaining excellent driveability. These engines make race-car power but can be driven on the street. Bob should know, as The Carb Shop has worked five years to perfect blow-through carburetor induction, and has learned what it takes to mate a Holley carburetor to a howling centrifugal supercharger.
The two elements-the blower and carburetor-are closely intertwined in the blow-through arrangement because the carburetor-which works on the principle of differential air pressure-must sense the boost the supercharger is producing. In other words, the carburetor must be exposed to boost pressure not only through its venturis, but also at the float bowls vents, and fuel pump. Furthermore, this boost pressure cannot leak out of the carburetor, or the all-important reference to boost will be lost.
Unlike blow-through systems where the carburetor is encased in a boost-tight box, the box-less blow-through arrangement requires that the carburetor itself must be prepared to seal in boost. To meet these requirements, The Carb Shop starts with new, never-assembled, Holley bare bodies, baseplates, metering blocks, and so on. Most of these parts are modified and many others are simply custom built by The Carb Shop, with the boosters built by The Carb Shop, main circuit, and transition circuit all optimized for blow-through duty. The throttle shafts are made boost-ready using a Chicago Rawhide seal made to The Carb Shop's specification. This seal keeps boost pressure from leaking out.
A pressure line is run between the two float bowls; Bob says detractors claim this line is superfluous, but the system absolutely will not work without it, as the two float bowls end up seeing different boost pressure without it.
Each carburetor is custom-built for its application, so the carburetor is sized to the engine displacement, desired power level, and boost pressure by The Carb Shop. The 950-cfm carb on Holley's 427 Windsor was listed at $985 by The Carb Shop, which, when compared to setting up an injection system, seems rather reasonable for a handbuilt carburetor.
Compared with the unique Carb Shop carburetor, the huge ATI-ProCharger F2 supercharger was installed stone stock, out of the box. While issues would soon arise with the geometry of the blower drive belt, ProCharger's kit proved perfectly engineered for the job and required little on Vrbancic Brothers' part other than attaching it to the big Windsor. This follows the Vrbancic brothers' long experience with ProChargers, which they appreciate for their heavy-duty mounts and prodigious power output.
The F2 system is intercooled, which is an absolute necessity when boost levels reach into the teens. While the air-to-air intercooler wasn't expected to help much on the dyno, it obviously would play a crucial role when this engine eventually lands in a car.
Here's where the extra effort was required to get the blower belt, the correct sprocket ratios (66 crank/52 blower), and the water pump to happily coexist. The belt is an 8mm pitch Gates unit and is tensioned by a long-travel, non-spring-loaded pulley.
As the bolting-on process evolved, it became clear the cog drive belt was passing right through the Meziere electric water pump. A smaller crank pulley was tried in order to package the blower belt past the water pump. This combination was run on the dyno, but it made boost pressure rise too fast-that is, the rate of boost increase was too great compared with the gain in rpm. The goal was approximately 18 pounds of boost at 7,000 engine rpm, yet the smaller crank pulley was making 18 pounds of boost at something more like 6,200 rpm, and something like 5 pounds down at 3,500 rpm. It would have been making 25 pounds of boost at 7,000 rpm, which would be too much, too fast. This would have made the engine squirty, far too hair-trigger to drive easily-exactly what the Vrbancic brothers were trying to avoid.
So, the 54-tooth crank sprocket and 56-tooth blower sprocket combination was switched to a 66/52 combination, which would accommodate the surprisingly small number of cog blower belt lengths available. This left the blower belt running through the water pump, so Vrbancic Brothers asked Meziere what it might have that would help. Meziere's answer was to incorporate one of its special belt-driven pumps and turn it into an electric pump with the pulley retained as an idler pulley. This allows the blower drive belt, which by now barely engaged the pulley, to slap along on the idler pulley as it needs to.
Meziere saved the blower-belt day with a one-off combination of its electric and belt-drive pumps. The hermaphrodite pump is electric drive, but it uses the belt-drive pulley as an idler. It allows the belt to flop along the water pump without hurting anything. | 
This is where the air torture begins-at the slicing, multi-angle entrance of the F2 blower's inducer and impeller. For this test the supercharger was run just as you see it, with no air filter, airbox, or tubing. It was also ingesting possibly rather hot air from just above the left-side headers, but this doesn't seem to have hurt anything much. | 
We were amazed at how much charge cooling took place in the ProCharger intercooler. We figured the air-to-air unit should have been only modestly effective without ram air running over it, but with just the fan-induced air movement through the dyno cell we saw temperature drops of 50 to 90 degrees of the charge air. While not tested directly, we believe the large end tanks and core design cost little in the way of boost loss. |
This overall shot illustrates the simple air path from blower to the carburetor. The tubing is mandrel-bent and 3 inches in diameter. In the elbow closest to the camera, you can just make out the probe used to determine the blower-discharge air temperature. | 
ATI's kit is complete and includes every part of the air plumbing, including this cast elbow at the carburetor. For testing, the elbow was plumbed with boost-pressure and air-temperature fittings. | 
Big blowers and big boost call for equally large bypass valves. Located between the intercooler and carburetor, this massive unit vents boost when the throttle blades snap shut in the carburetor. Otherwise, damaging volumes of air would be trapped between the carburetor and supercharger during high-rpm deceleration. |