Tom Wilson
June 6, 2006

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.

On the Dyno

Built from a 4150 Holley body, The Carb Shop's blow-through carburetors are handbuilt from all-new parts specifically for each application. This is the one on our test engine-note the balance tube running between the float bowls. It is critical to maintaining fuel-level equilibrium throughout the carburetor.

When Vrbancic Brothers installed the engine on its dyno, a small number of runs was anticipated. After all, the carburetor-as are all Carb Shop carbs-had been given a test session on Vrbancic's big-block Chevy dyno mule to verify functionality and the fuel curve. The rest of the engine was not breaking particularly new ground for the Vrbancics. Of course, things got slightly more complicated. There was the monkey motion trying out different blower belt/sprocket combinations as we already related, then the original camshaft ended up with too much duration. In essence, the Vrbancics could see they didn't need that much cam to reach the power goal and, by shortening up the duration, more engine efficiency could be realized. Therefore a second Lunati mechanical roller was installed, with lift remaining the same, but the duration shortened from 265/276 to 262/268 degrees.

With the blower drive properly configured and the camshaft optimized, the engine was run naturally aspirated to give a baseline of the basic powerplant. As the dyno chart shows, even without the big ProCharger, this stroker Windsor was ready to rumble, blasting out more than 600 hp all by itself.

Then the supercharger was hooked up, and with every possible twiddle knob turned to "baby safe" mode the combination belted out 1,047 hp. By "safe mode," we mean the air/fuel mixture was a blubbering 9:1 down low and leaning out to just barely 11:1 at the power peak. While blower engines obviously make good power running a little richer than naturally aspirated engines, this is rich enough to ensure the 91-octane swill that passes for premium fuel in California would not detonate. In other words, it was putting out the fire with gasoline.

Considerable work is required to seal the carburetor to keep it from leaking boost pressure. Here we see the outer edge of the special seals The Carb Shop built for the throttle shafts.

Also important to note is the water temperature was set to a frigid 120 degrees. Again, this is ridiculously cold to avoid detonation, and more power would easily be made simply by warming up the cylinder heads with 140- to 160-degree cooling water, which is more of a dynoing norm.

When run naturally aspirated, the cooling water was in the low-150-degree range, which did aid power compared to the blown run. Ignition timing was set at 31 degrees initial, with admittedly minimal retard-just half a degree per pound of boost.Just the same, the power is prodigious. The dyno chart starts at 4,500 rpm, where 599 hp and 699 lb-ft of torque are already on tap. This is with 7 pounds of boost measured at the carburetor, along with a fat 10:1 air/fuel ratio. More than 800 hp is available by just 5,500 rpm, 900 hp at 6,000 rpm, and 1,000 hp at 6,600 rpm. If this thing doesn't set you back in the seat, we don't know what will.

If you're wondering what the engine is like at lower rpm, look at the naturally aspirated data. There we see the engine opens with 306 hp and 459 lb-ft of torque at just 3,500 rpm-with zero boost, of course. Considering the blown engine is making 7 pounds of boost at 4,500 rpm, it's safe to assume it makes a couple of pounds of boost in the 3,000-rpm range. That's enough to up the power a little, so there's no lack of oomph down low. We'd say the only real issue here would be finding a tire and suspension that could hang onto this power.

What Could Be

Only the most advanced, high-flow bodies and other airflow critical parts are used in The Carb Shop carbs. This include the booster venturis, which are made by The Carb Shop to suit its needs.

With the engine set on double safe, it's interesting to conjecture on what the engine could produce if turned loose. There's plenty to play with-the coolant temperature could be raised and, obviously, the mixture could be leaned out. But the big gorillas are rpm and boost pressure. Note that the power peak was achieved at 7,000 rpm right at the end of the run. This engine is designed to run past 7,000 rpm, and simply turning it up a few hundred more rpm would deliver more boost and substantially more power. And, clearly, different pulley-and-belt combinations would deliver even more boost if anyone thought it necessary.

All of this is not to mention the benefits of simply running the combination on high-octane race gas, with all the boost and ignition timing increases that would allow. All told, this engine could easily whip out 1,300 hp with more aggressive tuning and boost in the low-20-pound range-something it could safely do with race gas, another degree of ignition timing, a jet change, and holding down the handle a few hundred more rpm. Wow!

The Engine

At one time we would have been surprised to see an oxygen sensor in a carbureted engine's exhaust system, but not now. It's there for the dyno's data-acquisition system only, and has nothing to do with feeding information back to the carburetor.

Holley's goal of bypassing the crate-engine route and assembling an engine from parts worked well, going together with a minimum of hassles and easily handling the impressive power and torque. Like most moderate-rpm, higher-boost blower engines, this motor uses high-quality parts-there's nothing low-buck about this approach. But there's also nothing here we'd label exotic or completely off the financial scale we'd have to call Department of Defense accountants to add up the numbers. In fact, on a dollar-per-horsepower basis, this engine is likely a bargain, and the internal clearances and specifications for things such as bearing and cylinder-wall clearance, ring thickness and placement, and piston weight would look rather standard to anyone with hot-rod V-8 experience.

The heavy architecture is from Dart Machinery, including a stout Dart block, Dart Pro 1 aluminum cylinder heads, a premium Lunati forged crankshaft, Lunati Pro-Billet connecting rods, Lunati forged-aluminum pistons with an 8.5:1 compression ratio, and an all-Lunati mechanical-roller valvetrain with mountainous 0.668/0.668-inch lift and week-long 262/268 degrees of duration at 0.050-inch valve lift. A Jesel belt-drive connects the crank and camshafts.

Airflow and fuel-delivery issues key to making power were handled by Holley and Wilson Manifolds. Wilson took a Holley Nitrous Pro Flow single-plane intake, gave it a full port job, fitted a 1.5-inch-tall, four-hole carburetor spacer, then turned to the cylinder heads. These were comprehensively CNC-ported by Wilson, all the way to breaking through into the pushrod passages and having to sleeve them to retain the port's airtight integrity.

Keeping with the straightforward theme, this engine's ancillary systems are blessedly simple. The wet-sump oiling is provided by a Melling high-volume pump and somewhat trick Milodon deep-sump pan. The ignition is all standard MSD gear, and the dyno exhaust is large-tube 4-into-1 dyno headers blasting into Vrbancic Brothers' immense dyno mufflers.

Yes, there's more to this than rebuilding an old 5.0. But if you're past the point of having a jar of parts left over after you assemble an engine, you might consider tackling this one yourself. It would be quite an accomplishment.

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