Boost is gooood. Boost makes big power. But for carbureted applications, it can mean big ugly if you’re not the hole-in-the-hood type, because it’s the Roots huffers that have been the most practical source of horsepressure ever since Barney Navarro first strapped one to his El Mirage roadster. Centrifugal blowers have also proven themselves since the ’30s; the blow-through McCulloch VS57 was introduced in ’53 and was OE on Kaisers in ’54. However, it’s been almost 40 years since Paxton first enclosed a carburetor on a Shelby, and nearly as long since an aftermarket company seriously promoted blow-through applications. But everything old is new again: Vortech’s got the hookup for centrifugally blown carbureted applications. This return to low tech opens up a new world for hot rodders unwilling or unable to install and tune a $3,000 fuel-injection system just to keep their blown engine under the hood.
What Do You Get?
Vortech’s pretty proud of the new carbureted system, and you’ll see it in plenty of magazines in the near future. Our test goes right to the obvious: a small-block Chevy, for which there’s a complete new kit. Vortech’s Race and Media Relations Manager Ricky Best was quick to point out, “It’s a universal setup, not an application-specific kit. There are so many small-block manifolds, engine sizes, and front-drive accessories that we provide the basics and you may need to do some work from there.” Even so, the kit is set up for long-water-pump applications, and it’s pretty easy to mate it to any common intake setup. The set lists for $2,495.95 satin or $2,695.95 polished; we’ve seen ’em cheaper by mail-order. For that you get a basic S-trim blower, brackets to mount the blower, a crank pulley, the carb enclosure and linkage, ducting from the blower to the enclosure, and most of the hardware. You’ll also need to buy a blower pulley (the Vortech rep can help you decide which size works for your application) and the belt to go with it. We suggest you order Vortech’s fuel-line kit, lower accessory pulley, and 10-rib drivebelt, too. Beyond that, you’ll need a Holley or Demon carb with mechanical secondaries, a fabricated air inlet, some sort of throttle linkage fiddling, and a complete boost-referenced fuel system.
Can My Engine Handle It?
The blower can run on pump gas, depending on boost, with engines that have a static compression ratio up to 9.25:1, and don’t forget that you could swap to thicker head gaskets or to heads with bigger chambers to reduce your current engine’s compression. Vortech guys tell us you can actually run up to 14.5:1 on race gas if you limit boost to 5 pounds or so. In any case, we’d really like to see forged pistons in your boosted engine. Hypereutectic or cast pistons can be used, but you need to be really, really sensitive to the tuneup to prevent lean-out or detonation, which will kill ’em quick. If you’re building from scratch, good rod bolts should be considered a minimum. A supercharger does a great job of covering up poor-flowing cylinder heads, so if you’re building specifically for the blower, you could use stock heads and spend the bucks with Vortech.
Regardless of the engine and boost level, we highly recommend either engine-dyno time or a chassis dyno with a Lambda sensor for air/fuel ratio for any supercharged application. At the minimum, get an O2 sensor with an air/fuel readout. It’s critical that the engine not go lean under boost, so carb jetting becomes very important to engine longevity. However, centrifugal blowers build boost more gradually than Roots positive-displacement blowers and are therefore much easier on engines and less prone to detonation.
So What’s It All Worth?
On a typical street engine that’s properly tuned, expect the Vortech blower to deliver between 1.5 and 1.85 hp per cubic inch, or a gain of 30 to 60 percent. Car Craft just ran an H.O. 454 GM crate engine that went from 535hp to 779 (1.716 hp per cube, a gain of 244 hp or 46 percent), and our 383 grew from 475hp to 677 (1.772 hp per inch, a gain of 202 hp or 43 percent). We know of a 514 Ford that’s likely to make 1,000. All of that is contrary to Vortech’s modest prediction of “about a hundred-horsepower gain” on a typical V-8. Of course, the power gains are totally dependent on the boost numbers you’re willing to run, but our testing revealed gains of 15 to 20 horsepower per pound of boost.
The Hot Rod HT383 Combo
We tested the Vortech on the same GM Performance Parts HT383 crate engine that was on the March ’02 cover. In that issue it made 463 hp at 5,600 rpm and 503 lb-ft at 4,200 using an Edelbrock RPM Air Gap intake, a 750-cfm Speed Demon carb, Hedman Tork-Step headers, a Comp Cams XE282HR hydraulic roller cam (230/236 at 0.050, 0.544/0.555 lift with 1.6 roller rockers) and a load of Royal Purple synthetic oil. We had also swapped the head gaskets to thin Fel-Pro shims to bump the compression to 9.6:1, but in preparation for the Vortech, we reinstalled the standard, 0.041-inch-compressed Fel-Pros to bring the squeeze back to 9.1:1. We also changed the headers to larger 1¾-inch pipes and swapped the carb to a 750-cfm Mighty Demon because Vortech highly recommends adjustable air bleeds for tuning with the blower; that’s a feature the Mighty carb has that the Speed Demon does not. Finally, the old Pertronix HEI distributor would not clear the Vortech carb enclosure, so we used an MSD Pro Billet unit instead. Despite the reduction in compression ratio, the header and carb swap brought our HT383 to 476 hp at 5,500 rpm and 505 lb-ft at 4,400 rpm—a 13hp gain with no sacrifice to torque anywhere in the curve. Cool.
Vortech Power, Step By Step
But that’s not half as great as the performance with the S-trim blower on board. We started with a 6-inch crank pulley and a 3.47-inch blower pulley, an external drive ratio of 1.73:1. You may notice that’s drastically higher than with a Roots blower, where pump-gas drive ratios are commonly below 1:1. But the Vortech S-trim even uses a 3.45:1 internal step-up ratio. Combine that with the 1.74:1 pulleys, and the impeller spins almost 36,000 rpm when the engine is running 6,000. And that’s actually low for a Vortech, delivering barely 3 pounds of boost to our 383. Even so, it picked up nearly 100 hp.
Greedy, we swapped the 3.47-inch blower pulley to a 3.125-incher, delivering a 1.92:1 ratio and an impeller speed approaching 40,000 rpm at 6,000 crank rpm. The resulting increase in airflow put the boost at 7 pounds peak and the power at 643 hp at 6,000 rpm. The torque curve was a dead flat 600 lb-ft between 3,500 and 5,500, peaking at 606 and 4,900 rpm. The rpm at peak hp and lb-ft was increased by 500 rpm over naturally aspirated, an indication of the blower’s ability to deliver air; boost is sort of an aftereffect we use to judge that airflow.
We always want more, so on went the 2.95-inch drive pulley (2.03:1 for 42,000 peak blower rpm). Guess what? A bit over 8 pounds of boost and 677 hp! With just a 1-pound gain in boost, this thing grew fat power—the improvement was never less than 20 hp at any rpm with a net gain of 34 at peak. The torque hung at 625 lb-ft forever and peaked at 627 at 5,000 rpm. This is an engine you could get used to in a big hurry. In our final tuneup, the Vortech picked up our 383 Chevy by 202 hp and 122 lb-ft.
The Tuneup
Sadly, the HT383 uses hypereutectic pistons, so we were very, very careful with the tuneup. And frankly, that’s a bit of a hassle with the carb enclosed in a big, cast-aluminum box. We ran all our baselines on 76 Performance Products 100-octane pump gas, then once we felt each tuneup was safe, we confirmed the power with a few pulls on 76’s more street-common 91-octane fuel. We’re comfortable recommending any of our tested boost and power levels for pump gas, especially if your engine has forged pistons. On this particular 383, we might stick with the 7-pound, 640hp setup to be extra safe with the hyper pistons. Regardless of the power levels, we retarded the timing from a naturally aspirated best of 35 degrees total to a supercharger-happy 30 degrees. Retarding further to 28 total killed about 10 power numbers everywhere, but it’s there for you if you get a tank of cheap gas.
As for the carburetor, the Vortech reps are sold on the new Mighty Demons. Ours was a 750, and Westech dyno operator Steve Brulé tuned it differently than he would with a Roots-blown mill: “The way the centrifugal builds boost, instead of hitting it right off idle, I thought about tuning the carb like two different engines. It’s like, just a little richer than a naturally aspirated engine on the primaries, and like a blown engine on the secondaries. It seems like the centrifugals are OK with a brake-specific [fuel consumption] that’s not as rich as I like for a Roots engine.” As a result, the primaries were much leaner than the secondaries, allowing part-throttle driveability without blubbering and full-throttle performance with safety. From an unblown best tuneup of 88/91 jetting, the blown applications ended up at 88/97. However, the Mighty’s adjustable high-speed air bleeds were also used. These work like fuel jets, but they meter extra air rather than fuel; therefore, smaller bleeds mix less air for a richer mixture and larger bleeds suck more air for a leaner mixture. They are finer than jets and also much easier to get at once the carb is bolted into the blower enclosure. After flogging several combos of bleeds and jets, Brulé settled on the 88/97 fuel jets and reduced the secondary high-speed bleeds from 0.040-inch stock to 0.028. With this setup, the fuel curve seemed dead-on at every boost and power level we tried; the more boost and power, the more fuel the engine pulled.
Carbs Under Pressure
Totally enclosing the carburetor in a sealed box and feeding it pressure above atmospheric brings a few questions and solutions. First, the fuel pump needs to be boost-referenced to deliver more pressure as boost increases. Here’s why: If the fuel pump is set at 7 psi at the same time that the blower is producing 7 pounds of boost into the float-bowl vent tubes, then the net fuel pressure is 0. You don’t want that. The fuel pressure needs to increase in direct proportion to boost, so if the desired fuel pressure is 7 psi, then it needs to dynamically increase as boost increases with rpm: 2 pounds of boost needs 9 psi of fuel, 4 pounds boost requires 11 psi, and so on. Vortech recommends pumps and regulators from MagnaFlow Fuel Systems.
We also questioned the function of the carb’s power valve, which is designed to open and richen the fuel mixture when vacuum drops below, in the case of the Mighty Demon, 6.5 inches of vacuum. But what happens under boost? Nothing special. Boost pressure is “below” the 6.5 inches of mercury, and the boost pressure is equal on both sides of the power valve diaphragm, which nets out to zero difference, so the power valve operates as usual. Even in a blown application, there is still manifold vacuum at idle and at low-speed part throttle. For these same reasons, a vacuum-secondary carb should also work in the supercharger enclosure, but Vortech does not recommend them, and neither do we.
Finally, Vortech advised that, at boost pressures above 6 psi, it would really be best to run one of the company’s blower bypass valves. While decelerating in gear, the blower is still spinning the same rpm and moving as much air as it would be if the engine were at WOT. But when the throttle is closed the air backs up and can cause impeller damage as the blower is trying to work against itself. The bypass valve can also quiet down blower chatter under deceleration.
Conclusions
What conclusions? That’s it—we’re sold. So sold that we’re dying to try the Vortech blower on the little 4.3L V-6 we built elsewhere in this issue. If we can figure out how to do it (a crank trigger is required because the airbox will not clear the V-6 distributor), then we’re all over it. Come to think of it, why not fab your own brackets to try the Vortech on any engine? Let us know if you try.
