Sometimes all you can do is stop and say, "Damn." That epithet was all I could manage as we fired up our newly Whipple-supercharged 383 small-block on Westech's engine dyno. As Steve Brul brought the force-fed mouse up to 3,100 rpm in preparation for a pull, the screen already showed 594 lb-ft of torque. Damn. And that was just our initial, cautious run. In short order we were trouncing that number, throwing down well over 700 hp in the bargain, and this with 91-octane swill and less than 10 psi of boost to boot. Suffice it to say that our stroker mule and Whipple Superchargers' new carbed small-block blower kit are a brute-force combination we totally dig.
We asked Dustin Whipple to elaborate on the heart of this positive displacement system, the compressor. "The twin-screw supercharger is a true compressor," he began. "It actually compresses the air in the housing." This leads to superior airflow into the intake manifold and cylinders with little leak-back or reversion, he explained, because the air discharging from the screw compressor is a higher pressure ratio than that in the intake manifold. This differs from the air in a roots-style blower, which only has positive compression below the supercharger, "because it's a pump." The high pressure air in the manifold actually attempts to flow back up into a roots-style supercharger, a phenomenon called leak-back or reversion. The supercharger recirculates the air back and forth, raising air temperature and forcing the system to overcompensate. "The screw compressor allows higher boost on a higher compression motor without the extra heat," Dustin summed up.
In practice, installation onto our trusty 10:1 compression 383 stroker small-block was straightforward and fairly intuitive. With our mule easily accessible on the engine dyno, installation only took about two hours. Mixing duties for our supercharged setup were handled by a Holley 950-cfm Ultra HP. Dustin told us that because we're dependant on airflow, "for the most part, a bigger carb is better." Our 383's timing needed to be retarded to run with the boost, of course; Brul took the advance down to 31 degrees. Whipple suggests that timing be set in consultation with themselves and the engine builder, but as a rule they figure on 24-31 degrees for a stock small-block and 28-32 degrees for a built. Our figure fit in nicely. With that, we were ready to give it a rip.
And damned if the hype wasn't believable. Using the largest-diameter pulley and 100-octane fuel to be sure of ourselves, we made some preliminary runs, which returned 624 lb-ft with a 609 average, along with a stunning 668 hp-and it was climbing when Brul backed it off. While we were concerned about exceeding the limits of our cast-crank 383, the mule wasn't showing any hardship when supercharged, and we wanted to see what it could really do. So we upped the ante in a couple of ways-first by increasing the boost with a smaller pulley, and second by running regular 91-octane unleaded to represent a real street setup. So what happened on the swill? This beast grew stronger, belting out 646 lb-ft and cracking the 700 barrier with 712 hp. We then took it a step further, bolting on a 4.250-inch pulley-this amounted to an average boost level of 8.3 psi. We got back 680 and 738. The torque curve is tremendous, showing 664 lb-ft at 3,200 rpm and averaging a mind-blowing 653 lb-ft. And there's plenty of horsepower upstairs; maybe too much, if there is such a thing.
Having already far surpassed what we expected-and what we expected a mild four-bolt-main motor with a cast crank to survive-we called it a day. Our combo was excellently formulated and responded massively to the Whipple supercharger. It makes loads more steam throughout the whole powerband and churns out phenomenal torque figures right where they can be used. All we can think about now is finding something to put this little twin-screwed monster in so we can stomp the pedal, and good.
What We Did:
Dyno'd a 383 stroker equipped with Whipple Supercharger's carbureted small-block tuner kit
We made ridiculous levels of bolt-on power
$4,499 in black finish with intercooler
What's It Flow?
|BRODIX KC IK210 CYLINDER HEADS|
Picking The Stick
As we often do when undertaking this kind of project, we consulted with one of our favorite cam gurus, Comp Cams' Billy Godbold, to pick a suitable bumpstick to go with our force-inducted 383. We added the appropriate lifters, pushrods, and Ultra Gold rocker arms to go with it. Godbold suggested a piece from the company's newest line, the XFI series. Specifically, we went with PN 08-468-8, grind CS 292XFI HR-13 for our blower application. Why an Xtreme Fuel Injection (XFI) cam? "You want to take overlap out in a fuel injection setting," Godbold explained. "You don't need the signal to pull fuel out, since the injectors squirt it in. You're only concerned about moving air. It's similar to a positive-displacement supercharger in this respect. It doesn't need the vacuum signal provided by overlap to move air, as you have the pressure in the manifold." Godbold also explained that the forced charge would "blow right through" if a cam with a tighter lobe separation angle (LSA) were used.
According to Godbold, the duration specs for this cam also work well with a positive-displacement supercharger. "You have 6 degrees more exhaust duration than intake, which is enough to help scavenge exhaust gasses," he continued. "With a centrifugal supercharger, you need less intake duration and longer exhaust duration so it makes torque down low and works well up high." With positive-displacement, he told us, "You can just put a bigger cam in it." And finally there's the fact that the XFI features Comp's newest, optimized intake and exhaust lobe designs. "It's a great all-around cam," he summed up. "It's as good as we can do." We're not inclined to argue.
An Insider Look
We've mentioned that the Whipple supercharger is a positive-displacement compressor based on a twin-screw design. The rotors, or screws, which never touch each other or the case, produce compression through intermeshing helical lobes. A male rotor with usually three bulbous lobes is complemented by a female rotor with usually five valleys. The valleys are similar in contour to the lobes, but different. In this typical three-five rotor combination, the male rotor turns 1.66 times for every turn of the female rotor. These numbers can vary, but the female rotor will always have more valleys than the male rotor has lobes. The number of lobes dictates three compression cycles per revolution. This means that the compressed air has pulsations when compared to a reciprocating compressor, though Whipple says the air is pulsation-free by the time it leaves the compressor unit. The male and female rotors go through three phases: inlet, compression, and discharge. The compression sequence continues on all rotor segments simultaneously, and the compressor we used can be spun up to 18,000 rpm. This high-efficiency, positive-displacement design is intended to deliver large power gains throughout the entire rpm range-and it did just that for us.
HOLLEY 750 STREET HP
|Intake||Weiand Stealth |
|Headers||1 3/4-inch Hedman |