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Centrifugal & Roots Blower - Blown Away

Superchargers: The Power Of Forced Air Induction

By Mike Petralia

The only other part we changed was to go to a bigger, 950-cfm Holley HP blower carb. This was definitely a good move because with the blower pulling tons more air, the engine now wanted more than 970 cfm according to the Superflow dyno printout! Sitting on top of blowers is one place where a bigger carb is definitely better. Since we were still running strictly pump gas, we backed total ignition advance down from 36 to 28 degrees because just like with nitrous, too much timing can kill a supercharged engine.

Mini-Rat Supercharged

3000 563 322
3500 575 383
4000 600 457
4500 608 521
5000 604 575
5500 588 615
6000 559 638
6500 529 655
7000 469 625
PEAK HP 656 @ 6700
PEAK TQ 610 @ 4400

As we also learned with our nitrous test this month, there's a limit to all that's good making horsepower on pump gas. We pushed past that limit and the Mean Little Rat suffered the consequences. We think the damage was caused by a combination of too much combustion chamber heat and our using the wrong pistons for the application. When running power-adders on big-block Chevys, it's pretty common for small-bore, 396-402-cid pistons like these to fail at the weak spot next to the intake valve relief above the top ring.

Although our pistons were forged, they were not the low-silicon 2618-aluminum alloy forgings recommended for blower use. Regardless, there are now holes in the tops of pistons No.4,6, and 8, and we don't think any amount of race gas or reduction of timing could have totally eliminated the inevitable destruction like this. Bottom line, if you're going to run hard with a supercharged, pump gas motor, lower its compression to 9:1 or less with low-silicon, 2618-alloy forged race pistons.

Centrifugal Spins Up Some Power
This is one of the first centrifugal blower tests we've witnessed that was not designed to make max power for racing. Instead, Vortech's engineers put together a street blower package for use on carbureted engines. The engine test we witnessed was a 383-cid small-block belonging to our June '01 cover-car's owner, Doug Kruse. It will power Doug's awesome '57, and he didn't want to go EFI because everyone else in his neck of the woods already had. Unfortunately, at print time we didn't have all the specs on Doug's powerplant, but suffice it to say that his motor was properly built with the blower in mind.

3000 315 180
3500 359 240
4000 458 349
4500 490 420
5000 508 484
5500 509 533
6000 502 573
PEAK HP 573 @ 6000
PEAK TQ 525 @ 5700
By Mike Petralia
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