Whatever type of blower you're running, the proper amount of additional fuel must be added at the right time or you'll suffer total destruction. Carburetors must be specially prepped to work best with any high-power blower system, and both Holley and Demon carbs are making out-of-the-box carbs for these kinds of engines. Whatever type of blower you're running, the proper amount of additional fuel must be added 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 DYNO POWER RPM TQ HP 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 PEAK BOOST 5.8 PSI MAX CFM THROUGH CARB 977 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 PowerThis 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. CARBURETED/CENTRIFUGAL 383 POWER RPM TQ HP 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 PEAK BOOST 11.4 PSI Spark plugs and your own ears are the best indicators of your blown engine's health. When advancing the timing or increasing the boost for more power, listen carefully and check the plugs often for little blacks specs on otherwise clean porcelain that will indicate dangerous detonation. Spark plugs and your own ears are the best indicators of your blown engine's health. When Octane boosters like this from Justice Brothers could be your best defense against detonation. Don't add octane booster unless you're already experiencing detonation that cannot be cured with other methods or your power may suffer. Octane boosters like this from Justice Brothers could be your best defense against detonat Both centrifugal and Roots-style blowers make boost dependant on how fast they're driven. You can change the drive speed simply by changing pulleys. Drive ratios are calculated by dividing crank pulley diameter by blower pulley diameter. Ex: 7-inch diameter crank pulley used with a 3.5-inch blower pulley equals a 2:1 ratio, or 100 percent overdrive. Both centrifugal and Roots-style blowers make boost dependant on how fast they're driven. « | 1 | 2 | 3 | View Full Article By Mike Petralia Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!