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Blown Small Block Chevy: Weiand 6-71 Blower

How To Make 700 HP With A Small-Block Chevy And A Little Help From A 6-71 Blower

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There's a place in this world for wretched excess. We're talkin' in-your-face, old-school stuff like a pair of Holley carburetors stuck on top of a monster blower sticking through the hood with a cogged beltdrive that sings like an high-pitch chainsaw. This is classic car crafting, harkening back to the days when you had to look around that huge blower to know when the stoplight turned green. Somehow we've gotten away from that, and there are those who mourn those days. Sitting on the shop floor at the giant Car Craft shop (where Hot Rod has to ask us to use the hoist) was this huge Weiand 6-71 blower just daring us to bolt it on something. It was just like the Siren's song in Greek mythology, and we're one of those ancient mariners who shipwreck onto those giant 6-71 supercharger rocks. Yeah, sail on, brother. Weiand 6-71 Blower whine—give me all you got!

Blown Small Block Chevy 383 So after getting all pumped up on supercharger bench racing, we decided to bolt this big Weiand 6-71 blower on Editor Douglas Glad's much-abused 383ci small block Chevy. Way back in the September '07 issue (“Are Premium Parts Worth the Price?”), we tortured that Lunati rotating assembly 383, with its big Comp Cams grind and Dart aluminum heads, by running a couple of dozen parts across it. The motor never missed a chance to shine and eventually made an impressive 571 hp at 6,900 rpm. Since it's bad karma to let a powerful engine like this just sit around, we ignored common sense and bolted the Weiand 6-71 blower on, hoping that the tight lobe-separation angle and the high compression would kinda cancel each other out. We could have changed the cam (or the compression with different heads), but we didn't. Save your poison emails telling us how wrong we are—we already know. Sometimes you do stuff in spite of the consequences. We just wanted to see what different pulley ratios would do to the power. Most car guys think that more ratio is better. We decided to find out. If you subscribe to the “more is not always better” approach to life and high performance, you might like what happens. But enough rhetoric. If you listen carefully, you can hear the Sirens singing. Crashing is optional.

Blown Small Block Chevy Boost Chart
Blower speed is a direct result of the combination of the lower (drive) pulley and the upper (driven) pulley. While this may seem simple, small changes in tooth count can have a drastic effect on power. Below is a simple chart we created based on the four pulleys we had. Unless you have deep pockets (each pulley costs a minimum of roughly $150), three or four pulleys can give you a wide range of boost potential. In the tech service section on Holley's website (, under supercharger instructions, you can find a chart in which Holley lists effective boost combinations based on engine family and displacement. For our blown small block Chevy 383 SBC, for example, the chart says that a 10 percent underdrive should produce 10 psi of boost. Our test showed 7.5 psi, which is relatively close, especially considering our cam has a tight (110-degree) lobe-separation angle, which tends to reduce the boost level compared with a 112- or 114-degree lobe-separation angle. Also, note in test 3 that adding nine teeth to the lower pulley changed the underdrive/overdrive ratio from -11.5 percent to +3.3 percent and caused the boost to increase by 4 psi. The total tooth count is important because there's a range of pulleys that can use the same belt length. Holley lists a complete ratio chart of all its pulleys in the blower instructions.

Test Bottom Pulley Top Pulley Total Teeth Under/Over Percentage Max Boost (psi)
1 51 63 114 -19 3
2 54 61 115 -11.5 7.5
3 63 61 124 +3.2 11.7
4 61 55 116 +10.9 14

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