I've read alot of blower threads but can't figure it out. I new to blower and would like to build a 671 for a 331 hemi im building. Ive always heard about the boost but all I read is about underdrive or overdrive. And I not real sure what it means. Can some one explain it to me please? Im wanting to build a blown street hemi and would like the boost around 4-5 pounds. So how do I calculate/translate the teeth count or the underdrive/overdrive? Thanks in advance.
Underdrive-less than crank speed, overdrive-more. B.D.S. and Dyers have charts that can get you in the ballpark.
The boost depends on 3 things: Blower size, Engine displacement, and Drive Ratio. There are charts you can find to figure out what drive ratio you need to get a certain amount of boost on a certain size engine with a certain blower. When you figure out the drive ratio you want, which will probably be around 30% or so under for such a big blower, small engine, and low boost, you figure out the pulley sizes. You run a small lower pulley and large upper pulley to get underdrive, for example 33 teeth lower, 44 teeth upper for about 33% under drive. This will still probably give you more boost than you want, but it'll run pretty good.
Squirrel has it right...I get a lot of parts & info from Good Vibrations. www.dragparts.com. Very helpful.
I just called dyers yesterday and asked about running 5lbs of boost on my 377 small block Chevy with a 6-71 and he told me I would be around 25% underdriven.
yeah, or maybe a little more underdriven. It wouldn't hurt to talk to whoever is building the blower...ask what they think.
1/2" pitch or 8mm belt ? You have to decide on the belt, then someone will be able to tell you the pulleys to use...
I would run about 6 lbs boost 8 lbs tops for street driven motor. also think about the pistons/rods also I would double key crank hub. The under drive is because a 671 will handle 426 cubic inches. you have 331 inches
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