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Old 06-20-2009, 12:00 PM
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Blower pulleys

How do you tell or how can you make a blower under-driven or over-driven ? And amount of pounds ? Do you count the number teeth on the crankshaft pulley and the blower . Our if the crank pulley is smaller make it under-driven ? Like mine has 32 on the crank and 35 on the blower. I am figuring like a 10-12lb boost. On a small block with a 8-71 BDS. I like to look for some pulleys. They have lots on ebay. But have no idea what numbers or sizes to look for. Also I have only one vacuum port and that is for my trans. modulator. I have it hookup to the intake near the dist. But was told it needs to hookup to the bottom of one of my carbs. Which is correct ? THANKS !

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Old 06-20-2009, 12:29 PM
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Old 06-22-2009, 08:39 PM
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At the base of the carbs you will always have vacuum, if you tap into the manifold which is below the blower you will have vacuum at times and boost at other times, this is not a good place to connect anything that is looking for strictly vacuum.

If the blower pulley is smaller than the crank pulley the blower is overdriven. If the blower pulley is larger than the crank pulley the blower is underdriven.

The size of your engine will come into play and be a large factor in how much boost will be made. I would guess you will need to be ~6-8% underdriven to be in the ~10Lbs range assuming it's a 350ci engine. You will have to adjust that according to your compression ratio, fuel type, and what your parts will support.

There are other factors that will come into play as well, boost is the measure of back pressure in your manifold, cam timing can and will effect your boost, head flow will effect boost as well.
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