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fitman 06-30-2012 11:22 PM

Blower pulleys configuration?
Hello everyone!
I have a 78 vette with a basic stock/cast 350. Recently I obtained a weiand 6-71 blower w/ 2-600 holleys. My question is can I run this blower underdriven at just 5-7 lbs? My mechanic is a forced induction freak. He assures me that i'll be ok with running the blower on my current engine as long as i dont get "foolish" with it. So my question to you all is what blower pulleys do I need to run to reach my target boost of 5-7lbs?
The blower came with a 39-cog upper pulley and a 31-cog lower. The person I purchased this from advised me that pulley configuration is what Im looking for, just need some confirmation from you guys first please.
Any help is greatly appreciated!

68NovaSS 07-01-2012 06:18 AM

2 Attachment(s)
Welcome to the board.:thumbup:

Here are Weiands' tech sheets, these blowers come underdriven to make 5 to 7 pounds. You have the older style it looks like, 1/2" pitch pulleys vs. the 8 mm. I think your one pulley must be 32 not 31, unless it's not a Weiand part.

NOlowrider 07-01-2012 09:21 AM

If your pulley's are 1/2" pitch (square cog pattern) with a 39 upper and a 31 lower you will be at roughly 20% underdriven. I don't think you will make your target boost with a 20% under drive. If you swap them to 39 lower and 31 upper you will be at roughly 25% overdriven, which I don't think is a good idea on a bascially stock engine. Most people run a 10% underdrive which would be a 34 lower and a 38 upper. I would shoot for that combo to get your target boost. The pic below is a 8mm pulley.

1/2" pitch


fitman 07-01-2012 04:46 PM

Thank you gentlemen for all your responses.
I definitely have the 1/2" old style/square pulleys.
Where can I find a 31-32 pulley with a triple groove?

NOlowrider 07-01-2012 05:21 PM


Originally Posted by fitman (Post 1570127)
Where can I find a 31-32 pulley with a triple groove?

You are not going to find a blower pulley with any grooves for the accessories. (water pump. alternator, PS pump, etc). This is where the tricky part comes in. If you want a three groovelower acc pulley you have to offset that extra distance with a longer blower snout. Meaning, if you have 3 grooves on the lower acc pulley then the blower snout will have to be longer than the snout for 1 or 2 groove lower acc pulley. Hope I didn't confuse you. You basically have to set the blower on the engine, hang the blower belt on the blower upper pulley and see how far it is from the harmonic balancer.

Kinda like this below.

fitman 07-01-2012 07:45 PM

thanks NOlowrider for your post. (BTW...gorgeous motor!)
I guess i'll prob have to go with a single belt set up that'll work PS, water, alt, & harmonic balancer. As i mentioned above, my 31-tooth lower pulley has a single groove.

fitman 07-02-2012 12:28 PM

FBird88-can I still run my alt, ps, water, & hb with my single groove pulley that I already have? Is it bad to have that many pulleys running off 1 belt?

68NovaSS 07-02-2012 02:07 PM

2 Attachment(s)
I was able to run two groove pulleys on a short WP and crank, behind the lower blower pulley on an 8-71 Weiand, SBC. I used ZOOPS adjustable brackets for the alt and power steering. The alt bracket is actually a high mount ZOOPS turned upside down and clearanced for the valve cover front. my configuration is wp/alt/crank and wp/ps/crank.

The outside V-Belt is pretty close to the blower belt, about 1/4 inch, but does clear. The pictures here are hard to identify, and I have the motor apart right now.

NOlowrider 07-02-2012 04:57 PM


Originally Posted by F-BIRD'88 (Post 1570323)
Most people end up ditching the P/S and install a manual steering box.

Yep, that's what I did. Sucks, but I guess I can live with the trade off.

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