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fitman 07-01-2012 12:22 AM

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 07:18 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 10:21 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


F-BIRD'88 07-01-2012 03:16 PM

That blower pulley combo is a good starting point.
you want low modest safe boost. If its not enough you can up the program later with a pulley change.

recurve the distributor to limit the spark advance under boost to 28-30deg @WOT.

blower motors like a healthy amounto of idle spark advance (20-26deg at idle)
but must use limited WOT spark advance under boost on pump gas.
(28 to 32deg @WOT under boost)

vacuum advance ( if used ) goes to the intake manifold below the blower.
the holley carbs will need rejetting and the power valves will need to be boost referenced. simple Carb main body modification.

The stock GM cam in your base 350 vette motor (L-48 or LM-1 350) is not blower friendly and will really limit the performance with a blower.
You want to change that.

get some thing that is street blower friendly

224-234@.050 112 to 114LSA .450" to .490" lift
EG crane 113801 H-278-2 ( HMV278-2)
OR Isky 201271281-12
or summit cam #1105
or Howards 110951-12

or something similar.
Get some hi perf replacement valve springs too.

fitman 07-01-2012 05:46 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 06:21 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 08:45 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.

F-BIRD'88 07-02-2012 01:17 PM

Usually with the 6-71 871 style blowers and drive setups there is no pulley provision for air conditioning.
The SBC kits use a short style water pump.

If you were to integrate more pulleys behind the crank blower pulley you would need to space out the top blower pulley. This will put more stress on the engine's front crankshaft main bearing. Its bad enough already.

And more stress on the blower drive snout bearing.

All these blower drive kits should have been upgraded to the newer flat belt style alternator/water pump pulleys.
much better than the Vbelts and V groove pulleys.

fitman 07-02-2012 01:28 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?

F-BIRD'88 07-02-2012 02:31 PM

Not the power steering.

The one belt will do the alternator and water pump with the right brackets and pulleys and the short pre 1969 style passenger car SBC water pump.

You may have to contact the blower shop/BDS, Dyers etc to see what they got for Corvettes and PS and AC.

A custom built drive/accessory drive may be required. This has always been an issue with the typical off the shelf roots blower kits on street cars w P/S. and A/C

Most people end up ditching the P/S and install a manual steering box.

There may be a ready solution. You are not the only one with a Corvette than wants to run this blower, for sure.

68NovaSS 07-02-2012 03:07 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 05:57 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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