crank hub / balancer question for a blower

another thing to help snap off balancer is how tight you have your drive belt,alot of guys run them to tight so they can hear the whine, but as the engine heats up it will expanded the alum. causing exessives pressure on the front of the crank...and then snap.. have seen alot of guys come in to dyer's shop with parts becuse of this.

 

Blowers are known to "sneeze back" on start up, especially when cold. This is the most likely time for a failure to start as it causes sudden shear forces on the snout and key. I would highly recommend double keying whenever possible. Most street motors can get by with a cast crank that is double keyed and a steel hub. I would never build a serious blower motor without a steel hub or balancer and a double keyed steel crank.

 

Im definetly buying the steel hub for the 400 small block, Only reason Im trying to do this without the second key is the motor is already in the car and running. If the motor was sitting on the stand, I wouldnt hesitate on pulling the crank and having it cut for 2 keyways. Im going with the smallest 1/2" pitch pulley on the crank, which is 30 tooth. and the blower pulley will be 37 tooth. I even considered drilling a hole parallel with the crank where the hub meets the crank and tapping it and installing a 3/4" long set screw in it to act like a second key. But I dont think I want to mess the crank up. jim

 

I have had issues in the past. I blew the balancer right off of my blown 6-71 SBC. I had to dish out a grand for a new steel crank and machine work to double key the crank. If you can avoid issues now it is worth the extra time to do it.

 

You do not need a double key way with a 6-71 and sbc cranks, cast or forged. I have drag raced and street driven blown sbc and bbc for years. Unless you plan on running a high helix teflon stripped blower thats making 20lbs of boost your single key is plenty. Alot of people have balancer problems with blown engines, You must have the proper press fit on the crank, some balancers and hubs require a slight hone to acheive this press fit. Buy a nice balancer or blower hub. None of our blown injected engines have ever been double keyed. A sbc snout is so short and thin, you do not want to remove more material. The reasoning for a extra key is that at high rpm and the resistance of boost in the intake along with the resistance it takes to turn the blower at a given rpm, the key is keeping the blower belt from blowing the key out.

 

As stated earlier, almost every blower engine i see on the street has the belt to tight. It should be loose, 3/4 to 7/8 of flop on the pull side when the engine is off

 

When you get to about 10% under drive with a 6-71 , you should highly consider double keyway
At 1-1 you should double key for sure
At 10% over with a 6-71 , you are ok with a double keyway, but with a 8-71 you should consider two things : A Crankshaft support , and/or a Big Block Snout on your crank.
At anything higher than 15% over with a 6-71 and 10% over with a 8-71 all small blocks should be equipped with a crankshaft support.
At 20% over with a 8-71 , 10-71 14-71 all Big Blocks should consider a Crankshaft support.
* Why ? - From the minute a blower begins to turn, it pulls the crankshaft up against the top main bearing- the more the boost or speed , the worse it gets. On a small block, you can snap the snout off the crank, on a big block, you will eventually beat the top main bearing out of the block-
More speed,,,,, more of a problem.
The Crankshaft support acts like a 6th main bearing out in front of #1 main bearing, causing the crankshaft to be supported as if it is in a cradle .
We sell crank supports for Big Block and Small block Chevies, and we are about to introduce our LS Chevy support next month.