underdrive pulleys on the cheap

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Badvert65

Badvert65

Long Time Mopar Nut
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OK, this is for racing only, drag racing in particular. There was a Hot Rod magazine article about the aluminum slant 6. When they drag tested it, they discovered that if you remove the belt on the water pump, you will gain about 20 HP. With that in mind, I figure that under drive pulleys should be of some benefit. since they are not readily available, I looked at some items I had lying around. This is what I came up with for practically zero cost.
IMG_1386_zpsekz85qnd.jpg

The crank pulley is the pulley off of the AIR pump from my 79 Volare super six. I just had to drill the holes so it would bolt to the balancer. This will yield approximately 25% reduction. This was zero cost, but you can buy the small crank pulley for a Chevy V8 that will bolt up and do the same.
The water pump pulley is actually a slant six accessory crank pulley. I used a steel washer (9/16 I think) in the center and reamed the hole out to 5/8 for the water pump snout. then I just drilled the holes for the water pump bolts. This pulley is too large to be powered directly by the balancer, but the offset clears the balancer with room to spare. I did use spacers on the modified AIR pump pulley to align things. The larger pulley on the water pump will give an additional 10% reduction to the water pump.
My alternator has a double pulley on it and the front most groove will line up with the modified pulleys. Total cost was one flat washer.
It isn't for everyone, but for the drag racer out there, every little bit helps.
 
Here is the top pulley, before and after modification.
IMG_1387_zpsr362gdxu.jpg

The flat washer was welded in from the back, but it was such a snug fit, it could probably be glued or staked in place.
 
For those of you wondering...
Factory balancer is 7inch dia, water pump pulley is 6.5 inch dia and alternator is around 2 inch dia
This means at 1000RPM (crank), the water pump is going 1077RPM, the alternator is going 3500RPM
If you put a large diameter 'hemi' pulley on the alternator (3 inch dia) the alternator RPM drops to 2335.
Changing only the small crank pulley (5.25 inch) the water pump RPM drops to 800 and the 2 inch alternator drops to 2625 RPM while the 3 inch alternator drops to 1750 RPM.
With the small crank pulley and the extra large water pump pulley (7.125 inch) the water pump RPM will drop to 740 at 1000RPM crank speed.
Make what you will of it, these are just the numbers.
 
Certainly accessories rob a lot of power. That is why the engine ratings dropped severely in the 1970's when the feds made them change the power ratings. Previously, they would run an engine on a dyno w/ no belts, supplying water and power from external sources.

I have read that some Hemi truck owners had the integral "toner ring" on the crankshaft shatter from using under-drive pulleys. That is the pickup slots for the crank sensor. Not sure why they call it that, but in this case it set up a resonance like a tuning fork, which made that name very appropriate. At least you have no such concerns.
 
This is the kinda stuff that makes hot rodding great. I am very impressed. You took junk and turned it into something for a nothing cost. Great job.
 
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