overdrive pulley question please help!!

beeturbo

Registered User
ive got a 90'SC auto! ive already did the exhaust with flomaster cat back system now considering a 10% overdrive pulley ! im kinda confused .. i understand that the smaller overdrive pulley is designed to spin the supercharger faster for a boost increase ; ihave 3 questions in this matter?
1.) how much more boost will i see with the 5% pulley or the 10%pulley in compared to the stock pulley.
2) i having it hard understanding this concept.. ifthe overdrive pulley for the jack shaft is designed to speed up the supercharger for more boost , their are available underdrive pulleys for the accessories that whereas the crankshaft is allowed to spin the accessories slower to retained robbed horsepower. so then if that is tru in slowing down the accessories with the underdrive pulleys the super charger would turn slower as well ,but adding the 5 or 10% pulley seems to me it would only bring the jackshaft back to the normal ratio.. please someone helpo me wit this cause im fairly lost!
3) how much of a horsepower gain can be expected using the 5% or 10% overdrive pulleys.
 
My $0.02....

The crank pulley in a set of "underdrives" is the same diameter as the stock unit. The water pump pulley and the alternator pulley are a slight bit larger than stock in the underdrive set. The larger diameter is what "slows" them down. The SC is driven off the jackshaft pulley, which is driven off the crank pulley. If the crank pulley diameter is the same for both stock and underdrive setups, then either reducing the diameter of the SC Jackshaft pulley, or reducing the diameter of the SC pulley will overdrive the SC. I think reducing the diameter of the SC pulley is the more common approach....
 
The blower is run off the jackshaft pulley, which is run off the crankshaft pulley....

I installed 5% jackshaft and 5% blower pullies as this is suppose to reduce slipage of the blower belt....

The jackshaft to crank dimention remained the same and all accesories are driven as they were originally intended.....

I was able to see an increase in boost from 7 lbs. up to 12 lbs., but this is from an old blower with 125,000 miles on it...... :eek:

Eaton says that our blowers can take the additional rpm's, as well....

I wish I had a dyno run before and after, as there is a noticable increase in power....... :D
 
Beeturbo, You are being confused by a basic misunderstanding of the way the system is designed, and also by some incorrect info.
The drive for all the accessories ie: water pump, alternator, a/c, powersteering are driven by a separate serpentine belt. The supercharger is driven by it's own belt "system". The pulley on the harmonic balancer is a double pulley. One half drives the accessories and the other half drives the supercharger. When you buy a underdrive pulley set the crankshaft pulley for the accessories is indeed much smaller than stock, but the diameter of the supercharger pulley is the same size as stock. So the best way to overdrive the supercharger is to either overdrive, (smaller pulley), the jackshaft pulley or smaller pulley on the supercharger itself. So you can see that it's possible to underdrive the accessories without affecting the supercharger in any way. BTW, if you want to stay away from detonation issues the 5% pulley is the one to use on a mostly stock engine even with exhaust upgrade. The 10% will require a better intercooler and possibly a chip and tuning. John
 
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