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Discussion Starter · #1 ·
so I'm installing my 6lb pulley ring over my 4lb ring. first thing is i take my 4lb off and put it next to the 6lb and only a 1/4 in of difference. 8.75 in vs 9 in. did they make a 5lb pulley?? also I'm swapping out my 100mm single idler off of my LFP aux idler to a double bearing 90mm and the second bearing has nothing to sit on, do i need to buy a different cap for the bolt to fit the double bearing idler?? thanks in advance and don't worry guys I have a BA2600 and custom tune waiting the install. :bigtu
 

· Stock block 11 second club
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The factory lower is 7.5". The general rule is that every 1/2" increase in diameter size equates to 2# of boost. Although there are slightly variances in pulley measurements depending on the brand, the generally accepted measurements are as follows.

OEM upper Lightning pulley = 2.93"
OEM upper HD pulley = 3.16"

2# upper pulley = 2.78"

2# lower pulley = 8.0"
4# lower pulley = 8.5"
6# lower pulley = 9.0"
8# lower pulley = 9.5"

Yes, there are some 3# and 5# pulleys out there.

Boost is relative so the best yardstick to gauge pulley combinations is use of pulley ratio which is exactly what the name implies, the ratio of revolutions made by the SC pulley in relation to the number of revolutions made by the crank pulley.

To determine pulley ratio divide the diameter (in inches) of the lower pulley by the diameter (in inches) of the upper pulley.

You can even go one step further and determine the SC speed at any given engine RPM by multiplying the RPM by the pulley ratio.

As you might already be aware, refer to the pulley diameter at the belt surface, not the lip-to-lip measurement.
 

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Discussion Starter · #3 ·
thanks Throttjockey, got everything installed, the for the double bearing pulley i needed a different bearing cover to included the front bearing, i bought that from metco. now just waiting on a response from JJ about the idle datalog.
 
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