Does anyone know if the lower pulley modification gets more HP then upper SC pulley upgrade?

 

The lower crank pulley is a night and day difference , the upper will slip and a waste of money
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Thanks for the video and info. Lower pulley seems to be the better bang for the buck in HP.

 

It depends on which XF SC engine you are talking about.
If the AJ133 5.0 V8 then not a helluva lot of difference both power and cost wise between the two pulleys although there is less hassle fitting the lower/crank pulley and it is fully reversible while the upper/SC pulley is not usually.
On the other hand if the AJ126 3.0 V6 then definitely more bang for your buck with the crank pulley as the stock SC pulley is already near to as small as you can go so not much leeway or improvement with a smaller SC pulley. Also there is the slip problem although Velocity AP now sell a new smaller SC pulley with a new "Grip-Tec" coating which they claim eliminates any slip.
If the older 4.2 V8 SC then I don't know much about one pulley vs the other although I suspect it would be similar to the 5.0.
Last but not least VAP now do a package for the 5.0 using both pulleys for 660 hp.

 

If it's the 4.2 V8 then the top pulley is the one to change, not the crank.