I’ve called Whipple myself about this and you can to just to confirm what I’m about to say. So technically whenever you are changing the air to fuel ratio or timing? Yes. BUT after speaking with Whipple. Per Whipple customer service. It is safe with the Whipple canned tune to go down one pulley size because the ECU and variable timing will learn how to adjust to the extra 1 pound of boost going from size 4.0 down to 3.875. They said since it’s only 1 pound of added boost that it’s ok. So I pushed the needle a little bit and asked if I could go down to a 3.75 size pulley from a 4.0. They said that would be the lowest, safest pulley I could go down to. Any lower would risk engine problems and possible damage. BUT since I had purchased and planned to install my Stainless Works Hi Flo Catted Long Tube Headers. I went ahead and not only dropped down from a 4.0 size pulley to a 3.75 but I also went with a Reische 170 degree performance thermostat, new pulley belt and a tune from Ken over at Palm Beach Dyno.
When I installed a new pulley I just reused the bolts. My Whipple came with a 3.875 pulley from the factory and I installed a 3.75 when I went got a custom tune from Oz.
Thank you for the info ordered the new pulley today. Whipple said I didn’t need to get a new tune for one size down.
