When is a bigger idler necessary. TVS/Whipple

[The Red Monster]

Hey guys just wondering when a bigger idler pulley is necessary and how can you tell that you have belt slip on these setups?

I am running a 3.00 pulley on my whipple and I was just wondering if and when it would be necessary to run the 90mm idler.

 

[vortecd]

It is never a bad idea to run the biggest one you can to get the most belt wrap. Someone may say they never get slip and someone else with the same setup does. I have always had the best luck with Napa/Gates belts slipping the least

 

[Vert]

I would think it would be a good idea since you are running equivalent to a 76mm pulley.