Ok i have a 05 mini cooper s, and it has a 17% alta pulley...and at 5500 rpm i lose boost and i hear the belt squealling. it has a new gater back 1365 537 belt on there now and it had a new napa 539 and a napa 535 and same problem....anywhere else i could look? bad tensioner maybe???
i am stummped...
cheers

 

From what I remember the 535 should be the smallest and work well for you.

I had a tensioner that looked fine, but was crap in all possible ways. The damper did nothing. If you see any fluid or staining on the damper, it may be doing nothing too.

I am not a fan of parts changers, though often that is what I end up doing. I don't want you to waste money or time on it, so wait for confirmation of belt size, but it the damper looks like it is leaking, it may just be the issue.

 

Yup, I second the 535 length belt....that's what I have on my 17%...perfect length!!
One word of warning...belts do vary in size a bit...so don't get too caught up in the which hole should I see on the tensioner debate...just put a good tight belt on...and if using an NON-OEM belt...you are...change it more often than recommended...IMO 1 per year on a 17%, every other year for a 15%...and all should be good!!

Some folks like the Gator Back...IMO try the NAPA/Gates etc belt first. It is very common, and matches the OEM profile....the gatorback is different...and is designed to solve a problem....specifically sequel for belt alignment....something that should be fix mechanically.
Check the tensioner...I bet the gas strut on it has failed....just a guess...but it is kinda common.