I'm being a good community member here and passing on the imperial shit ton of learning I have had over the last few days in discussions with Gates... This info in blower independent... rib count independent... but all about belts for all blowers. And yes, this is all about Gates belts - and yes - there are other belts out there - and it's fine if you love your belt manufacturer with a zealousness, I'm not trying to convert you... I'm just trying to get good info I never knew out to other folks to help good decisions be made, that's it... I have been wrestling with belt slippage on my Whipple 2.9 - not unique to the blower - all blowers can do it. So under the advice of a tuner I got a slightly shorter Gates belt of the standard V Groove variety. It helped a little but didn't fix it. So Whipple sent me the newer tensioner - and I installed the a shorter Gates racing RPM belt... and promptly stranded me and my wife less than 10 miles from home with a broken belt. So I started talking with the folks at Gates - and this is where the learning began... Most belts, like the 6 rib, are standard poly corded belts... they stretch by design - a lot. That's why we have these huge auto tensioners on supercharger setups - the stretch is massive and needs to be managed to keep the belt tracking. So here's an underhood video of a standard Gates belt (on a Camaro / Whipple setup)- which is really similar to almost all standard belts in terms of how it stretches... https://www.instagram.com/p/BTDAN8fDF6u/?hl=en Lots of people run these kinds of poly belts and have lots of success. Not bad. The typical rule of thumb is to set your tensioner in the 70-80% of its travel range so as the belt stretches (like in this video) the tensioner keeps it tight... and then when the throttle is let off - the tensioner can react back in that last 20-30% of travel and not hit the stop and get things back to where they should be. Then there's the RPM belt that is marketed for supercharger / racing... the one I broke. Now watch this video. https://www.instagram.com/p/BTHu2sCDAVe/?hl=en Very different behavior... these RPM belts use a core material that does not stretch (it does but very very little)... so the belt is more linearly stable - but it's also more "brittle". Also if you watch in the video when the guy lets off - the kinetic energy in the belt "bucks" the tensioner back a bit more... which makes tensioning very different. The correct way to tension a RPM belt is almost the exact opposite of a standard stretchy "poly belt" - only apply 20-30% of the tensioner travel to the belt... because it won't "stretch - that's fine when the belt is under load - but then when you jump off the gas and the belt unloads - there is a long way for the tensioner to move before hitting the full open stop. Here's a marked up image from my manual I made to show where / what I mean by these numbers Gates gave me. Regardless of your belt routing / SC type - if you have an automatic tensioner these %'s of adjustment apply to your tensioner with the different kinds of belts. The key with these RPM belts is, in metallurgical terms (which has nothing to do with a belt), since they are "harder" they are also more "brittle" - and unlike a stretchy belt that can bounce all over the place - these RPM belts don't like being hard stopped by a tensioner at all and a single bounce off a stop can damage the belts core and make it break - so proper tensioning is SUPER important. Finally from this guy's Instagram feed - here's a full dyno pull with the RPM belt - compare that to the YouTube Camaro video at the strip of the tensioner and a green belt flopping all over the place (yes I know they are very different conditions as the guy on the strip is murdering it to go and the dyno is more controlled until 4th - but the difference is pretty freaking obvious even so between these and the above links). RPM belt on dyno pull https://www.instagram.com/p/BT7V0rsDT6r/?hl=en Standard poly belt on drag strip The thought crossed my mind that a belt with less give is "harder" on the components, so I asked that - and it was pointed out - the belt isn't / was never designed to be a shock absorber - that's what the tensioner and the dampeners inside the tensioner are supposed to be doing. The reason it's a belt and not a chain is so that when you let off hard there's give and controlled slippage - so says the belt company that makes both types of belts... so - I think there's likely a lot of truth to that. Finally - there IS a break in process for a RPM belt... it's very involved... Put it on, tension it to that 20-30%... start the car... let it idle for 1-2 minutes... turn off car... verify you are still in that 20-30% range, if not adjust, if you are - go win races. I can tell you (if anyone reading this has followed any of my "issue" threads) that my car, with that RPM belt, for it's short life I gave it - pulled harder and showed more boost on the gauge than any other time out. This is probably why Whipple (and others) are switching their kits over to these RPM belts. I'm 100% putting another RPM on the car - it's in the mail as I type this, actually.