Have some questions about BPS idlers/setup

I called BPS and spoke to Matt because I had some questions about idler size, he said they actually sell a complete LW pulley kit for the GT500's. He said you get the choice of a 76mm or 90mm idler and he said you use the 90mm if you don't have an auxiliary idler kit, and a 76mm if you do. Also said they don't make a replacement idler for the stock tensioner, only for ARS and Thump.

So I get on their website, find the kit, and have more questions. Matt was busy so I talked to another guy. I wanted to know if with a Whipple with a 3.0 pulley if I need the auxiliary idler (currently have the Metco kit with my stock blower), or if I'd be better off going with the 90mm and not running the auxiliary idler. This guy didn't even answer my question and told me to run the 90mm if I had an auxiliary idler which conflicts what Matt said, and everything I've read. I had some other questions, but when I got that answer I didn't even bother to ask them lol.

So my questions are:

1. For the SVTP audience, is the kit worth it? I need to replace the grooved pulley above the AC compressor anyway since it has a lot of play. The BPS is actually less than the Ford part so that's a no brainer. I'm sure there's some minimal power gains but I'm more interested in the fact other bearings will probably start having issues too (haven't even checked all of them) and that these are serviceable in the future too.

2. I've read in several threads including their 4 page thread on here that you can replace the stock SC tensioner pulley with one of their 76mm's, but Matt says they don't replace that one and it doesn't fit. Can anyone confirm one way or another? Is the ARS or Thump really worth the upgrade?

3. What's your thoughts on a auxiliary/snub idler for my setup (2.9L Whipple most likely with 3.0 pulley and 10% lower)? I currently have a Metco, the OCD in me would kinda like them to all match though and it's not like it would be hard to sell the Metco for a few bucks, hell it might even almost pay for a BPS. Is the auxiliary idler needed for my setup though? Personally I think it would look cleaner without an idler up there, but is there any doubts as to whether I'd have any belt slip?

4. If I go the BPS route for a auxiliary idler kit, which one do I want? The regular one or the Whipple/KB one? Their site says the Whipple/KB one was designed for the 4.0 Whipple but works with any Whipple/KB but I've read conflicting info elsewhere and to run the regular one with a 2.9.

5. Can anyone clarify which idler pulley (76mm or 90mm) works with or without the auxiliary idler since I got conflicting info their too?


Thanks in advance.

I'm running quite a few of the BPS pulleys. They're top notch pieces. As are as different pulley diameters, there are different reasons for bigger and smaller pulleys. The 90mm idler and the auxiliary idler on the supercharger are intended to take up belt slack when using a smaller upper pulley. The auxiliary idler promotes better belt wrap with a pulley the size of a 2.4", but the larger the pulley diameter, the less you need one. I'm running a 3" upper on my Gen 3 TVS and I removed the auxiliary because the pulleys were too close together. The bolts for the auxiliary pulley are a bit too undersized for a lot of stress too and I don't think the 3" needs it, not with 18 psi anyway.

The smaller pulleys on the belt tensioners are intended to make more clearance for oversized lower pulleys. The aftermarket belt tensioners have better springs than the stock tensioner and a lot stronger than the cast stock piece. I'm running all stock sized idlers, and my Thump tensioner has the 63mm stock idler it came with. I'm running an ATI 10% lower. I'm running an 85.3" belt.

I have no experience with BPS lightweight pulleys, but I believe GT Premi installed the full set on his GT500. For a 2.9L Whipple with a stock lower, 3.00" upper, and stock length belt, use a 90mm idler and no aux idler. With a Gen 4, you will see 18-20#s of boost with upper only depending on supporting mods and RPM, probably higher if spinning past 6500 RPM. I recommend speaking with your Tuner to see what pulley combo they recommend for the application and type of fuel you will be running before purchasing a new lower balancer. Personally, I prefer the clean look without an aux idler, and when I removed it years ago the bolts were working loose. My new Whipple has been installed since January, but I haven't been able to take my car to my Tuner for a dyno tune revision... maybe in the next couple of weeks. Here is a 3.00" upper with 90mm idler... plenty of belt wrap:

Thanks guys, I knew this would be the place to ask. I’ll go ahead and get the 90mm and delete the metco auxiliary idler. I too like the look better without the idler in place, I think it’s pretty fugly personally.

Already talked to Lund and with my current ATI 15% lower he recommended a 3.25” upper so with the 10% IW it should be perfect with a 3.0”. He said that would put me in the 18-19psi range. I wouldn’t mind it being more because I may run Torco anyway.

I’m thinking hard about the ARS tensioner but I’m not sure I could get it off if it had any more tension than the stock one, that ****er took 3-4x the amount of force to get it to cam over than a terminator tensioner.

Can anyone confirm if any of the BPS idlers indeed work with the stock tensioner? I’m more concerned about more bearings going than I am the tensioner not being strong enough.

Just an FYI, IW's 10% is actually a big bigger than 10% and equates to 12% overdrive. 7.1" x 1.1 = 7.81" IW is 7.95"

If your tensioner was that tight, your belts too short. You want some range of adjustment. No adjustment and you'll start breaking stuff.

Might explain why the AC clutch bearing and grooves idler bearing went tits up lol. It seemed to rest about in the middle of its travel, the print is just really damn stiff compared to what I’m used to.