I think my belt is slipping, but don't want to over tighten it. The brackets I am using have a jack screw tensioner, not a spring loaded type. The install guide says "set the belt tension," but doesn't give a deflection/lbs or tension figure. A Dayco belt table says 270 lbs of tension, but that seems like a lot. Anyone using a 12-rib have some tension numbers they go for? (4.75" top pulley, 7.625" bottom pulley I believe.)

we install them at the shop never done a 12 rib but the 8 rib belt u want just enough tension so you can barely "twist" the belt with your fingers thats how we do it no problems so far good luck

Hi, I am building a blower motor for my 85 firebird. I am going to use a procharger. The choices are F1R or F2, if the F2 fits. Would you be willing to post a pic of your engine bay with the F1R?
Thanks. My e-mail is calabria1@prodigy.net

I am using the F1R on a big block (standard deck height), so I'm not sure how helpful this will be. The SC plate came with the holes in it for the F1 and F2. I milled off the outboard section with the F2 holes to make room for an air passage underneath to feed the SC. I also removed the wind shield washer tank, and cut out and flattened the raised portions of the inner fender that mount that tank. Even with that done, I had to curve the inner fender in a bit near the strut cap to make room for the impeller housing, which now clears just barely. I am running 275/35 18's on the front, set in to where they just about hit the struts, and don't have tire to inner fender clearance problems. I think I would have had to work a lot harder to fit the larger F2 in there, and might have had unsolvable hood clearance issues. (I am using a factory hood, and had to clearance some of the reinforcing sheet metal so it wouldn't rub on the SC pulley.

Maybe you can compare the big block bracket dimensions Procharger gives to the small block ones to determine if the added size of the F2 will fit in your case. From the charts though, I went with the F1R because it looked like it had all the flow I would ever want. With the pulleys I mentioned, which are a really mild step up ratio, on 496 ci, I had 6 psi at about 4000 rpm.

Will still try to post some pics when I bolt it back on to the new (slightly larger ) motor.

I know mine used all the travel possible in the tensioner. Proper belts would almost not go on, and the tensioner would have to go as tight as it could.

Cranks out 7.5psi with the bigger motor, and I think the belt is slipping, cause the boost drops off consistently at 4300 rpm. Currently, the tension is only tight enough that you can turn the belt 90 deg. by hand on the long side. I'll be tightening it up a little tonight to see if the boost drop off changes.

2 runs before:
7.5 psi @ 4500 (4 - 3 psi 5000 to 5600)
7.0 psi @ 4250 (4 - 3 psi 4800 to 5500)
with the boost falling off somewhat sharply, and even a slight rpm spike in the 2nd pull.

1 run after tightening the belt some:
9.2 @ 5200 with the boost falling off more gradually after that to 5.2 @ 5600

I'm guessing I still don't have the belt tight enough, but it is hard to tighten with the jack screw covered by intake parts. Wonder if I did any damage to the belt with 100 miles of track time only getting 5-6 psi?

I don't think a slipped belt would be an ideal thing once it has slipped. You have 4 options left now: 1. Tighten belt further, 2. More belt wrap around the pulleys...add an idler, 3. Use a stickier belt...Gatorback belts seem to grip better, 4. Cog belt/pulleys

What I've done to prevent slippage
- Spring loaded tensioner
- 12-rib
- new bracket, stock ATI setup for serp-belt setup was flexing.
- I use cheap dual 6ribs now with "beltgripper" spray

Will order gatorback belt now When I know the lenght

Thanks for the info fast82z and gta324, I may make up a spring loaded tensioner or an idler and put it up close to the SC pulley. I have ~56% wrap on the pulley now, and with a close 3" idler, I could get up to ~63%, 13% increase in wrap. I just maxed out the tension adjuster, so one or so more test runs should show if it slips before my leisurely 5500 shift point.

With my 12 rib setup I have to tighten it up pretty hard I even had a new tensioner pulley made thats bigger cause like you I ran out of adjustment.

If you're very concerned about that F1R slipping, you could just give it to me.

That's a nice setup you've got there..

Yea, it might be time to make a longer jack screw, or an extender, as it is now maxed out, but I think I got enough tension for now. It is to where (like tom86iroc said he had success with) you can barely twist the belt. Now I have no drop off, and 11.2 psi at 6000. Thanks for the confirmation Tony89GTA on the tightness, I figured with the surface area of the belt, and the size of the pulleys, that it wouldn't need this much tension. I guess it takes a few HP to spin it 52,000. Now I see why a cog is about necessary for the F1s or bigger, or to spin it 68k.

On my set up I use the stock tensioner pulley to tighten up the belt and the "realese" the spring loaded one. The Spring loaded is placed so that when its maxxed out and the stock tensioner is all the way up, the belt wont touch "itself" thats a big NO,NO

Belt takes a nice trip and the long stock bracket to head bolts may go off