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Discussion Starter #1
I was at the dyno yesterday for a tune and my gates green belt was slipping badly above 6k RPM. I have a carbonite treated 2.75 upper on my 3.4 Whipple but the belt just wouldn't hold. I was told about the RPM belts but before I go buy one, I wanted to see if anyone here has ever used them.

The other thing I am going to have done is to have the upper pulley cross grooved to see if that helps. I hate leaving 50-100 hp/tq untapped.

BTW, this is on a 04 4v 4.6 Mustang if that makes any difference.
 

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How big is the crank pulley?
 

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I've started using an RPM belt recently and like it. Biggest difference between it and the Gates green belt is stretch. The RPM doesn't stretch. Works great with my automatic tensioner.

I always mark it where the tensioner sits right after install when I put a new belt on. The Gates and Dayco belts I used previously always stretched right after use. The last Dayco Gold Label I bought stretched so much that it ended up rubbing against itself when the automatic tensioner took up the slack.

Unless something weird happens, I'll be using RPM belts from now on. Not seeing any more boost with it versus Gates Fleetrunner or Dayco Gold Label but the durability with my set up is better with the RPM.
 

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Discussion Starter #5
This is an 8 rib setup with a stock lower pulley for an 04 Cobra(7.5"). I was running a 774 but the RPM belts don't come in as many sizes. I ordered a 780 and will take up any extra slack with a bigger idler. Thanks for the input BigDumbRedneck.
 

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I have never had an issue with the Gates Green FleetRunner belts. I spin a 2.8LQ Kenne Bell S/C over 24K rpm. Thats with a 9.5 crank pulley and a 3.1 blower pulley at 8K engine rpm. 8 Rib, never slips and one belt usually lasts me most of a season. I use a stock belt tensioner as well.
 

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Cross grooving the pulley will do nothing more than eat the belt faster. I've tried it. You're reducing surface contact by a small amount and adding damaging edges to the pulley. The secret is in the tensioner. Ever since I went nuts building a fully adjustable, hydraulically damped, ball bearing tensioner, I can dial out an belt slip with ease and with minimal belt tension (like half of what I used to run with a manual tensioner).
 

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Goodyear Gatorback belts are like rubber bands. The worst ones I've tested. Very "thin" relative to the other belts we're talking about and stretch by far the most - just holding one side by side with a fleetrunner or rpm belt will instantly show you the difference. They stretch more than even Dayco Polycog and other ubiquitous parts store belts. I have one, but don't even keep it in the car as a backup. I keep an off-brand belt (made in Belarus! ugh) as a backup. Even that slips and stretches less than the gatorback I've got. They're probably fine for accessories, but certainly not good for blowers. Unless you're looking purely from a longevity of the hard parts perspective - the slip and stretch is good for bearings and keeping blower speed down (and power, for that matter).
 

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Discussion Starter #13
^^ That's good to know. I picked up a RPM belt but haven't had the chance to get the car out due to 8+ weeks of travel and a surgery. I ordered a manual tensioner from BG Racing almost a month ago but it's still awaiting fulfillment.
 

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I've had the green belts work great and had them slip. As alex said it's got a lot to do with tensioner and I've found that pulley size is the most critical part to the function of the system. Running a very small upper pulley with stock idlers and tensioner and stock lower is a formula for slippage in my experience. Run a larger top pulley and increase crank pulley as well as idlers for more belt wrap and you will get the boost you want. Bottom line, simply a belt change won't cure your issue. Have you looked at the Thumperrrrr tensioners at all? Not sure they even sell them any longer.
 

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Discussion Starter #20
I just installed a Brettco Cog Drive designed manual tensioner. I just haven't had a chance to really stress the belt yet to see if it helps. I also installed the largest possible upper idler to get as much belt wrap as I could. I am hoping this will do the trick.
 
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