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Old August 16th, 2007, 11:20 AM   #1
Join Date: Jul 2005
Location: Orlando/Daytona, FL
Posts: 936
Default The 2.6" pulley and idler debate, fact or fiction?

I've seen many posts on here where people insist the belt will slip if you run a 2.6" pulley without the auxillary idler bracket made by Metco (and now a few other companies as well). Some have even warned of damage. Based on the testing I've done this is untrue.

I datalog boost using my Dynojet, I just don't believe you can safely and accurately watch the boost gauge and RPM while accelerating a 500+RWHP car. Anyways, the boost gauge in our cars is based off inferred boost calculated by the computer, there is no sensor.

I've ran GT500s on my dyno several different ways with the 2.6" SC pulley. I've found that average boost is the same regardless of no idler, 90mm idler in place of stock 75mm idler, and with metco aux idler bracket.

You can see that average boost is the same regardless of idler used. The car with LTs was down about .5psi over everyone else.

If you are running a KB with a 2.5" pulley then the extra wrap of the aux idler with bracket may be of benefit to you. However, with a stock blower and 2.6" pulley I don't see the need for it.

Some have claimed that belt stretch over time will result in slippage, but I believe there will be very little stretch due to the massive 10 rib belt (a design originating from the Ford GT).

Ford designs that belt to last 100,000 miles.

My car is coming up on 3,000 miles, each time I have it on the dyno I'll continually check for belt slip and boost loss. At this rate its usually up there every few weeks to try different parts and combinations.
Justin Starkey - VMP Tuning - Ford & GM Tuning
Mobile DynoJet 224xLC Load Control
Justin@VMPTuning.com 321-206-9369

2007 Shelby GT500 Ford Racing TVS Blower, VMP 2.6" Pulley, IW 10% OD lower, TB, 697RWHP/702RWTQ
2006 F250 6.0 14.3@93
2000 Mustang 3.8 A4 TT 450RWHP@17psi & 10.9@127.8 1.74 60ft@17psi stock motor
2006 Mustang 4.0 M5 Vortech 15psi 395RWHP 12.5@110 wife's car
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Old August 26th, 2011, 05:39 AM   #2
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Join Date: Oct 2010
Location: Netherlands
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I forget to replace the 90mm idler when swapping the 10% OD and have a lot of belt slip. As it is my daily driver, i ride it and with slipping results.

Then i replaced my 90mm idler and the stock tensioner with a upgraded version and used the old pully from the stock tensioner, i leave the belt i just replaced and ride for 500 miles with slip, and the car now has again belt slip.

Can i have ruin my belt and will this problem go away when swapping to a new belt agian?
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