belt slip?

Hello everyone. I just had my brothers 96 Cobra dyno'd, Vortech T trim with stock 6 rib belt setup with a RR 3.2 pulley. It made 529rwhp @6800rpm....on 93 octane....tuner wanted to pull timing out cause of sketchy ethanol pump gas these days...with 16 degrees it came to 493rwhp/405rwtq with about 13psi. The belt shows no sign of wear, no belt dust and the Motec data acquisition doesn't show any obvious signs of slippage. My problem is that the boost builds real slow. Is it posssible for the belt to be slipping(especially low rpm)even though there are no obvious signs? I'm ready to go 8 rib but do'nt want to be wasting my time. Thanks in advance.

13 psi is a lot for a 6 rib setup. I would imagine you'd see alitlle improvement w/ 8 rib. Plus you will probably see consistent psi.

However, the 3.2 RR pulley is a good pulley to prevent slip. If your not seeing really any dust and don't plan on more boost then stick w/ what you have.

If you see inconsistent 13 psi readings later upgrade.