I'd recommend about 14-15 degrees. I have the E-303 and I run my car at around 12 on 93 just because mine doesn't like the extra timing. I don't really feel any noticeable power increase, and the car runs noticeably rougher. It won't even run 16 degrees without upping the fuel rail pressure to stop it from pinging badly.
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'91 Mustang GT 5.0 AOD Daily Driver
Current stats: NEW 302 short block, E-303 cam (@.050, 220/220, .498/.498, 110 degrees), 1.6 roller rockers, Performer RPM heads (aluminum, 1.9/1.6, 60cc chambers), Cobra intake, Granatelli 75mm MAF, Billet AFPR, Equal-Length Shorty Headers, Off-Road X Pipe, Flowmasters, 4.10 POSI
13.73 at 103.8, ~2.2 60'
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