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We were stoked to see the AFR heads deliver peak-to-peak gains of more than 50 rear wheel horsepower. The stock heads produced max hp at 5,400 rpm, while the AFRs were still climbing at 6,200 rpm when the tach light lit, causing us to automatically lift. At 6,100-6,200 rpm the AFRs were making nearly 80 more rear wheel horsepower. On the first few pulls we noticed the A/F mixture was too lean at 13.5 (12.9 to 13.0 is usually ideal). We came armed and ready for tuning with boxes of Holley tuning parts.