He's got an inaccurate reading dyno.
The dyno shop clearly needs to calibrate the dyno correctly, or they just like customers to feel they have super special cars and hope that brings in more customers who want dyno glory for bragging rights.
You could take his car with it's supposed 402 rwhp and a guy who took his stock 5.0 to a different dyno that gave him 350 rwhp and do some rolling runs and guess what.....if they have the same gearing and same octane, they'll run pretty much side by side with each other.
50 rwhp difference on a SAE certified, balanced, computer spec'd and built motors is not going to happen.
50 rwhp would mean one 5.0 could pull multiple car lengths on another in a rolling 40-120 mph rolling run.
It ain't going to happen with equal cars and equal drivers guys.
It's purely bad calibrated dynos.
Same goes with Shawn's or anybody elses (like Inside Line, etc) whose dynos gave them 390 rwhp stock. It's just a high reading badly calibrated dyno, plain and simple.
i totally agree, normally ill dyno at 2 places, add the two numbers and divide by 2 and that should be close enough to your power
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