So here's the uncut graph. Three sets of runs all overlaid. Same dyno, same car.

2018-09-04 All Together.jpg

Stock Car Peak Numbers:
542.23 hp, 500.57 ft-lb
549.30 hp, 501.64 ft-lb
553.67 hp, 504.19 ft-lb

The temperature in the shop was +92°F. IATs not monitored.

Belanger Headers & Mufflers, with high-flow cats, no check engine light Peak Numbers:
555.73 hp, 509.51 ft-lb
535.49 hp, 505.84 ft-lb
541.20 hp, 504.90 ft-lb
556.05 hp, 516.46 ft-lb

The temperature in the shop was +73°F. Again, IATs not monitored.

Belanger Exhaust and Arrow Controller (canned tune) Peak Numbers:
560.70 hp, 512.51 ft-lb
548.85 hp, 515.59 ft-lb
543.84 hp, 508.98 ft-lb

The temperature in the shop was +90°F. No IATs.

SO,

I see no statistical significance in any of these numbers to definitively claim there was a power gain. The top run from each session was only separated by 7.03 hp, and when back to back runs in the same session differed by as much as twenty horsepower, I can't see a reliably-measurable difference.

2018-09-04 Best Of Each.jpg

The red line in the above graph is the Arrow line. There's a massive apparent loss of power between 4,200 rpm and 5,000 rpm, and the peak power only happens north of 6,000 rpm. You can't see this in the "all in one" graph, since it's lost in the noise.

Where am I going with this?

Nowhere.

I like the sound of the Belangers and I really like the pop of the Arrow. The sills are only warm to the touch, the skip shift is gone, and the car feels smoother to drive and shift at all speeds. Even the Dyno tech asked what I'd done to it since it sounded so different when he fired it up. I don't think any of these mods are a waste of money and not worth doing. My biggest takeaway from this, since I'm relatively new to dynoing things, is that if you can't measure the same thing twice in a row and get the same answer, you just can't verify any claim of power gains. There's just too much experimental error here. You'd need an engine dyno in a climate-controlled lab to knock out some of the uncertainty.

To those of you who have dynoed your cars and reported numbers, have you only taken the best run? Do you see as much variance as I did during multiple runs? Do you have this big dip in power, consistently, around 4,700 rpm?