In your test of Pro Circuit’s 2019 Husqvarna FC350 exhaust pipe, you said that the Pro Circuit pipe made 2 more horses than the stock pipe at peak and was 3 horses better at 11,000 rpm. That makes no sense. Does it make 2 horsepower more or 3 horsepower more? It can’t be both.
Of course it can be both. You are confused about the difference between “peak horsepower” and horsepower measured at random spots on the rpm curve. Peak horsepower is defined as the maximum horsepower that the engine makes. In this case, the Pro Circuit pipe makes 2 horses more than the stock exhaust at peak (55.03 horsepower to 53.15), while at different points on the curve—take 10,300 rpm for example—the Pro Circuit FC350 pipe makes 3.45 horsepower more (53.48 horsepower to 50.03).
Peak measures the highest horsepower, but often a pipe, piston, cam or mod makes larger gains, albeit at lower horsepower numbers when compared to the stock engine, at different parts of the powerband. This can be very pronounced when an engine doesn’t sign off as soon. For example, the 2018 KX450 makes 56.43 horsepower at peak compared to the 2019 KX450’s 55.86 horsepower at peak. That would lead you to believe that the 2018 KX450 is one horsepower stronger than the 2019 KX450. It is, but only at peak; however, after peak, the 2019 KX450 made 3 more horses than the 2018 model (albeit at 50.65 horsepower versus 47.14). If we kept going to sign-off at 11.500 rpm, the 2019 KX450 would be making 5 more horsepower than the 2018 KX450, but even though the advantage over the stock engine would be 5 more horses, it would not be making more horsepower than the peak number.