I'm enjoying all the comments about the rocky curve, and I think a lot of them are right on the mark. Now, take a look at the graph below, and tell me what you think of this:
Believe it or not, this is the same engine. In fact, the curve is from exactly the same data as I posted a little earlier. The difference is, the curve smoothing algorithm in the dyno software has been turned on. This lowers the whole curve a little bit but smooths out the numbers, so that the discontinuities in the curve completely disappear.
I have a lot of things to say about this, but primarily I think that the curve smoothing algorithm hides potential problems with the engine. And almost every dyno operator uses it, because it makes the curve look better to the customer. For this reason I NEVER run the curve smoothing option on my dyno. Sometimes the curves look choppy, but I think this is telling you something. Yesterday, while dynoing Jim's engine we noticed that jet changes were not really having the expected effect on the A/F numbers and the horsepower/torque curve. Experience points to the carb, as quite a few folks here have pointed out. But it would have been a challenge to see that, or really understand what was happening, with the curve smoothing turned on, because the torque and horsepower curves would have looked fine.
Now, one other thing to consider is that the dyno is an electrically noisy place, and electrical noise can mess up the signals from the dyno's tachometer output, and the torque sensor output. This can also lead to discontinuities in the horsepower and torque curves. However, I have dynoed enough really strong engines to know that when everything is right, those curves look very smooth, with only an occasional glitch, even when the curve smoothing algorithm is turned off.
There are options for the curve smoothing, called first order, second order, and third order, referring to the mathematical formula used to modify the data. Below are four dyno charts from one pull, the first with no smoothing, next with first order smoothing, next with second order, and finally third order:
The moral of the story is that it is important to understand the data that you are looking at, if you want to really see what's going on with your engine - Jay