512 has a good start on the problem. A water brake engine dyno, like every other tool has its issues. If you don’t understand them, you get bad data. The sweep rate is controlling how fast the engine accelerates. Let’s take 300 RPM per second. You’d be hard pressed to find an engine that will RPM that slow, especially in low gear. So the dyno is now holding the engine back from what it actually does in its normal operation. I just heard Darin Morgan say in a webinar that a Pro Stock engine will RPM at 1600 RPM per second in low gear. If you test that engine at 300 RPM per second, the data will have flaws. I don’t know how the Pro Stock teams test on the dyno or how they evaluate the data if they are using slower sweep rates, but they do it. I do know of at least 3 teams that at one time had an inertia dyno that allows the engine to RPM at whatever rate it does in operation. In fact, there was a report done comparing the Revolution inertia dyno against a water brake dyno. I recall the test being 6 or 8 different intake manifolds for a circle track application. The upshot was both dyno’s gave conflicting data. What looked good on the water brake looked bad on the inertia dyno and what was good on the inertia dyno looked bad on the water brake. After a ton of dyno pulls on each intake on each dyno, they took the car to the track to test them. As I expected, the manifolds that looked the best on the water brake dyno were the slowest in the car, and the manifolds that looked the best on the inertia dyno were the best in the car. How could that be? It’s because the water brake dyno controlled the RPM that the engine accelerated. The inertia dyno allowed the engine to RPM at its natural acceleration rate. So from what I see, if you slow down the engine’s acceleration rate you will get higher horsepower numbers than if you run the test at a higher acceleration rate. In other words, if you run a test at 300 RPM per second and then test that same engine at 600 RPM per second, the 600 RPM per second tests will show LOWER horsepower numbers. I also believe that at slower acceleration rates you see things like a single plane intake manifold lose power to a dual plane, when in reality the single plane, if tested at a higher acceleration rate will should a totally different result, with the result being the single plane is just as good or better than the dual plane everywhere. And that should translate to the same result in the car. Unless you have the chassis not matched to the engine. Like not enough gear and/or converter. When I was 15 years old I was told by a guy who I had a ton of respect for and who had earned that respect with his on track performance (and his street stuff too) that the water brake dyno didn’t measure the ability of the engine to RPM. And he’s not wrong. That doesn’t mean the dyno isn’t a useful tool. You have to recognize what it does well and and what it doesn’t do. Guys who like a big number on the dyno will test at 300 RPM per second. Guys who want more factual data will speed up the acceleration rate and live with the data.
