Why is it so hard to measure HP?

In reply to bradyzq:

Well technically inertial dynos only measure drum rpm, the hp calculation is even further removed than calculating from torque. Drop an apple from a tree and you measure distance and time, not acceleration.

In reply to Keith Tanner:

I mean more chassis tests are transient tests, and the certified power tests is steady state. Many boosted engine run transient tests (regardless of how measured) with more boost and less compensation for issues. So for a cold 1/4 race, the chassis test may be correct. But run that for a few min, and lots of things happen to keep the engine alive.

Even a lot of the shop power tests on engine only dynos are transient tests. I'm sure the Car Craft of HR ones are just like the tests that you see on TV. Which is fine, and repeatable for quite a few engines, even on a SS dyno. (a carbed V8 with mechanical advance will probably be pretty close on a steady state vs. transient measurement)

but transient v steady state opens a whole different issue with me.

The main reason we (and a lot of other tuners) use chassis dynos is because it's simply too much of a PITA to pull the engine How well it lines up with a 1/4 mile run isn't a factor. I think the main reason so many chassis tests are transient or sweep tests is because Dynojets are cheap and popular, and they can't do steady state.

The Car Craft/ Hot Rod tests are published as a table of HP/torque at various RPM points, so I've always assumed they were steady state tests.

Dynojets can do steady state, they've been able to for quite some time... Whether or not the old haggard units that are still in use in many shops have that capability is likely a different matter, though.

Ours couldn't - at least, not really. I seem to recall it involved an internal drum brake and a huge mess and smoke.

Keith Tanner wrote: Ours couldn't - at least, not really. I seem to recall it involved an internal drum brake and a huge mess and smoke.

LOL!!! Story time?

Not really. But follow an inexperienced semi driver down a mountain pass as he rides the brakes. You'll get the general idea.

In reply to Keith Tanner:

I think we are talking about different things.

I'm pointing out why boosted engines can show higher "real world" dyno numbers than what is published, when the published number is an SAE witnessed test.

And I don't think Car Craft/Hot Rod tests are steady state. Just because they publish speed vs. power/torque does not mean it's steady state, a few min of sitting at a setting test.

I do understand why tuners use chassis dynos. And I also understand why most hot rod shops also use a transient test. DC dynos are very expensive, and the test is very time consuming.

I don't have any real knowledge of exactly how the OE tests are performed. A few minutes at a certain load and RPM, okay. The "few minutes" part is more than I knew before, I assumed it was enough time to get a reading and that's all. On our dyno, we can park an engine at a certain load and speed for as long as we want.

I know that there are transient boost behaviors. In your lab, everything is boosted. For the rest of us, not everything is And we're not all dealing with current generation tech. Nothing I've ever run on our dyno, for example, has an overboost function except by accident. So the majority of stock cars (which can include anything made in the last 25 years easily) will not do anything weird on a transient.

grpb wrote: In reply to bradyzq: Well technically inertial dynos only measure drum rpm, the hp calculation is even further removed than calculating from torque. Drop an apple from a tree and you measure distance and time, not acceleration.

Well, technically inertial dynos only measure voltage, the drum RPM is a calculation based on knowledge of how the hall effect sensor or reluctor or some other device works. :-)

Load-bearing dynos only measure voltage too -- a load cell converts a force into a resistance, and you measure resistance by measuring voltage drop across it.

codrus wrote:

grpb wrote: In reply to bradyzq: Well technically inertial dynos only measure drum rpm, the hp calculation is even further removed than calculating from torque. Drop an apple from a tree and you measure distance and time, not acceleration.

Well, technically inertial dynos only measure voltage, the drum RPM is a calculation based on knowledge of how the hall effect sensor or reluctor or some other device works. :-) Load-bearing dynos only measure voltage too -- a load cell converts a force into a resistance, and you measure resistance by measuring voltage drop across it.

Fair enough, I've missed the point by now anyway. I've spent too many hours sitting behind the glass at dynos to care about absolute numbers, each dyno has it's own character anyway. Torque curves are like the fine female form, it's the shape that's important, too easy to be fooled by the number shown on the scale.