Horsepower as a Function of Manifold Pressure

here is on that does it. You could ratio down the torque at part throttle based on vacuum at manifold.

http://www.bgsoflex.com/roughhp.html

Also search for "turbo-charger" and you will get a wealth of information about the effect of raising the manifold pressure.

The ScanGauge2 provides a HP readout using more or less this logic. Such a gauge needs to be calibrated to the particular engine, and a mass air flow sensor gives a more accurate indication than manifold pressure alone, but the principal is valid. There are of course many limitations, with engine wear, timing, mixture control, etc affecting HP for a given rpm and manifold pressure.

Yes you could figure out a rough engine HP based on known or measured values such as displacement, manifold pressure, air flow, etc. However it would be difficult to get an accurate value because of changes in spark timing, drivetrain losses, flow characterisitics which affect cylinder filling, etc. For all that trouble, its probably more effective to use an accelerometer, which would give you the horsepower after all those losses given the measured velocity and weight of the vehicle. Several companies make such devices just for this application, and I've even seen people hack Nintendo Wii-motes to do this.

"...use an accelerometer, which would give you the horsepower after all those losses given the measured velocity and weight of the vehicle."

iPhone has an app for that. Costs around 15 bucks, also works on iPod Touch. A cubicle neighbor used it to get an empirical number on his VW GTI. (Was within 3% of the factory published number)

Thanks for your answers. I appreciate your answers.

Bob Clark