My calculator gives slightly different results:

 

I get a change in air speed of 1.58% rather than 1.95%. I think you may have used IAS rather than TAS.

 

Also, correcting for OAT and MAP gives 1.60% predicted change in air speed

 

If you are going to correct for accessory losses, then you should probably also be correcting for the throttling loss. This power is the throttle depression (inHg), 1" in this case, multiplied by RPM x displacement (in^3)x volumetric efficiency(VE) divided by 122150 to get horsepower. I would guess that VE for a naturally aspirated engine is about .92, somebody out there may know the real value for a specific engine. VE will also change slightly with RPM.

 

Rob,

 

What correction for OAT (should be TAT) and MAP?  What about the correction to induction air temp after it passes thru the hot oil sump?

 

Throttle depression?  I am measuring MAP (Manifold Absolute Pressure) in the manifold, after the throttle.  MAP at wide open throttle (WOT) is without (theoretically) any induction system vacuum.  Assuming instrument accuracy and assuming we could actually measure the true ambient pressure we could also measure the Ram air rise.  I think the only measure of "power" would be delivered thrust.  I would tie my tail to a fish scale attached to a SUV but something's different when the prop is screwing thru the air.

 

I guess the only way to do these things is side by side until I pull ahead!  All other things being equal!

 

Scott Krueger AKA Grayhawk
N92EX IO320 Aurora, IL (KARR)

OK, looks like brain farted in using OAT as relating to intake air temp. It made the results look real good though didn't it?

 

However, the mass air flow through the engine is the important value when operating ROP. Temperature and Pressure both have direct impact on the density of the air and hence mass flow rate.

 

The MAP and Temp relate to the energy/power that is being fed into the engine. The throttle takes power away from the engine twice. The primary way is by reducing the manifold pressure and hence mass flow and power. The second way is by forcing the engine to operate against a differential pressure. In an ideal world with perfect efficiencies, at WOT the manifold pressure would be equal to the turbine inlet pressure. Closing the throttle results in reduced intake pressure relative to the exhaust. This applies a braking effect to the engine and explains why the engine operates with such a relatively high MAP at idle.

 

This is part of the reason that diesels are generally more fuel efficient than gas. It also partially explains why the 400 ci engine in my Firebird gets substantially poorer fuel economy than another with a 326 ci, even when driven by someone with a substantially smaller right foot than myself!

 

Rob