Pick any power where the thetaPP is at
16dATDC, and ANY change in timing, advancing or retarding will decrease
HP output.  The problem is that to do this, you really need a closed
loop system that MEASURES the thetaPP and adjusts the timing
accordingly.  This is only marginally successful when you calculate the
thetaPP.  I only now of one system that is capable of measuring the
thetaPP in real time and adjusting the timing on the fly--PRISM. 
Computer mapping is only a guess.  If one maps for anything other than
thetaPP and peak pressure, there are potential problems.

Walter,

 

Thanks for the response and additional details.

 

I agree completely that a closed loop system could be the best way to handle this.  I am investigating O2 exhaust sensors since now there are ones that can survive in a leaded environment.

 

Disallowing for any bias on your part, PRISM may not be the only answer since all thru history we have been able to manage engines by indirect measurements that imply other conditions (i.e. F/A ratio via EGT).  A predetermined map utilized by an EI computer may do an adequate job as indirectly measured by increased airspeed and decreased fuel consumption whilst the engine remains within reasonable operating limits, LASAR notwithstanding. 

 

Even utilizing an engine map, one can make a best guess at ignition timing, fuel flow, etc. by using direct measurements of such things as barometric pressure, temperature, mass air flow, MAP, RPM, other temperatures (depending on how the engine is cooled), exhaust O2 content, etc. and then refine the control of efficient combustion producing elements with cylinder pressure, the so called theta PP.

 

Sometimes I am just amazed that my water cooled twin cylinder motorcycle engine (110 ci, 4" bore, 4.4" stroke), utilizing two iridium plugs per cylinder, 12 port electronic fuel injection, 9:1 CR, 3 valves and a 3D mapped computer that manages the 107 HP (@5000 RPM) engine with refinements from coolant temperature (startup), Baro, MAP, RPM, induction air temp, and O2 feedback.  Works at any altitude with a road.  Why aren't our aircraft engines so controlled?  Ahhh, don't answer that - it was a rhetorical question.

 

I am patiently waiting for a better system.  The only problem is that I am rapidly aging to TBO whilst waiting.  Arrrrrgghh.