[ACE-OF-ACES]

So I guess when the AAC started using the higher octane fuels they had to install high compression pistons and retime the powerplant and tune the supercharger controls to take advantage of the higher octane fuel. Because if the engines were not tweaked the higher octane rating could result in a lower power output since it is slower burning than the lower octanes and only is effective when the compression ratio is increased and timing changed. Manifold pressure is controlled by the throttle plate and if the manufacturer recommmends a miximum boost of lets say 65" Hg for three minutes then no matter what the octane rating of the fuel you should only pull the 65".

Here is an explanation found on line that is pretty accurate.

Many high-performance engines are designed to operate with a high maximum compression, and thus demand high-octane premium gasoline. A common misconception is that power output or fuel mileage can be improved by burning higher octane fuel than a particular engine was designed for. The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density. Since switching to a higher octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power.

BTW PING and detonation are two different things PING will not necessarily destroy a powerplant whereas detonation will, think of PING as having the ignition timing too far advanced so the mixture ignites before the maximum compression is reached where as detonation is an uncontrolled explosion of the fuel/air mixture rather than a controlled burn. As this controlled burn starts at two different areas in the combustion chamber the flame fronts should meet as the piston arrives at TDC.

A good read for gasheads: http://www.jcmmachine.com/PDF%20files/J ... %20ch3.pdf

The engines were 'tweaked'

But as far as I know the pistons were not replaced with high compression pistons.

As for tweaks, here is a link to the changes made for 150octane fuel

http://www.wwiiaircraftperformance.org/ ... i-1659.pdf
http://www.wwiiaircraftperformance.org/ ... 1jul44.jpg
http://www.wwiiaircraftperformance.org/ ... -150-1.jpg
http://www.wwiiaircraftperformance.org/ ... -150-2.jpg
http://www.wwiiaircraftperformance.org/ ... -150-3.jpg

[.041 Safety Wire]

The increase in manifold pressure will most likely raise the powerplants critical altitude but I dont believe that you will see a significant increase in power output, Its interesting to note that the P&W,s Wrights, and Allisons pretty much remained the same which leads me to believe that the RR,s were slightly derated from the gitgo.

[ACE-OF-ACES]

The increase in manifold pressure will most likely raise the powerplants critical altitude but I dont believe that you will see a significant increase in power output

belive..

http://www.wwiiaircraftperformance.org/p...28392.html
Flight test have been conducted at Wright Field on the P-38J-15 Airplane, AAF, No. 28392 at the request of the Power Plant Laboratory, Engineering Division. These tests were made to determine the increased performance of the airplane using the higher powers allowable by the use of 44-1 fuel as compared with the powers allowable with the standard fuel, grade 100/130

S!