What are the limits of pressure and temperature to avoid detonation with 100LL? This is specifically related to a carbureted, free-floating draw-through turbo setup without an aftercooler. Yeah, I know that's stone-age stuff. 
Consider these cases I calculated with an imaginary engine as an example:
1) Standard day, sea level
50 in Hg MAP (i.e. 20" boost)
Compressor inlet temperature = 8F (assuming full evaporation of the fuel in a 12:1 mixture)
Compressor discharge temperature = 110F
Cylinder pressure before combustion = 24.6 psi (50 in Hg) x 8.25 compression ratio = 203 psi
BMEP = 205 psi (14.1 bar) @ 100% rated power
2) Sea level, standard pressure, ambient temperature 95F
Compressor discharge rises to 150F; cyl. pressure remains 203 psi.
BMEP = 192 psi (13.2 bar) @ 94% rated power
3) 5,000 MSL, standard pressure, ambient temperature 95F
The same 20" of boost now yields 45" MAP.
Compressor discharge rises again to 167F; cyl. pressure is reduced to 182 psi
BMEP = 176 psi (12.1 bar) @ 83% rated power
4) 12,000 MSL, standard pressure, ambient temperature 70F (95F surface temp at 5,000)
The same 20" of boost now yields 40" MAP.
Compressor discharge falls to 158F; cyl. pressure is reduced to 163 psi
BMEP = 151 psi (10.4 bar) @ 74% rated power
So how does one determine detonation margins based on these data points of temperature, pressure, and BMEP? Is there anything resembling a hard-and-fast rule, or do we go by experience and precedent?
I suspect that cases 1 & 2 above might be asking too much of the engine (for comparison, I estimate a TIO-360-C1A6D is operating at about 180 psi BMEP at 44" MAP and max rated power, but its charge temperature should be much lower). Perhaps cases 3 & 4, with their more reasonable BMEP (although higher intake temps) might be a more realistic expectation for some detonation margin? (Assume it's an air-cooled engine with dished pistons for reasonable quench.)
Can someone point me in the right direction to learn more about this? I have studied Hugh MacInnes' book and unfortunately he doesn't address this specific question; what other resources would you recommend? (BTW, thanks @epijack for the Piston Engine Tech series--what a wealth of information. Sorry for using the D-word instead of "auto-ignition"!)
I'm aware that a fuel-injected or pressure-carbureted, blow-through system with an aftercooler would sidestep these concerns but I'd rather not take the thread in that direction for now as I'm hoping to learn more about the limitations on a draw-through setup.
Consider these cases I calculated with an imaginary engine as an example:
1) Standard day, sea level
50 in Hg MAP (i.e. 20" boost)
Compressor inlet temperature = 8F (assuming full evaporation of the fuel in a 12:1 mixture)
Compressor discharge temperature = 110F
Cylinder pressure before combustion = 24.6 psi (50 in Hg) x 8.25 compression ratio = 203 psi
BMEP = 205 psi (14.1 bar) @ 100% rated power
2) Sea level, standard pressure, ambient temperature 95F
Compressor discharge rises to 150F; cyl. pressure remains 203 psi.
BMEP = 192 psi (13.2 bar) @ 94% rated power
3) 5,000 MSL, standard pressure, ambient temperature 95F
The same 20" of boost now yields 45" MAP.
Compressor discharge rises again to 167F; cyl. pressure is reduced to 182 psi
BMEP = 176 psi (12.1 bar) @ 83% rated power
4) 12,000 MSL, standard pressure, ambient temperature 70F (95F surface temp at 5,000)
The same 20" of boost now yields 40" MAP.
Compressor discharge falls to 158F; cyl. pressure is reduced to 163 psi
BMEP = 151 psi (10.4 bar) @ 74% rated power
So how does one determine detonation margins based on these data points of temperature, pressure, and BMEP? Is there anything resembling a hard-and-fast rule, or do we go by experience and precedent?
I suspect that cases 1 & 2 above might be asking too much of the engine (for comparison, I estimate a TIO-360-C1A6D is operating at about 180 psi BMEP at 44" MAP and max rated power, but its charge temperature should be much lower). Perhaps cases 3 & 4, with their more reasonable BMEP (although higher intake temps) might be a more realistic expectation for some detonation margin? (Assume it's an air-cooled engine with dished pistons for reasonable quench.)
Can someone point me in the right direction to learn more about this? I have studied Hugh MacInnes' book and unfortunately he doesn't address this specific question; what other resources would you recommend? (BTW, thanks @epijack for the Piston Engine Tech series--what a wealth of information. Sorry for using the D-word instead of "auto-ignition"!)
I'm aware that a fuel-injected or pressure-carbureted, blow-through system with an aftercooler would sidestep these concerns but I'd rather not take the thread in that direction for now as I'm hoping to learn more about the limitations on a draw-through setup.
