[wildman10]

Following on . . . . . . .

The same paper gives optimum ratios of cooling mass air flow to charge mass air flow (M1/M2) of 5.7, 4.4 and 3.8 for pressure drops of 250, 500 and 750 Pa respectively for an intercooler efficiency of 80%. For a maximum charge air mass air flow of 850 CFM (64 lb/min, 0.48 kg/sec) these figures give cooling air mass flows of 2.7, 2.1 and 1.8 kg/sec respectively. For the core area of my intercooler, 500mm x 260 mm, without ducting, these cooling air mass flows imply air speeds at the intercooler face of 38, 30 and 25 mph respectively. Similar calculations show speeds of:
• 80% efficiency, duct area ratio 1: 38, 30 and 25 mph.
• 80% efficiency, duct area ratio 1.4: 53, 42 and 35 mph.
• 80% efficiency, duct area ratio 2: 76, 60 and 50 mph.
• 90% efficiency, duct area ratio 1: 59, 46 and 38 mph.
• 90% efficiency, duct area ratio 1.4: 83, 64 and 53 mph.
• 90% efficiency, duct area ratio 2: 108, 92 and 76 mph.

Putting all this together:
• The figures in the paragraph above are worse case figures as they are steady state peak power using my intercooler and derived from practical experiments.
• Full power will be used only intermittently below 60 mph as my car will accelerate rapidly and/or traction may be limited, and even then the intercooler structure will act as a heat sink to be cooled after gear changes.
• Cruising and most road use will involve charge mass flow rates much lower than those for peak power, thus allowing higher duct ratios.
• Track use is the worst case scenario, involving as it does high power for longer. Looking at the figures above, track speeds will give 90% efficiency with a duct ratio of 2 ie an inlet area of ½ the intercooler area.
• The duct area ratio of 2 is higher than the duct ratio of 1.4 for the lowest drag losses but gives better cooling at expected car speeds.