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The restrictor size for SBK got me thinking. So I thought do a few calculations since that's what I do most of the time anyway.

I started by calculating the maximum mass flow possible thru a restrictor with an area of 543 mm2. This is the area of 2 X 18.7mm or 1 X 26.3mm orifices. Some relatively straightforward isentropic flow relations yield a maximum flow rate that depends on the inlet conditions. For the standard atmosphere (T = 15deg C, P = 101.3 kPa, density = 1.225kg/m3) it they fix the maximum mass flow is 0.131 kg/s. Cool.

Next I approximated the amount of fuel you might want to burn with this amount of air. I choose 14:1 since it's normally around the mark, even though around 14.7:1 is the stoichiometric "ideal" ratio. So anyway, we burn fuel at a rate of .00936 kg/s. The average fuel has a heat value of 44 000 kJ/kg so you can just multiply it out. It yields 412 kW. Whoo-hoo.

The brake thermal efficiency is basically impossible to calculate with a significant degree of accuracy, so I'll just pick an average "high" value of 35%. What does all this give us. 35 % of 412kW is 144 kW. So there you go. This gives you feel for the approximate maximum power output of an engine breathing through said restrictor.

I thought I'd also waste a bit of extra time and consider the case of a "ram air" system travelling at mach no. of 0.24. This is about 295km/h at standard conditions. This, at best, helps us pick up 4.1% more pressure, 2.9% more density and 1.2% temperature. Repeating all the above calculations I get a grand total of 149kW. Or exactly 200hp in the old money. Fun hey.

So this wasn't an exercise in fixing the absolute power level since there are complications upon complication, but I think it does give a reasonable estimate. And it's probably pretty close to the sort of power the twins might be making in WSBK.
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