Quote:
Originally Posted by Old Tele man
Like this (1L = 61.024 cid) quick answer:
T = (hp × 5,252)/(rpm)
T = (12hp × 5,252)/(2,000 rpm) = 31.512 lb·ft
BMEP = 150.8 × (T/D)
BMEP = 150.8 × (31.512 / 61.024) = 77.871 psi
However, more specific engine data is needed for meaningful results--like bore & stroke, CR, VE, value timings--for determining piston velocities, dwell times, assumed intake/exhaust flow rates, etc.
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The quick answer is adequate.
With conversion into liter displacement input, the formula is simply:
BMEP = ( HP * 13000 ) / ( L * RPM ) , for a four cycle engine.
But the result is the same. 78.1 psi .
For those not familiar with BMEP, it is a quantity that represents an ideal, flat pressure head acting on the piston top across the angular range between TDC and BDC on the power stroke ( 180 degrees piston motion ). It's usefulness is in comparing ICE engines across a broad spectrum of design variables. It is usually introduced in undergrad engineering classes as a derivation exercise that is surprising in that the resulting pressure is dependent only on the variables of displacement, engine speed and power - three values that are easily found in literature for a particular engine, or measured in the college dynamometer lab.
We can use this equation to glean some insight into engine modifications.