Club Cobra - Crank HP, Rear Wheel HP and Air Density

The 351W was assempled by Eric Hindenburg, Performance Engineering in Ross Ohio. The dyno showed 469 horsepower at the crank. Two and a half years and 14,000 miles later, the well broken in engine was retuned in Billings, Montana. The dyno indicated 335 horsepower at the rear wheels.

The apparent disparity in results is explained by two factors: parasitic losses through the drivetrain and the significant diference in air density between Ross, OH at 558 feet above sea level and Billings, MT at 3648 feet. Folks who want a constant figure to use when estimating drive train losses usually end up with a figure between 15 and 20 percent, with 18 percent being advocated most frequently. 20% of 469 is 94 HP, giving a net of 375 HP.

Since the dyno in Billings measured rear wheel horsepower, the allowance for reduced air density due to greater elevation would be applied to this figure. If I recall correctly, air density drops by 4% per one thousand feet of elevation above sea level. (At 25,000 feet, air density is no longer a factor affecting lift. You could not, for example, operate a helicopter at 25,000 feet. A jet or a rocket would continue to be pushed through the air, but would not be supported by it.) At 558 feet, Ross Ohio would have 2.5% less density than a location at sea level; Billings is 3,648 above sea level for a loss of 12.5%. The net loss attributable to the difference in elevation between Ross OH and Billings MT is 10%.

10% of 375 equals 37.5 HP. The predicted HP at Billings is 337.5 HP, close enough, I think, to support the conclusion that the engine continues to perform very closely to the original dyno numbers.

I gained a bunch of horsepower when I moved from Bozeman, 4,400 feet above sea level, to Billings. The move alone cost me $3,600. Next time I'll opt for more efficient headers.