A direct drive situation, like a sprint car, is going to have considerably less loss of horsepower than a street car or drag car (which is what the blog is talking about). Through a standard transmission AND a differential, you’ll have slippage at the clutch as well as frictional losses in the gearbox and differential. With no clutch and no additional rotating gearing to deal with in a manual transmission, your drivetrain frictional losses will be limited to your quick change and your birdcage bearings. I don’t know specific figures, but I would be surprised if the losses were greater than 10%.

All that being said, none of that horsepower, at the crank or at the rear wheels, means anything if you don’t get it in the ground, and in dirt racing, that’s the proverbial $64,000 question.