Talk:Ford Model Y/Archives/2015

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REAL output, people!

So it's tax horsepower is 8... what would it's REAL output be? The preceding model A was noted as being 40bhp (claimed) and 24hp (tax), so we may presume the Y to be about 13-14bhp? It would fit with it's top speed being in the mid 50s. Maybe even 18, if they improved its specific output efficiency and the "108hp" claim for the Koln is but a typo... 193.63.174.10 (talk) 16:03, 30 October 2009 (UTC)

There is no direct correlation between tax horsepower and "real" horsepower. Around about 1900 the values for each may have been close together, but as engines became more efficient, they extracted more "real" horsepower from any given engine size. Tax horsepower, in England, was (as far as I remember) simply a function of the surface area of each cylinder multiplied by the number of cylinders. That made it easy to compute and impossible to disagree. Tax horse power in France and Germany and Italy was also based on cylinder size - can't remember if it was bore (piston diameter) or stroke (piston travel) or both (cc) - but I'm pretty sure each government's car taxers had a different formula, which no doubt made sense to each of them.

By contrast, then as now, there is plenty of scope for disagreement over the "real" power output of a given vehicle and the published power outputs claimed by the manufacturers. Disagreement because the impact of different components affects the real power getting through to the driving wheels (eg increased back pressure from the exhaust system (muffler) reduces by a small but measurable amount the power available to the wheels so the question arises of whether you measure the power output with the engine on a test bench or with the engine in the car running on a rolling road. Ditto for lots of other bits that you might or might not consider part of the engine.) Different because production tolerances mean that even when apparently identical cars are new, actual power outputs may well vary from car to car, especially with cars from before the 1980s by when machining tolerances were beginning to be much reduced. Disagreement because if your car is five years old, it almost certainly delivers less power to the wheels than it did when it was new.