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Real HP

Real HP is the actual power that you get at the rear wheel, rather than what a lot of material talk about which is crank hp which is an estimated power that the engine produces before going through the gears and rear tyre. Often when hp is stated in things like Vespa specifications and dyno results, it not stated whether the hp is at the crank or at the rear wheel, more often than not the crank value is used because it sounds bigger! Chasis Dyno's also confuse the matter because they measure the power at the rear wheel but then some add a fudge factor to the data to guess what the power is at the crank (again because people prefer numbers that sound bigger).

For example Dynojet is a common dyno used and why the fudge factor exists is in The Story Behind the Dynojet Chassis Dyno - The Truth Meter

"Dynojet's final number-fudge was arbitrarily based on a number from the most powerful road-going motorcycle of the time, the '85 1,200cc Yamaha VMax. The VMax had 145 advertised factory horsepower, which was far above the raw 90hp number spit out by the formula. Meanwhile, existing aftermarket torque-cell engine dynamometers delivered numbers that clustered around 120. Always a pragmatist, Dobeck finally ordered his Chief Engineer to doctor the math so that the Dynojet 100 measured 120 hp for a stock VMax. And that was that: For once and forever, the power of everything else in the world would be relative to the '85 Yamaha VMax and a fudged imaginary number."

The above means that although the dyno reads 90 hp they report 120 hp, so to interpret real hp (i.e. the actual power you end up with pushing you along) you need to remove 25% of the number!

Consistent Results

Here are some rules of thumb for getting consistent results and allowing you to compare apples with apples.

  • Rear tyre pressure should be @ 35 PSI (lower pressure increases rolling resistance, higher pressure is not what would be used on the road)
  • The engine should be warmed up to running temperature

Reading the Results

More to add... about it's not all about peak power with some examples