The Whp To Hp Calculator turns a dyno’s wheel-power reading into an estimated crank figure, in HP, kW, or PS, and checks a tune’s real gain against the factory crank rating directly.
Converting Dyno WHP with the Whp To Hp Calculator
The Whp To Hp Calculator turns a dyno’s wheel-power reading into an estimated crank figure, in HP, kW, or PS. An optional factory rating field checks a tune’s real gain at the wheel instead of stacking two crank estimates.
Three Power Units and an Optional Factory Comparison
Enter dyno wheel power, drivetrain loss percentage, and vehicle weight in Imperial HP, metric kW, or metric PS. Add a factory crank rating to unlock the fifth card, which compares an actual dyno pull against what that rating implies at the wheel.
Whp To Hp Calculator Math for Crank Power
Crank power comes from dividing the wheel reading by the drivetrain’s remaining efficiency.
$$ CrankPower = \frac{WheelPower}{1 – Loss\%} $$
This is the same relationship dyno tuning forums walk through directly — one thread works a 465 wheel-hp reading with a 20% loss out to 581 crank hp, then confirms it by multiplying 581 back down by that same 20% to land exactly on 465 again.
The single most common mistake here is adding the loss percentage back onto the wheel number instead of dividing by the remaining efficiency — a 15% loss means dividing by 0.85, not multiplying by 1.15. Those two operations only agree at very small loss percentages and diverge meaningfully above about 10%.
Drivetrain loss itself isn’t fixed. Dyno comparisons posted on tuning forums for the same car show it running anywhere from roughly 5% to 40% depending on the dyno, the gear it was tested in, and the drivetrain hardware, which is why this calculator asks for a loss percentage directly instead of assuming one.
Wheel power must be at least 10 (HP, kW, or PS), weight at least 500, and loss percentage is capped at 50 — anything past those limits is rejected rather than returning a crank figure inflated past anything realistic.
Where the PS Conversion Comes From
$$ 1\ PS = 0.98632\ HP $$
PS, or Pferdestärke, is defined as 735.49875 watts, against 745.7 watts for one mechanical horsepower. Both are fixed definitions, not estimates, so converting between them for the quarter-mile projection is exact.
Checking a Tune’s Real Gain Without Stacking Two Estimates
Entering a Factory Crank Rating converts that number down to an expected wheel figure using the same loss percentage, then compares it directly against the actual dyno reading.
This sidesteps a mistake tuning discussions call out by name: converting a dyno wheel reading up to an estimated crank figure and comparing that estimate against the factory-advertised crank number stacks two separate loss-percentage guesses on top of each other.
A widely discussed dyno thread on a tuned Audi S4 makes the same point — the wheel-level gain, 320 to 360 awhp, was the number worth trusting, while converting either side to an estimated crank figure to compare against Audi’s advertised rating was specifically criticized as marketing math, not measurement.
Leaving Factory Crank Rating blank simply omits card five rather than guessing at a default, since there’s no way to check a tune’s gain without a real baseline to compare against.
Questions About Reading a Dyno WHP Number as Crank Power
Why does dividing by drivetrain loss give a different answer than adding it back?
A forum worked example shows why: 465 wheel-hp at a 20% loss divides out to 581 crank hp, and 581 minus 20% of itself lands back on exactly 465. Adding 20% to 465 instead gives 558, a different and incorrect number.
Is drivetrain loss the same percentage for every car?
No. Dyno comparisons on the same car posted across tuning forums show loss running anywhere from about 5% to 40%, depending on the dyno used, the gear tested in, and the specific drivetrain hardware.
Should I compare my tuned car’s estimated crank horsepower to the factory rating?
Forum discussion of a tuned Audi S4 specifically warns against it, since that comparison stacks two estimates on top of each other. Comparing the actual wheel numbers before and after a tune is the more reliable measurement.
What’s the difference between HP and PS?
PS is metric horsepower, defined as 735.49875 watts versus 745.7 watts for one mechanical horsepower. One PS equals about 0.986 HP.
How much horsepower does a tune typically add?
It varies by car and tune. One documented dyno comparison showed a reflash adding 40 wheel-horsepower peak gain on an Audi S4, but that figure is specific to that car and tune, not a general rate.
Why do WHP numbers vary between different dyno shops for the same car?
Dyno-to-dyno variation is well documented on tuning forums. Different rollers, correction factors, and even which gear the car is tested in can shift the reading for an otherwise identical car.