Modification Cost per BHP Calculator

Add up everything the modification costs — parts, labour, mapping or tuning, and any other costs — then divide that total by the power gained. The before-and-after figures are converted to BHP from your chosen unit (BHP, kW or PS) so every option is compared on the same basis.

Worked example: £600 parts + £200 labour + £400 tuning = £1,200 total, taking power from 200 to 240 BHP. BHP gained = 40, percentage increase = 40 ÷ 200 × 100 = 20%, and cost per BHP = £1,200 ÷ 40 = £30 per BHP.

If the power gain is zero or negative, the calculator shows the actual power change but reports cost per BHP as “Not applicable — no power gain”. It never divides by zero or ranks a negative gain as good value.

Compare figures measured on a consistent basis, ideally using comparable dyno conditions — the same dyno, similar ambient temperature and the same correction factor. Do not mix wheel horsepower and engine horsepower, because drivetrain losses make wheel figures lower for the same engine.

Estimated gains are not guaranteed; they are a tuner’s or manufacturer’s prediction and can fall short in practice. Cost per BHP is a value-for-money signal only — it does not account for reliability, handling, drivability, emissions or other benefits a modification may bring.

For several modifications carried out together, use an overall before-and-after figure from a single dyno session rather than adding each part’s advertised gain together. Advertised gains are rarely cumulative and stacking them usually overstates the real result.