How to Go Fast Faster: The Math Behind Turbocharging. Part 2: Calculating Output + Boost

Part 1 is here.

     So now that we want to turbocharge an engine, where do we start?  First we need to know our desired output for our engine and then figure out the increase in output.  With that, we can figure out how much boost to run and then choose the right turbocharger.  Boost is basically how much extra air we’re stuffing into the engine to create more power.  This is how we’re increasing the P in Power = P x L x A x N.

     First we need to set a goal for our turbo plans.  This goal will be the basis for most of our math, so decide early how much power you want your car to end up with: your Desired bhp (hp).  You’ll also need to know your car’s Stock bhp (hp) also.  (Please use either Wheel Horsepower or Brake Horsepower consistently.) So first, we’re going to find out our Performance gain by using this simple formula:

Performance gain (%) =

This is a simple equation to find out the increase in performance percentage-wise.  We just divide our Desired power by the Stock power, subtract by 1, and then multiply by 100% to find your Performance Gain.

     Using that number, we can find out how much boost we need.  Boost is how much extra air we’re stuffing into the engine.  Remember, 14.7 psi at sea level is atmostpheric pressure, and boost is just the difference between the total pressure and the atmospheric pressure.  But rather than displaying the total amount of atmospheric pressure, we just display the amount of pressure over the atmospheric pressure.  For example, 5 psi of boost is basically 19.7 psi of air crammed into a cylinder.  Simply speaking, we want to see how much extra pressure we need to compress our air so we can meet our desired output.  We only need our Performance Gain (in decimal form) here.  Here’s our equation for our Boost Required:

Boost Required (psi) =

Here we’re multiplying our performance gain (in decimal form) by 14.7 (the atmospheric pressure).

     This is just the beginning of our long journey to mathematically choosing our turbo setup.  The next post will deal with the fuel system, and how to make sure it’ll be up to par with our increase in power.  Stay tuned.