Making horsepower

Making more horsepower can sometimes sound like a black art. The engine builder weaves his magic during the build, and out pops a hundred extra horses! But it isn’t. Making power is about science. Mostly physics, with a little bit of chemistry thrown in.  Here are the basics:

Principle One – The internal combustion engine

Yeah, I know.  We’re all rev-heads, so we all know how an engine works. Or at least we think we do.  But to really understand how to get the best out of any engine, we first need to have a basic grasp of how it works and the role of each key component. So, here we go…

Every four-stroke petrol powered engine goes like this:

  1. Intake stroke: Intake valve opens, piston moves down the cylinder and an air-fuel mixture is drawn into the cylinder.
  2. Compression stroke: The intake valve closes and the piston moves up the cylinder, compressing the air-fuel mixture.
  3. Power stroke: A spark from the spark plug ignites the air-fuel mixture and the rapidly expanding gasses force the piston back down the cylinder.
  4. Exhaust stroke: The exhaust valve opens as the piston moves back up the cylinder, pushing the combustion gases out of the cylinder.

Fairly simple really! And it is. Despite all the fancy electronics on modern engines, all the power adding features like turbocharges and induction systems, these basic components are at the heart of every engine.  It’s how they all work together that’s the key to making power…

Principle Two – Power comes from burning fuel

Self-evident really.  Simply put, the more fuel we can burn, the more power our engine will make. But think back to primary school science and you’ll

remember that it takes three things to make a fire burn: fuel, oxygen and an ignition source.  For power, fuel and the ignition source are easy. Oxygen is always the limiting factor. Once all the oxygen is used up in the combustion process, you won’t get any more power, no matter how much fuel you put in. The unburnt fuel just gets pushed out of the cylinder during the exhaust stroke.


Principle Three – Stoichiometric air/fuel ratio

Big word, simple concept. The stoichiometric air/fuel ratio (ARF) is 14.7:1 for a petrol powered internal combustion engine.  So to burn all the fuel in the cylinder, you need 14.7 parts air for one part petrol. An AFR of greater that 14.7:1 is called a lean mixture and some unused oxygen will be expelled during the exhaust stroke. An AFR of less than 14.7:1 is called a rich mixture, meaning some unburnt fuel will be expelled.

Also, try and think of the AFR in terms of mass, rather than volume.  14.7 grams of air to every one gram of fuel rather than 14.7 ml of air to every ml of fuel.  Remember, volume will change as temperature rises and falls. Mass will not.

Principle Four – More air equals more power

So basically, the more oxygen from atmospheric air we can get into the cylinder during the intake stroke, the more power the engine will make.  This is the basis of many, many performance modifications. Here’s just a few:

  • Make the engine bigger, either by increasing the size of the cylinder bore or the length of the stroke. Or both! Each stroke draws in more air, so we can make more power.
  • Force more air into the cylinder by using a turbocharger or supercharger.
  • Alter the camshaft to change the valve timing. By opening the intake valve a little earlier, we can get more air in during the intake stroke.
  • Change the valve lift by altering the camshaft lobe height or changing the rocker arm ratio. By increasing the distance between the intake valve seat and the valve when it opens, more air can flow past the valve.
  • Changing the intake port in the cylinder head so that more air can flow through the port during the intake stroke.

There are plenty more, but these are the main starting points for any performance upgrade.  We’ll cover some more later-on in the series.

Principle Five – cold air is more dense than hot air

Basically, cold air is heavier than hot air.  That’s why a hot air balloon lifts off the ground.  So when air temperature is low, the mass of the air drawn into the cylinder will be higher.  This means we can put in a bit more fuel while still maintaining the stoichiometric AFR, thus making a bit more power.  This is why modifications like cold air intakes and intercoolers on forced induction engines help increase performance.

Principle Six – Internal combustion engines are inefficient

Thermal efficiency is the term used to describe how much of the energy potential in the fuel is turned into usable power by the engine.  For petrol engines, this is only around 25%.  Most of the energy is lost as heat, either through the exhaust system or the engine’s cooling system.  A bit more is consumed through internal friction and even more from engine ancillaries such as alternators, A/C compressors, cooling fans and the like. And a bit is lost by having the piston push the exhaust gases out through a restrictive exhaust system.

While we can’t do much about the heat loss, all the other losses present opportunities for performance gains.  Reducing internal friction, power consuming ancillaries and modifying exhaust systems are all areas for attention.

Principle Seven – There’s no such thing as a free lunch!

When it comes to modifying an engine to make more power, there’s always compromises.  Here’s a few to think about:

  • It will use more fuel. Not one most rev-heads will care about, but worth pointing out anyway.
  • It won’t behave like a standard road car engine. It may become harder to start, rattle excessively when cold and use more oil. It may need to be warmed-up for a few minutes before being driven.
  • Power delivery will change. It may make more power, but that power will only be available at higher RPM.  Available power and torque will usually decrease at lower RPM making the car a bit of a pig to drive in traffic.
  • It will need more maintenance. An average road car engine should be good for at least 250,000 km if driven normally and properly maintained.  Fairly minor modifications or more aggressive driving can easily cut this in half.  Well-built but heavily modified street engines will need attention after about 5,000km. In circuit racing, a V8 Supercar engine lasts about 4,000 km. In drag racing, a GM LSX454 is good for about 600 passes, or about 240 km. And a top fuel dragster engine needs extensive maintenance after a single pass, or just 400 metres and less than four seconds…
  • It will make more heat, and this heat will need to be controlled. So higher horsepower engines will need bigger cooling system to cope. This not only means radiators, but oil coolers as well.

All things to keep in mind when you embark on your next project…

Next up, we’ll look at the induction system.