How Does Boost Work?

If you've seen the latest episode (CLICK HERE) then you'll see Marty has prepped his self-proclaimed Greatest Japanese Car in the History of The Universe for more boost. We know this is good, but what is boost and why does everyone want more of it?

All engines (even those strange triangle-shaped piston-less, God-less things called "rotaries") are simply air pumps which work on the old principle of "suck, squish, bang, blow". They suck air in, squish it with a bit of fuel mixed in, a bang known as combustion occurs, and then the engine blows the spent gases out the exhaust.

The more air and fuel you can ram into a given engine (within reason) the more power you will make (within reason). When air is sucked into the engine it is done so at one unit of "atmosphere", and anything more that you can push in using force is considered "boost". 

Normally boost-adders compress air as the most efficient way of craming more atmospheres into an engine.You will sometimes hear Japanese tuners refer to "bars" of boost, as they measure pressure in "bar", not PSI or how many pigeons can be fitted to an intake pipe (I'm looking at you here, MOOG).

One bar is 14.7psi, and this is considered to be double the atmosphere of any given engine. So, if you add 14.7psi to your Honda engine you have doubled the air inside that engine. If you add fuel to match, you make more power.

"But, Workshop Manuel, how does that work?" you may well ask. 

Well, pookie (do you mind if I call you pooks?) because squeezing this increased air into your engine effectively increases its displacement. This is because it's consuming more air than what it could naturally suck in, on its own. 

There are many ways to push more air into an engine, and they're all loosely referred to as "super-charging" as this intake air is known as "charge-air". Boost-adders come in many forms: belt-driven superchargers (known, traditionally, as superchargers), exhaust gas-driven superchargers (called turbochargers), chemical supercharging (nitrous), or "passive supercharging" where air pressure from the bellhousing or cowl area is fed into the engine. 

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All these different methods have positives and drawbacks depending on packaging, efficiency, weight, and more. One of the biggest considerations for choosing a method to ram boost into your engine is how much heat it will introduce, and how effectively you will be able to get rid of that heat. 

Old school "blowers" like the ones favoured by our mates from Roadkill (below), can push a lot of air but also take a lot of power to spin and heat up the air a lot.

"But Workshop Manuel, if boost is good then why is hot boost air bad?" I hear you ask. I'm glad you asked that question, pooks!

As air is compressed the molecules are jammed together and this creates heat through friction. Engines don't run great on hot air, so you really want to try and ciol the charge-air and keep it as cool and dense as possible. 

This is where intercoolers come into play, knocking the heat out of the compressed air so the air:fuel mixture is kept at its optimum, which is the best for power, fuel efficiency... and also for avoiding detonation which could have you driving over your own crankshaft (that's bad, pooks). 

Another important boost fact to get around is that the amount of boost you see in a gauge is actually a measurement of restriction. An engine's ability to process intake air is known as "volumetric efficiency" and VE teaches us you can't simply cram limitless amounts of air into an engine and have it keep making power at the same rate. 

Improving the engine's ability to breathe (through cylinder head mods, reducing manifold restrictions, or adding a more aggressive camshaft), will improve the engine's VE and you'll make the same power with less boost, as your engine can now process much more air than before.

Besides all the science and the learning, what boost is can be summed up in one word: fun. Naturally aspirated engines are awesome,but the silliness of a boosted engine singing its song is one of life's true joys. So, time for some PgUps...