Tuning the C20LET - BOOST: what is it anyway?

Boost::gimme, gimme, gimme!

If some is good, then more is better ---> and too much is never enough. Right?

Everybody talks about boost, hey, it must be a good thing. Upping the boost gives horsepower out of thin air - horses that the manufacturers tried to keep hidden from us.

They keep those actuators set low, mainly for insurance purposes, to please the Green party, the nanny state wants to protect us from ourselves - you name it, there's a conspiracy behind it. Some conspiracies are believed to be instigated by the governments themselves, to cover up (or discredit) other conspiracy theories...

Those with some technical understanding know that manufacturers will always err on the side of caution, to keep low those warranty claims, and account for weird driving conditions (extreme heat, abnormal altitude, etc.)

Those of even more technical inclination will concede that safety margins will not be compromised if fuelling and intercooling are kept within spec - all in the name of more boost!

 

So what is boost anyway? How does it relate to power?

 

Let's look at a normally aspirated 2 litre engine: The maximum it can theoretically pump every two revolutions is 2 litres of air. Expertly-tuned exhaust systems can raise this a bit above 2 litres, but that's only for a specific, narrow rpm band - and it's at the expense of breathing at other parts of the rpm range. Let's face it, in the real world it will always be below 2 litres, i.e. the Volumetric Efficiency(VE) will always be below 100%. 80~90% is not bad for modern engines (full throttle, max torque revs)

So the engine intake is always under vacuum, meaning that it never manages to see atmospheric pressure. If we'd want the engine intake to be experiencing atmospheric pressure any time the throttle is wide open, then we'd need a PUMP to be providing pressurised air, at a rate slightly higher than the engine can consume. If we can then adjust that pump's output so that the engine never sees vacuum under full throttle, then we're close to a n/a engine operating at it's theoretical maximum VE. Looking at the pump's output, we're surprised to see that it's pumping well over 2 litres/2 revs. Lots of losses there then...

Now we can turn up the output of this large airpump of ours, so that the engine (under full throttle) shows higher than atmospheric pressure. But why is that? Why can't the engine consume the extra air that's stuffed down it's throat and still remain in vacuum? That would be nice, wouldn't it? Stuffing more and more air, and still registering a vacuum on the intake - that would mean that our engine is more and more efficient, that it can inhale, process and exhale a lot of air. CFM figures from a head flowbench are an indication of this 'processing' ability. Cubic Feet per Minute, that's volume of air that can shift through the head per minute. More air will need more fuel, so that will mean more power. Of course intake and exhaust pipes will also need to flow more, so that they don't act as weak links in this flow chain. We get the picture.

But in reality, as we turn up the output of our large airpump, the engine shows vacuum no more, in fact it shows boost. Is that a good thing? Well, let's look at it calmly: It's an indication that we've reached the engine's flowing capacity, and now the extra pressurised air is piling up on the intake. It's stacking up in a queue, waiting to be consumed, because our engine has reached it's limits, the house is FULL.

So boost can be seen as the engine's reluctance to flow. Not very glamorous is it?

That's what most factory turbocharging was all about back in the dark ages: Engines were 2valve/cylinder designs. 4valvers were coming out, but they did cost more to manufacture. So the designers were trying to extend the lifespan of the older engines by turbocharging them. Either that, or creating a cheaper alternative to their bigger engines, for their own reasons (weight, legal capacity ranges etc.). Turbocharging was no more than a Band-Aid for obsolete engines (that couldn't breathe properly like the 4valve designs). Of course marketing used the word 'turbo' to portray a sporty and rapid lifestyle, but the truth remains: those with no financial constraints would always go for a proper, big engine. It's still the case today for all major auto manufacturers, although medium-sized and big-engined turbo cars do exist if one looks hard enough.

 

So what?

Nice one, now our life is richer, we know that positive pressure on our boost gauge is an indication that we've exceeded our engine's flow capacity and we're filling up the cylinders under pressure - what's wrong with that?

Nothing's wrong with that as long as the temperature of the pressurised air doesn't get out of hand - and the ultimate conditions in the combustion chambers are within spec. But let's compare two 2.0 litre engines:

1. 8v design, restrictive intake and exhaust, doesn't like revving at all.
2. 16v design, head is gas-flowed to the brim, intake and exhaust routes are short, wide, begging for more

Both engines are fed by our large external airpumps, fooling them into thinking that they live on a planet where atmospheric pressure is double that of the earth's (boost at 1 bar)

Trick Question: do both engines consume the same volume of air? After all they're both 2 litre engines running the same boost.

Obvious Answer: of course not! If we look at the output of the pump serving engine #2, we'll see that it works a lot harder than the other one. It has to flow a lot more air through the 16v engine, if it's to keep up with the 1 bar boost objective. It's also producing a lot more power at that 'same' 1 bar boost.

If we take this to an extreme, if the engine were bigger in capacity, could rev like a F1 design, and could swallow the air like Linda Lovelace, then our big, external airpump would never manage to register 1 bar boost, despite the monumental airflow it would have to provide (and the extreme bhp figures on the dyno). Our big airpump would run out of breath just trying to produce a single psi of boost. The engine would consume anything the pump could throw at it, and still beg for more. The bigass pump is working flat out and still no boost in sight - is that a bad thing? Or is it a sign of an extremely efficient engine?

Think about this, next time someone brags about doubling the boost on their stock, asthmatic engine.

Like flogging a dead horse...

Speaking of boost, take it easy - see what happened to this Scooby (local copy here)

Back to the Introduction