There are some very common misconceptions out there about tuning turbo engines. In particular whether improving flow through cylinder head and induction system porting makes any difference to power or whether all you have to do is turn the boost up. The simple answer is that improving flow can be even more important on a turbo engine than a normally aspirated (N/A) one. All engines make power in proportion to the amount of air they can breathe and anything that increases air flow into the cylinders increases power.
A N/A engine relies solely on the 14.7 psi of atmospheric pressure to push air into the cylinders when the valves open. With no way of increasing this pressure it's pretty well understood by most people that you have to improve the flow ability of the head and induction system to get more air through in a given time. So why the confusion about turbos? In fact a turbo engine is really only a N/A engine operating at a higher than standard inlet manifold pressure. If this were a larger planet with a thicker atmosphere it might well be that standard atmospheric pressure could be 30 psi instead of 14.7 psi. Engines would then produce twice the horsepower from the same capacity but tuning mods would still concentrate on improving induction system flow in exactly the same way as they do at 14.7 psi. Fitting a turbo to boost inlet manifold pressure from 14.7 psi to 30 psi just makes an engine on this planet run in a similar way to a N/A one on a bigger planet and the same mods will improve power on both.
To generate a pressure difference there has to be a resistance to flow. Try blowing as hard as you can through a straw and then do the same just through your open mouth. You can generate enough pressure through the straw to puff your cheeks out and almost no pressure without the straw. The amount of airflow is much greater without the straw of course. All the pressure rise is telling you is that there is a resistance to flow but it doesn't tell you how much flow is taking place. The same thing applies to a turbo engine. What the boost gauge is telling you is how much pressure it takes to generate a given amount of horsepower through the restriction of the head and induction system that the turbo is pushing against. If you reduce the restriction you'll get more airflow and power at the same boost level or alternatively the same amount of power at a lower boost level.
The Universal Gas Laws tell us that when we compress a gas we also raise its temperature. This is also discussed in the article on doing compression tests. Turbos increase the temperature of the air they compress and this reduces power and increases the engine's tendency to detonation. This excess heat has to be removed with an intercooler but this adds yet another restriction to flow. The more boost you run the bigger the intercooler needs to be which loses flow again leading to the need for even more boost and eventually you end up in a vicious circle which limits the amount of power the engine can generate.
So the trick is to get more air into the cylinders with as little heat rise as possible. The answer is simple. Reduce the restrictions to flow which then reduces the amount of boost the turbo needs to generate and also the amount of extra heat created. You get more power with less heat and kill two birds with one stone.
In simple terms if a ported cylinder head gives 20% extra power on a N/A engine it will also give 20% extra power on a turbo'd engine. Maybe even more once the heat drop is taken into account. However, the bhp amount of the power gain will be much higher on the turbo engine because the base power is higher. If you take a 150 bhp N/A engine that extra 20% will add 30 bhp but if the engine is turbocharged to 15 psi boost and produces 300 bhp on the standard head then the 20% extra power from the ported head becomes 60 bhp.
Ported heads and especially big valve ported heads are therefore just as important on turbo charged engines as they are on N/A ones.
One thing you need to watch out for on turbocharged engines is the spark plug gap. This needs to be smaller than for normally aspirated engines because it takes more voltage to fire a spark across a denser fuel/air mixture. The average 0.8mm to 1mm gap on a N/A engine will need to be reduced to maybe 0.6mm depending on the output of your ignition system or else the engine will give the feeling of running into a brick wall at higher rpm as the boost blows the spark out. If your engine runs ok up to medium rpm and then just stops dead or misfires under boost have a look at this area.
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